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How large (that is, radius) could a planet be?

How large (that is, radius) could a planet be?


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I know that normal planets don't get any larger than Jupiter (or 2 Jupiter radii if hot), as adding more gas just increases density, not radius, until you reach the point of being a brown dwarf star.

That compressability, I suppose, is a property of the overall average material that forms star systems, as beyond a critical size it accretes the ambiant gas as well as dust in the cloud. That is, it's mostly hydrogen and a quarter helium with a bit of other stuff.

But what about a body made of other stuff, such as rock or metal? Without worrying about planetary formation processes, just that atoms are piled together and feel self gravity. Perhaps it needs to be grown slowly so it has time to cool before adding more. Otherwise, no special techniques: just what would happen if material of suitable composition was heaped together?

I suspect that normal ideas of minerals would not exist under that kind of pressure, even if carefully allowed to cool. But can elements other than H and He still compress their volume in the same way, or would a rocky world be able to reach sizes of millions of miles?

What about more exotic cases, like the "puffy" planet I heard of that has the density of styrofoam?


Puffy planets tend to be Jupiter or Saturn like, probably lower mass than Jupiter, perhaps lower metalicity but the most important factor is heat. Either close to the sun or recently formed. Heat expands gas planets. You're correct that as you add more mass the planet of Jupiter mass tends not to grow larger, but if there's enough internal heat, gas giant planets can get a bit larger than Jupiter. Planets as much as 2 Jupiter Radii have been observed (though there's some inaccuracy in those estimates), but growing larger than Jupiter is largely a factor of high temperature.

Rocky worlds will never grow as large as Jupiter. The greater mass will prevent it, and past a certain mass, a rocky world is unlikely to remain what we consider rocky. Above a certain mass it holds onto hydrogen which is the most abundant gas in the universe, and that would give the massive rocky world an appearance more like a gas giant.

But to answer your question in theory, if you had a rocky world with Jupiter mass and a negligible atmosphere, it could never get close to Jupiter diameter. The mass crushes the inside of the planet. Mercury for example is made of denser material than the earth. Higher Iron content, but it's less dense than the Earth because the Earth's mass crushes its rocky mantle and metallic core. When you start to get a Jupiter-mass rocky world, the crushing becomes significant and you could never build a rocky world close to as big as Jupiter. As with hydrogen, it would reach a certain maximum size, then it would begin to get smaller under the crush of gravity. Even so called "non compactible" material, does compact at the pressure inside large planets. Iron has a density of 7.874 g/cm3 and slightly less than that at high temperature, but the Earth's metallic core has a density of 12.6-13.0, and that's primarily due to crushing. When you've reached Jupiter mass, the crushing and density is significantly greater.

At about the mass of the sun the planet would be smaller than the Earth, and it would basically resemble a white dwarf star, and no longer be distinguishable as a rocky world.

I can add a few links to back this up if needed and if someone wants to run the math or give an answer with more detail, feel free.


The maximum possible radius of a planet is probably unknown to date because current observation techniques not only will give us a biased view of the distribution of planet radii but it is also difficult to tell planets and brown dwarfs apart.

However current data suggest a limit of roughly 2 Jupiter radii.


Large exoplanet could have the right conditions for life

Astronomers have found an exoplanet more than twice the size of Earth to be potentially habitable, opening the search for life to planets significantly larger than Earth but smaller than Neptune.

A team from the University of Cambridge used the mass, radius, and atmospheric data of the exoplanet K2-18b and determined that it's possible for the planet to host liquid water at habitable conditions beneath its hydrogen-rich atmosphere. The results are reported in The Astrophysical Journal Letters.

The exoplanet K2-18b, 124 light-years away, is 2.6 times the radius and 8.6 times the mass of Earth, and orbits its star within the habitable zone, where temperatures could allow liquid water to exist. The planet was the subject of significant media coverage in the autumn of 2019, as two different teams reported detection of water vapour in its hydrogen-rich atmosphere. However, the extent of the atmosphere and the conditions of the interior underneath remained unknown.

"Water vapour has been detected in the atmospheres of a number of exoplanets but, even if the planet is in the habitable zone, that doesn't necessarily mean there are habitable conditions on the surface," said Dr Nikku Madhusudhan from Cambridge's Institute of Astronomy, who led the new research. "To establish the prospects for habitability, it is important to obtain a unified understanding of the interior and atmospheric conditions on the planet -- in particular, whether liquid water can exist beneath the atmosphere."

Given the large size of K2-18b, it has been suggested that it would be more like a smaller version of Neptune than a larger version of Earth. A 'mini-Neptune' is expected to have a significant hydrogen 'envelope' surrounding a layer of high-pressure water, with an inner core of rock and iron. If the hydrogen envelope is too thick, the temperature and pressure at the surface of the water layer beneath would be far too great to support life.

Now, Madhusudhan and his team have shown that despite the size of K2-18b, its hydrogen envelope is not necessarily too thick and the water layer could have the right conditions to support life. They used the existing observations of the atmosphere, as well as the mass and radius, to determine the composition and structure of both the atmosphere and interior using detailed numerical models and statistical methods to explain the data.

The researchers confirmed the atmosphere to be hydrogen-rich with a significant amount of water vapour. They also found that levels of other chemicals such as methane and ammonia were lower than expected for such an atmosphere. Whether these levels can be attributed to biological processes remains to be seen.

The team then used the atmospheric properties as boundary conditions for models of the planetary interior. They explored a wide range of models that could explain the atmospheric properties as well as the mass and radius of the planet. This allowed them to obtain the range of possible conditions in the interior, including the extent of the hydrogen envelope and the temperatures and pressures in the water layer.

"We wanted to know the thickness of the hydrogen envelope -- how deep the hydrogen goes," said co-author Matthew Nixon, a PhD student at the Institute of Astronomy. "While this is a question with multiple solutions, we've shown that you don't need much hydrogen to explain all the observations together."

The researchers found that the maximum extent of the hydrogen envelope allowed by the data is around 6% of the planet's mass, though most of the solutions require much less. The minimum amount of hydrogen is about one-millionth by mass, similar to the mass fraction of the Earth's atmosphere. In particular, a number of scenarios allow for an ocean world, with liquid water below the atmosphere at pressures and temperatures similar to those found in Earth's oceans.

This study opens the search for habitable conditions and bio-signatures outside the solar system to exoplanets that are significantly larger than Earth, beyond Earth-like exoplanets. Additionally, planets such as K2-18b are more accessible to atmospheric observations with current and future observational facilities. The atmospheric constraints obtained in this study can be refined using future observations with large facilities such as the upcoming James Webb Space Telescope.


Rocky versus gaseous planets

In our Solar System, we have two kinds of planets: small, rocky, dense planets that are similar to Earth and large, gaseous planets like Jupiter. From what we astrophysicists have detected so far, most planets fall into these two categories.

In fact, when we look at the data from planet-hunting missions such as the Kepler mission or from the Transiting Exoplanet System Satellite, there is a gap in the planet sizes. Namely, there aren’t many planets that fulfill the definition of a “super-Earth,” with a radius of one and a half to twice Earth’s radius and a mass that is five to 10 times greater.

So the question is, why aren’t there any super-Earths? Why do astronomers only see small rocky planets and enormous gaseous planets?

The differences between the two kinds of planets, and the reason for this super-Earth gap, has everything to do with a planet’s atmosphere – especially when the planet is forming.

When a star is born, a huge ball of gas comes together, starts to spin, collapses in on itself and ignites a fusion reaction within the star’s core. This process isn’t perfect there is a lot of extra gas and dust left over after the star is formed. The extra material continues to rotate around the star until it eventually forms into a stellar disk: a flat, ring-shaped collection of gas, dust, and rocks.

During all of this motion and commotion, the dust grains slam into each other, forming pebbles which then grow into larger and larger boulders until they form planets. As the planet grows in size, its mass and therefore gravity increases, allowing it to capture not only the accumulated dust and rocks – but also the gas, which forms an atmosphere.

There is lots of gas within the stellar disk – after all, hydrogen and helium are the most common elements in stars and in the universe. However, there is considerably less rocky material because only a limited amount was made during star formation.

Comparison of confirmed super-Earth planets compared to the size of the Earth. NASA/Ames/JPL-Caltech


Solar System to Scale: Sun and Planets

It's often hard to fully grasp just how big the planets in the solar system are. 1.3 million Earths could fit in the Sun, but that is hard to picture. A good way to help with this problem is drawing the planets to scale. This dataset has the Sun as the background and then has a picture of the solar system drawn to scale. The Sun is also to scale with the rest of the planets. By far, Jupiter is the largest planet with Saturn the second largest, but they are certainly no where close to being as big as the Sun, which has a radius of 432,000 miles (695,000 km). Now that Pluto is no longer classified as a planet, Mercury, the closest planet to the Sun, is the smallest planet with a radius of 1516 miles (2440 km). The second planet in the solar system, Venus, is the third smallest planet with a radius of 3761 miles (6052 km). Earth, of course, is the third closest planet to the Sun and the fourth smallest with a radius of 3963 miles (6378 km). Just past Earth is Mars, the fourth planet in the solar system. Mars is the second smallest planet with a radius of 2111 miles (3397 km). The first four planets are called terrestrial planets because they are made mainly of rock and have thin wispy atmospheres.

The outer planets are called gas giants because they are large and consist mainly of hydrogen and helium. The gas giants also typically have a large number of moons, while the terrestrial planets have limited moons. The first gas giant, Jupiter, is certainly a giant. The radius of Jupiter is 44,423 miles (71492 km). More than 1300 Earth's could fit inside Jupiter. The next planet past Jupiter is Saturn, the second largest planet. The radius of Saturn is 37,449 miles (60268 km). The rings of Saturn would be the width of a credit card and would extend much further than the picture shows. The edge of the brightest rings extends 75,900 miles (122,200 km) from Saturn and the faintest rings extend out 300,000 miles (483,000 km) from Saturn. The seventh planet from the sun is Uranus. Uranus ranks as the third largest planet with a radius of 15882 miles (25,559 km). Now considered the eighth and final planet in the solar system is Neptune, the fourth largest planet with a radius of 15,389 miles (24,766 km). Uranus and Neptune are very close in size. The final object in the picture is Pluto, which is now classified as a dwarf planet. Pluto only has a radius of 715 miles (1150 km). It was demoted from planet to dwarf planet in 2006 when new guidelines were created to classify planets and Pluto didn't meet all of the qualifications.


Astronomers Puzzled by Giant Planet at Large Distance From Sun-Like Star

A direct image of the exoplanet YSES 2b (bottom right) and its star (center). The star is blocked by a so-called coronagraph. Credit: ESO/SPHERE/VLT/Bohn et al.

A team of astronomers led by Dutch scientists has directly imaged a giant planet orbiting at a large distance around a sun-like star. Why this planet is so massive and how it got to be there is a mystery. The researchers will publish their findings in the journal Astronomy & Astrophysics.

The planet in question is YSES 2b, located 360 light-years from Earth in the direction of the southern constellation of Musca (Latin for The Fly). The gaseous planet is six times heavier than Jupiter, the largest planet in our solar system. The newly discovered planet orbits 110 times more distant from its star than the Earth does from the sun (or 20 times the distance between the sun and Jupiter). The accompanying star is only 14 million years old and resembles our sun in its childhood.

The large distance from the planet to the star presents a puzzle to astronomers because it does not seem to fit either of the two most well-known models for the formation of large gaseous planets. If the planet had grown in its current location far from the star by means of core accretion, it would be too heavy because there is not enough material to make a huge planet at this large distance from the star. If the planet was created by so-called gravitational instability in the planetary disk, it appears to be not heavy enough. A third possibility is that the planet formed close to the star by core accretion and then migrated outwards. Such a migration, however, would require the gravitational influence of a second planet, which the researchers have not yet found.

The astronomers will continue to investigate the surroundings of this unusual planet and its star in the near future and hope to learn more about the system, and they will continue to search for other gaseous planets around young, sun-like stars. Current telescopes are not yet large enough to carry out direct imaging of Earth-like planets around sun-like stars.

Lead researcher Alexander Bohn (Leiden University): “By investigating more Jupiter-like exoplanets in the near future, we will learn more about the formation processes of gas giants around sun-like stars.”

The planet YSES 2b was discovered with the Young suns Exoplanet Survey (YSES). This survey already provided the first direct image of a multi-planet system around a sun-like star in 2020. The researchers made their observations in 2018 and 2020 using the Very Large Telescope of the European Southern Observatory (ESO) in Chile. They used the telescope’s SPHERE instrument for this. This instrument was co-developed by the Netherlands and can capture direct and indirect light from exoplanets.

Reference: “Discovery of a directly imaged planet to the young solar analog YSES 2” by Alexander J. Bohn, Christian Ginski, Matthew A. Kenworthy, Eric E. Mamajek, Mark J. Pecaut, Markus Mugrauer, Nikolaus Vogt, Christian Adam, Tiffany Meshkat, Maddalena Reggiani and Frans Snik, 19 April 2021, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202140508


Or is it the bigger the planet the less the gravity?

If the composition stays the same -- and that's a very big "If", -- then the bigger the planet, the stronger is gravity. Specifically, increasing the radius by a factor F increases mass by F^3, and decreases gravity due to larger distance from the center by F^2. Ergo, given constant composition, gravity is proportional to radius.

The above holds true for asteroid-to-moon size bodies, Above that things get more complicated because the pressure in the planet's center becomes great enough to increase any material's density, and the bigger is the planet, the greater is compression. Which means that increasing the radius by a factor F increases mass by more than by F^3, and gravity increases faster than radius. How much faster, I do not know.

However, since very large planets also have increasing proportion of light elements, their surface gravity ends up rather less -- Uranus, Neptune and Saturn all have surface gravity less than Earth. Beyond Saturn's mass adding more mass mostly increases density without adding much to radius, so gravity rises quickly. Beyond about twice Jupiter mass adding more material compresses the planet so much that its radius actually decreases, and surface gravity rises VERY quickly.


How large (that is, radius) could a planet be? - Astronomy

I was always under the impression that for an object to be a planet it had to have a satellite orbiting around it, a moon, that is why Pluto can be called a planet even though it is so small. My question then is, why are Mercury and Venus planets and what are the parameters required for planet status.

Since the original submission of this question (in 1999), Pluto has been demoted to a dwarf planet, instead of just a planet.

Not all planets have moons (you've pointed out that Venus and Mercury do not), and that's not a requirement.

In 2006, the International Astronomical Union (IAU), which has the final say on matters of astronomical nomenclature, voted on a formal definition of what makes a planet. (The official press release is here.) According to their decision a planet must satisfy the following three criteria:

  • It must be an object which independently orbits the Sun (this means moons can't be considered planets, since they orbit planets)
  • It must have enough mass that its own gravity pulls it into a roughly spheroidal shape
  • It must be large enough to "dominate" its orbit (i.e. its mass must be much larger than anything else which crosses its orbit)

Because Pluto is not large enough to "dominate" its orbit, it is not a planet. (Neptune is about 8000 times more massive than Pluto, so Neptune is a planet and Pluto is a dwarf planet.)

This page was last updated on July 24, 2015.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.


Super-Earth Exoplanet K2-18b Could Have Right Conditions for Life

A team of astronomers from the Institute of Astronomy at the University of Cambridge, UK, has found K2-18b, a planet of almost nine Earth masses in orbit around the red dwarf K2-18, to be potentially habitable.

This artist’s impression shows planets K2-18b and c and their host star. Image credit: NASA / ESA / Hubble / M. Kornmesser.

K2-18 is an M-type star located some 111 light-years away in the constellation Leo.

Also known as EPIC 201912552, the star hosts two massive planets: K2-18b and c.

Discovered in 2015, K2-18b has a radius of 2.6 times that of Earth and is about 8.6 times as massive.

The planet orbits the parent star every 33 days at a distance of approximately 0.15 AU and has an Earth Similarity Index of 0.73.

In 2019, two different teams reported detection of water vapor in the hydrogen-rich atmosphere of K2-18b. However, the extent of the atmosphere and the conditions of the interior underneath remained unknown.

“Water vapor has been detected in the atmospheres of a number of exoplanets but, even if the planet is in the habitable zone, that doesn’t necessarily mean there are habitable conditions on the surface,” said Dr. Nikku Madhusudhan, lead author of the study.

“To establish the prospects for habitability, it is important to obtain a unified understanding of the interior and atmospheric conditions on the planet — in particular, whether liquid water can exist beneath the atmosphere.”

Given the large size of K2-18b, it has been suggested that it would be more like a smaller version of Neptune than a larger version of Earth.

A ‘mini-Neptune’ is expected to have a large hydrogen ‘envelope’ surrounding a layer of high-pressure water, with an inner core of rock and iron.

If the hydrogen envelope is too thick, the temperature and pressure at the surface of the water layer beneath would be far too great to support life.

Now, Dr. Madhusudhan and colleagues have shown that despite the size of K2-18b, its hydrogen envelope is not necessarily too thick and the water layer could have the right conditions to support life.

The astronomers used the existing observations of the atmosphere, as well as the mass and radius, to determine the composition and structure of both the atmosphere and interior using detailed numerical models and statistical methods to explain the data.

They confirmed the atmosphere to be hydrogen-rich with a significant amount of water vapor.

They also found that levels of other chemicals such as methane and ammonia were lower than expected for such an atmosphere.

Whether these levels can be attributed to biological processes remains to be seen.

The researchers then used the atmospheric properties as boundary conditions for models of the planetary interior.

They explored a wide range of models that could explain the atmospheric properties as well as the mass and radius of the planet.

This allowed them to obtain the range of possible conditions in the interior, including the extent of the hydrogen envelope and the temperatures and pressures in the water layer.

“We wanted to know the thickness of the hydrogen envelope — how deep the hydrogen goes,” said Matthew Nixon, co-author of the study.

“While this is a question with multiple solutions, we’ve shown that you don’t need much hydrogen to explain all the observations together.”

The scientists found that the maximum extent of the hydrogen envelope allowed by the data is around 6% of the planet’s mass, though most of the solutions require much less.

The minimum amount of hydrogen is about one-millionth by mass, similar to the mass fraction of the Earth’s atmosphere.

In particular, a number of scenarios allow for an ocean world, with liquid water below the atmosphere at pressures and temperatures similar to those found in Earth’s oceans.

Nikku Madhusudhan et al. 2020. The interior and atmosphere of the habitable-zone exoplanet K2-18b. ApJL, in press doi: 10.3847/2041-8213/ab7229


Ep. 79: How Big is the Universe?

We’re ready to complete our trilogy of discovery about the universe. We’ve learned that it has no center rather everywhere is its center and nowhere. We discovered that the universe seems to be flat. It’s not open, it’s not closed, it’s flat. If that doesn’t make any sense, you need to listen to the previous show because there’s no way I could give that an explanation.
So now we want to know: How big is it? Does it go on forever or is it finite in scale? How much of it can we see?

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The Visible Universe — map from the Atlas of the Universe

Transcript: How Big is the Universe?

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Fraser Cain: We’re ready to complete our trilogy of discovery about the universe. We’ve learned that it has no center rather everywhere is its center and nowhere.
We discovered that the universe seems to be flat. It not open, it is not closed, it is flat. If that doesn’t make any sense, you need to listen to the previous show because there’s no way I could give that an explanation.
So now we want to know: “How big is it? Does it go on forever or is it finite in scale? How much of it can we see? All right, I know you wanted to define some terms before we got rolling so why don’t we break this down.
Dr. Pamela Gay: The first thing we have to figure out is what is the visible universe. The part of the universe we can see is probably only a very small fraction of the total universe. But it’s not what we think it is.
Fraser: Well, what do I think it is?
Pamela: So, most people ask, “How far away do you think we can see?” They reply, if they’ve been listening to the show long enough, well the universe is 13.7 billions years old and so that means we can probably see things that are 13.7 billion light years away.
Fraser: In all directions?
Pamela: In all directions. And its sort of valid logic, but the problem is that the Universe is expanding. We can see objects with light that has traveled a path that is 13.7 billion light years long, but the problem is the starting point of that photon that traveled that 13.7 billion years isn’t 13.7 billion light years away.
Fraser: You’re right. If you had asked me, that would have been my answer. But now that I think about it, it’s all red chipped it’s all spread out. So, it’s actually been traveling further than that. Okay, someone must have done the math.
Pamela: Someone has done the math. In fact, it’s more like 78 billion light years away is where that starting point for that furthest photon that we’re able to see started.
Fraser: Seventy billion light years?
Pamela: Seventy-eight billion light years away is the starting point of that photon.
Fraser: So we can see a sphere around us 78 billion light years radius, so 156 billion light years – a bubble, I guess across.
Pamela: And that’s all because the universe is expanding. The light travels one distance but the objects are moving away from us so what we’re actually able to see represents a larger sphere than the sphere the light traveled on. It’s really weird.
Fraser: But, wait a minute. I’m having one of those moments where the questions are bubbling up faster than I can enunciate them. So let me just sort of harness this.
I can just imagine let’s say we have the most distant the thing that happened at the Big Bang and it’s moving away from us and it’s emitting light. We’re seeing the light that came to us at the moment of the Big Bang, but how are we still seeing light that is coming from it as it’s moving 80 billion light years away? Help.
Pamela: Here is how I think of it: Imagine that you’re standing at the very end of a moving walkway that’s moving away from you. You start off with the moving walkway turned off. There is someone right in front of you with a bag full of marbles. They’re able to roll the marbles a little bit faster than the walkway is moving.
So you turn on the walkway and you tell them to start firing marbles. The marbles that they’re throwing at you are having to travel further than the original distance between you and that person, but less than the distance between you and that person when the marble gets to you.
So that person keeps moving away from you and that marble that’s moving just a little bit faster than the walkway is moving gets to you eventually but it has to travel this increasing distance with time.
Now if that person keeps rolling marbles, you’ll get a steady stream of marbles from that person the same way we get a steady stream of photons from the cosmic microwave background. But each of those marbles has to travel a little bit longer and a little bit longer as the universe expands.
Fraser: And that would be all well and good except we now have the problem of the accelerating expansion of the universe thanks to dark energy. So there could be a time when that poor person rolling marbles is going to roll marbles and they’re just never going to reach you.
Pamela: Exactly. One of the problems that we have is that even ignoring the fact that the universe’s expansion is accelerating the moving walkway kind of breaks down because the universe isn’t causing people to move at a constant or causing galaxies as the case may be to move away at a constant velocity from us.
Rather, objects that are further and further away are moving away faster and faster. So objects that are coming from greater distances actually have to travel even more than you’d expect from our simplistic moving walkway.
What you might imagine is instead you have this expanding floor where you’re each standing on a different edge of a tile and the tiles are getting bigger and bigger. The more tiles there are between you and the person rolling marbles, the faster they have to roll the marbles. So, it gets really mathematically complicated very quickly.
Fraser: Or maybe they step on to faster moving walkways every five minutes, right? So the furthest people have the hardest trouble. What implication does this have? I can imagine when cosmologists are even doing their calculation, which we went into last episode, talking just about the shape of the universe.
The math must come in pretty heavy. I’ve read articles about how astronomers have found a galaxy and we see it, as it was a mere one billion years after the Big Bang. This implication must really come into their calculations.
Pamela: Well one of the things that gets most complicated is trying to state what you mean by distance. When I say a galaxy is some distance away, I could be talking about the distance between where it was and where the Earth was at the moment that the light was given off. That’s a really tiny distance.
I could be talking about the distance that the light traveled. That’s the middle distance. I could be talking about the distance between where the object was when it gave off the light and where the Earth is now, which is a little bit bigger.
I could be talking about the distance between where the object is now and where the Earth is now which is the largest distance.
Fraser: I get this complaint from a lot of people when they’re reading articles and we talk about something like right now this star is giving off puffs of stellar material. It’s giving off its outside envelope.
But the star is 12,000 light years away. People will complain, don’t say the words right now because it happened 12,000 light years away. This is just sort of a bigger version when as you say, there are four different ways to talk about right now.
It’s not fair to pick one arbitrarily and so it’s almost like the one we default to when we’re doing the writing is right now for the light as we see it today.
Pamela: Yeah. It’s like if you look at this moment at this place in the sky this is what you would see.
Fraser: Because that’s the only one we can really know for sure. All the others we are projecting and right now we don’t know what’s going to happen in 12,000 years. On the converse we could say when we look at this object it’s not really what is was doing 12,000 years ago. That’s just what the light was doing 12,000 years ago.
So you almost need to get into a cosmology conversation with the person when they have that argument with you. We just default to this is what we’re seeing right now and that’s what it’s doing right now. Yes we know that it happened in the past but this is the only way that we can write about it.
Pamela: And it gets particularly confusing when we’re looking at objects that its light got to us from two different directions. Imagine that you’re looking at a quasar where you’re seeing the light that went on the straight-line path in our flat geometry of the universe and came straight toward the planet Earth.
Fraser: We’re also looking at a bent image of this quasar where the light perhaps was headed off somewhere else and then was bent by a large galaxy, cluster of galaxies or some sort of a gravitational mass that lensed the object and bent its light to come to us from a different path.
That different path is going to be longer so it took more time for the light to get to us. So we see two different images, neither of which is now but we’re seeing them now. And we can see the galaxy do something in the straight line image and then look a little while later and see it do that exact same thing again in the bent line image.
That really mucks up when is now when you’re observing and what is the quasar doing now? ‘Now’ has way too many meanings.
Fraser: Did I mention people should keep some more of that ibuprofin this week for this weeks’ headache. Let’s go back then. We said that the observable universe is about 156 billion light years across.
That begs the next question: “What percentage of the real universe are we actually able to see?”
Pamela: We don’t know.
Fraser: Okay, well then I guess that depends. When I see a percentage I guess that depends on whether a universe is finite or infinite. So why don’t we talk about that and then we can come back around.
Pamela: This is where all cosmologists just sort of hang their heads and say, “We’re working on it. We have limits. We have ideas and we may never know the exact answer.”
Fraser: So let’s define then an infinite universe first. That would mean that wherever you go, in whatever direction you look, whatever direction you travel, it’s galaxy after galaxy star after star dark matter after dark matter. It’s turtles all the way down.
Pamela: It’s turtles all the way down and you can never get from the top turtle to the bottom turtle.
Fraser: Right. And one of the implications for that is I think we talked about multiple dimensions. When you have an infinite universe, not only is everything possible, but also everything has to exist.
If you travel in one direction far enough you will run into another Fraser and Pamela sitting on a planet that looks like the Earth recording a podcast talking about this situation.
In fact there will be an infinite number of Frasers & Pamelas having this conversation because if it’s an infinite universe, everything has to exist an infinite number of times.
Pamela: And it starts to get really screwball.
Fraser: Like it starts like the things that I just said isn’t screwball enough.
Pamela: So here’s where you start to go augh! Because like you said, in a truly infinite universe you start getting into infinite numbers of the same thing happening over and over again no matter how small the probability you eventually find it.
Now if you have a small enough finite universe, the light wraps around and we can see us in the past.
Fraser: Right. I believe we talked about that since the universe is flat parallel lines move in the same direction. You look one direction and once you hit the edge of the universe as it were, that’s just the back of your head.
There is no edge and so in all directions at the most distant thing, what you will see is where you started. But then I guess you would look past your own shoulder and see further, right?
Pamela: Well, what’s so cool about that idea, which doesn’t work out it turns out, is we could in principle if the universe were finite and small enough, look and see what the Earth looked like 10 billion years ago. That would be so cool to actually watch the evolution of the planet.
Fraser: But if is was small enough wouldn’t we it a billion light years and the just past the Earth there’s another Earth and that one we’re seeing at 2 billion years ago?
Pamela: That’s if you’re really tiny. And we really would have noticed that by now. It’s not that tiny. But there was some hope, dashed horribly by the Wilkinson Microwave Anisotropy Probe that perhaps we lived in a universe that was either a four-dimensional toroid or some hyper-dodecahedron which is just fun to say.
If the universe was small enough we could look out at the cosmic microwave background and see in different places on the cosmic microwave background identical splotches of light. Identical places where we’re basically looking out the front door of the universe and seeing in through the back door.
Fraser: And we don’t see that.
Pamela: We don’t see that.
Fraser: So then wouldn’t that just say well fine then, that’s not true? When physicists say you’re looking out the front door and you’re seeing the back door shouldn’t that evidence from WMAP have said no?
Pamela: Well, so we looked out and astronomers stared at WMAP with great dedication and computer software trying to find identical splotches and they couldn’t do it. Now the thing is that doesn’t tell us that with certainty the universe is not a hyper-toroid It is not a hyper-dodecahedron, not a soccer ball, not a donut.
We can’t say that because it could be that it’s just such a big donut or soccer ball that the two identical splotches, the front door and the back door that see in through one another lie further apart than the size of our visible universe.
Fraser: So they moved away too far?
Pamela: Yeah. And we just can’t see the two identical splotches on the sky.
Fraser: So, seeing identical splotches would give you the confirmation, so it’s still unknown. Let’s go back to the infinite universe for a second. How would the Big Bang fit into that concept?
Pamela: The idea is that with certain geometries, for instance the saddle shaped geometry, you can’t create a geometry that is saddle shaped and closed in. The edges are just reaching out and it has to be infinite to get to that geometry.
With flat you can start to get the things like toroids, which are finite. But in general flat in most of the ways we work the math with you end up with an infinite universe. It’s just the geometry of space that you can always take that next step and fall out of the universe but you’re still in it because it’s infinite. You can just keep going and the universe has no edges in its particular geometry.
Fraser: So the discovery of the flat universe supports the infinite universe.
Pamela: It supports it but it doesn’t require it because we could have this hypertoroid. We could have some other really crazy geometries. And there is also the question of if our universe is really big enough it could have all sorts of crazy shapes, crazy geometries where it’s just the little tiny part of the universe that we’re in appears to be flat.
That’s one of the annoying things when you start dealing with really big things. You stretch anything out and it looks flat. You stretch anything out and it looks like it’s constant consistency.
Fraser: But I guess with the Big Bang like imagine an infinite universe, 13.7 billion years ago, the universe was a singularity.
Pamela: Right.
Fraser: And then it started expanding from that. But yet if it’s infinite, was it a singularity of infinite size? I guess.
Pamela: It’s a singularity with no edges. This is where the finite/infinite thing starts to break the human brain. Because part of the definition of a finite universe is that you have a closed geometry. The definition of finite is that you go in a straight line and you move back to the back of your own head.
And with an infinite universe, the geometry, you just keep going. And so you start to get into the, well it’s not finite and infinite in the sense of six dice versus an uncountable infinite number of dice. It’s a does the thing have an edge or not type of boundary.
Fraser: Now we talked a bit about it that WMAP helped close off some of the possibilities that would have said that it is absolutely finite. But left open the possibility for both, then where is the research going now to define it?
Pamela: So now we start looking and saying okay, so we’ve ruled out these possibilities. Smaller dodecahedrons. So with the next generation microwave probe, the Plank mission, we’re going to have higher resolution and greater sensitivity.
So, let’s start figuring out what are the patterns in the hot spots and cold spots in the microwave background that we’d see if you made the universe a little bit bigger and just starting to get the edges of these doors and windows looking in through one another.
What are the patterns in the light if you have it going one way or another way? What are the geometries we haven’t thought of yet?
The ones that get the most attention in the media that people have talked about are basically a soccer ball type shape and a donut type shape that are both highly unsatisfactory. We don’t want the universe to be shaped like that. It hurts our little brains.
So, let’s start thinking outside of the box. Let’s start by figuring out what are the different ways that we can mathematically define something as flat. Which strictly means two parallel lines stay parallel and two lines that start out at right angles to each other stay at right angles to each other.
What are different ways that we can start getting at those geometries and what are the different signals that we would find in the cosmic microwave background if we had these alternative geometries? And let’s start thinking in lots of dimensions.
Let’s start thinking with a box that’s rotated in all sorts of crazy ways that we can’t generally visualize but we can program a computer to create. It becomes the world’s ugliest geometry problem. Then you start trying to figure out what are the limitations if the universe is this big what does it mean?
Unfortunately, all we can ever do is put a lower limit on the size of the universe. If the universe truly is infinite, we’ll never be able to say that with certainty. That’s one of the sad things about this.
At most we’ll be able to say: “The universe is bigger than,” and then state some number.
Fraser: Right, because with this increasing sensitivity of our instruments we will be able to measure the hot and cold spots better and better. Each time that we don’t see the mirror images, then we know that the universe has to be bigger than ‘X .
Then we’ll produce a spacecraft that is even more sensitive and it will have another look at it and say okay, it has to be bigger than ‘Y’.
That’s the best we can do? We can never know for sure?
Pamela: That’s the best we can do.
Fraser: However, when I talk to you, your thinking seems to be that it’s finite, right? That seems to be your synthesis of the science so far. Why is that?
Pamela: At a certain point when you look at multiple lines of evidence you just sort of have to pick the one that your stomach is happiest with. This is where I have to admit that there is no consensus and more people are doing the math for an infinite universe – a little bit easier to do the math that way – than are doing it for a finite universe.
I have problems with the idea of an infinite universe that could be parallel to other infinite universes. That sort of breaks my brain.
Then there is just the notion of “How do you create a singularity that contains everything in an infinite universe that currently has a density of around six hydrogen atoms per cubic meter on average?” How do you create an infinite version of that?
It seems to make more sense in my brain, which favors the cosmology of multiple parallel universes, to have a finite starting point that has a critical density that is evolving as the volume changes and everything balances out but you’re dealing with finite amounts.
Packing multiple infinities side-by-side, my brain says no to that idea. However, I can’t scientifically say that my decision to like that theory is anymore valid than someone else’s decision that the universe should be completely infinite and that there is only one of them and someone else’s idea that there are infinite branching universes.
There could be someone else’s idea that there are finite branching universes. There are so many different things that could be coming together and we don’t observationally have a way of saying who is more right than whom.
Fraser: But you don’t need to bring in all of the multiple universe stuff into this, right? That’s ways of trying to answer problems in quantum theory. Which is a whole other show. Maybe that will be part four?
Then even so, no matter how insane the possibility is, all you have is the evidence. We’re so far beyond where your intuition can help you out. You left your intuition back home in the Savannah in Africa a million years ago. It’s not ready for contemplating an infinite versus a finite universe.
So, what lines of evidence could we possibly have to try and determine and narrow this? Is that all we’ve got is looking at the cosmic microwave background radiation and seeing the edge.
Pamela: Right now, yes. Right now all we can do to define the size of the viewable universe and to place limits on the size of the total universe is to look at the cosmic microwave background. And not place on it some size of the total universe and to study the rate of recession of the objects that are between us in the cosmic microwave background to measure the evolution of the expansion of the universe.
If we don’t actually understand how the rate of expansion has changed with time, we can’t accurately say where that point that admitted the photon from the cosmic ray background that we’re seeing is currently.
So we need to constantly work to refine our values for the expansion rate in the universe at various points in time. And we need to study the microwave background in as much detail as possible.
Through those two sets of observations we can figure out just how far away is that wall of the cosmic microwave background in terms of where are the points that admitted the light that we see now with our current location compared to where those points were when they admitted the light.
We can place limits on well we know the universe is bigger than what we can see. And that’s as good as it gets.
Fraser: All right, so let’s assume then that the universe is finite. I’ll give you this. How big is the universe?
Pamela: Well, a lot of people working on theories are coming up with numbers on the order of it’s about a hundred times bigger than what we can see. So that’s the current mainstream cosmology. About a hundred times bigger than what we can see.
Fraser: Okay, so that would then put us at about 1.5 trillion light years across. Not bad for only 13.7 billion years of expansion.
Pamela: It’s that period of inflation during the first second or so that really allowed us to get to where we are. Without that, we’d probably be able to see everything, but that period of inflation really stretched things out and allows us to only see a small corner of where we live.
Fraser: How much of that was inflation? How big was the universe by the end of inflation? Or was it just like the big accelerant?
Pamela: It’s the big accelerant.
Fraser: Right, so what then finally, percentage of the whole universe is our observable universe? In a finite universe?
Pamela: Oh, so now you’re starting to deal with cubes. You have to figure if the radius is 100 times of what we’re able to see, you take one over 100 cubed and that’s the fraction we see. So 100 cubed is 10 to the 6th so one over 10,000th. We see one over 10,000 the universe.
Fraser: One 10,000th of the universe? At our current minimum size of the universe based on what we see with WMAP.
Pamela: Kinda cool?
Fraser: Kinda sad. I just want to see more of the universe. I like to travel I have to say so….
Pamela: It gets worse though, think of this. In the part of the universe we can see, we only see four percent of the mass. What we can see is visible matter.
Fraser: Right, the rest being dark matter, dark energy and just black matter that we can’t see. It’s regular matter just not shiny.
Pamela: So we see four percent of one ten thousandth of the universe.
Fraser: And we only see what’s happening now or recently in a tiny little sphere around the earth. The further back we look, it’s old news. It would be like the only newspaper that shows up is news from the 14th century. Not really helping.
News from Japan is what happened in the 18th century. That would suck. Could you make it any sadder?
Pamela: No, I’m happy.
Fraser: All right, so just to reiterate, we can see just a teeny tiny fraction of the universe and an even teeny tinier fraction of what actually is and a teeny tiny fraction of what’s happening right now.
Well, I think we are done with this trilogy of the center, the shape and the size of the universe. I know we’ll get some questions. So send them in and maybe we’ll queue up our next question show to try and run through all of the headaches we’ve caused people and try and give you some relief.

This transcript is not an exact match to the audio file. It has been edited for clarity.
Transcription and editing by Cindy Leonard


How Big Is Earth?

Earth, the third planet from the sun, is the fifth-largest planet in the solar system only the gas giants Jupiter, Saturn, Uranus and Neptune are bigger. Earth is the largest of the terrestrial planets of the inner solar system, bigger than Mercury, Venus and Mars. But how big is Earth, exactly?

Radius, diameter and circumference

The radius of Earth at the equator is 3,963 miles (6,378 kilometers), according to NASA's Goddard Space Flight Center in Greenbelt, Maryland. However, Earth is not quite a sphere. The planet's rotation causes it to bulge at the equator. Earth's polar radius is 3,950 miles (6,356 km) &mdash a difference of 13 miles (22 km).

Using those measurements, the equatorial circumference of Earth is about 24,901 miles (40,075 km). However, from pole to pole &mdash the meridional circumference &mdash Earth is only 24,860 miles (40,008 km) around. Our planet's shape, caused by the flattening at the poles, is called an oblate spheroid.

Those numbers make Earth just slightly bigger than Venus, whose equatorial radius is about 3,761 miles (6,052 km). Mars is much smaller than both Earth and Venus, with an equatorial radius of just 2,110 miles (3,396 km).

But Earth and the other rocky planets are much smaller than the gas giants. For example, more than 1,300 Earths could fit inside Jupiter.

Density, mass and volume

Earth's density is 5.513 grams per cubic centimeter, according to NASA. Earth is the densest planet in the solar system because of its metallic core and rocky mantle. Jupiter, which is 318 more massive than Earth, is less dense because it is made primarily of gases, such as hydrogen.

Earth's mass is 6.6 sextillion tons (5.9722 x 10 24 kilograms). Its volume is about 260 billion cubic miles (1 trillion cubic kilometers).

The total surface area of Earth is about 197 million square miles (510 million square km). About 71% of our planet is covered by water and 29% by land. For comparison, the total surface area of Venus is roughly 178 million square miles (460 million square km) , and that of Mars is about 56 million square miles (144 million square km)

Highest and lowest points

Mount Everest is the highest place on Earth above sea level, at 29,032 feet (8,849 meters), but it is not the highest point on Earth &mdash that is, the place most distant from the center of the Earth. That distinction belongs to Mount Chimaborazo in the Andes Mountains in Ecuador, according to the U.S. National Oceanic and Atmospheric Administration (NOAA). Although Chimaborazo is about 10,000 feet (3,048 m) shorter (relative to sea level) than Everest, this mountain is about 6,800 feet (2,073 m) farther into space because of the equatorial bulge.

Everest and Chimborazo are nowhere near the tallest mountains in the solar system, however. The peak rising from Rheasilvia Crater on the asteroid Vesta, for example, is about 14 miles (22.5 km) tall. Mars' huge Olympus Mons volcano is nearly as high, at 13.6 miles (21.9 km), and it covers an area the size of the state of Arizona.

The lowest point on Earth is Challenger Deep in the Mariana Trench in the western Pacific Ocean, according to NOAA. It reaches down about 36,200 feet (11,034 m) below sea level.


Watch the video: Dimitar Sasselov: How we found hundreds of Earth-like planets (January 2023).