Scientists Find The Size of Smallest Habitable Planets Possible

Hello and welcome! My name is Anton and in this video, we will talk about a new study that discovers the smallest limit for a habitable world - how small can a planet be and still be habitable?

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[andrewjohnson]

Serious question: we spend a lot time looking for water and the habitable zone for exoplanets. Shouldn’t we be looking for ways to detect a magnetosphere? That’s clearly the key element to sustainable livability.

[wayneshirey]

The existence of life near thermal vents deep in Earth's oceans makes the idea of life in ice covered oceans on small moons feasible. Sunlight is not required, heat is supplied by the gravitational effects of the nearby gas giant, and the liquid water is protected by the ice shell. No trees or skyscrapers msybe, but life could very well exist there.

[psykkomancz]

Imo existence of habitable moons protected by magnetospheres of giant planets who they orbit is more probable than planets like Earth doing it for themselves. For single planet, possibility of developing maghetosphere depends very much on composition of given protoplanety disc.

[geoffhalsey]

There was a NASA article I read, some time ago, that proposed a means of creating an artificial magnetosphere for interplanetary space vehicles that could be theoretically could be scaled up to protect Mars. The proposal was to position the device at Mars L1 lagrange point, to able it counteract the solar wind. Possibly, such a device might also be used to provide the Earth with additional protection from the effects of the Quasar galaxy scenario, outlined in one of your previous videos, or even another Carrington event from our own Sun. So and idea not without merit during the next Pole reversal either.

[JORDIIMusic]

Love your videos Anton. Thanks for sharing this information about our solar system!

[Raptor]

Why do I always get existential dread whenever I think about a habitable planet dying, even though I know it will live 10 million times longer than me?

[donm-tvcm]

It's an interesting first study, and it produced a number far lower than I would have ever guessed. But it's flawed in that it only looked at the solar/temperature effects versus mass. I think a critical parameter would be the thermal mass of the planet: how quickly a planet of given size cools down and solidifies inside, making a magnetosphere impossible. Smaller planets cool quickly, larger planets cool over long periods. They say that that was Mars' downfall -- its magnetosphere went away after it cooled, and the solar wind stripped away most of its atmosphere and water. At 0.10 Earth's mass, Mars was done within its first billion years. Conversely, a "super-earth" should still be tectonically active with a magnetosphere billions of years longer than our own Earth will last in this regard.

Also, a small planet with low gravity closer to the Sun shouldn't last very long at all – the combination of low gravity, “taller” atmosphere, and quick loss of magnetosphere should make it very vulnerable to the more intense solar flux, producing a “hot Mars” outcome or even something like Mercury.

[Coolcleverstone]

We must increase the mass of Ganymede! Start pushing rocks at it!

[andrewbridson]

A very interesting piece of research. Your students, and YouTube, are very lucky to have you as an educator.

[ArdaKaraduman]

AFAIK another problem with planets close to the star is that they tend to get tidally locked, which stops core rotation and eventually removes the magnetic shielding effect. (which will eventually lead to loss of atmosphere)

[JeffDrennen]

cool video 3 things come to my mind here.

1.) I was disappointed that liquid water could show up on smaller bodies in our Solar System. Think about it, a body floating around in our Solar System that has not been yet been betected and harbors extraterrestrial life. that would have been very cool.

2.) Concerning planets that are 0.286 mass of earth's, I would think such a planet would not have a long lasting magnetosphere, Considering smaller masses don't hold in heat as long as larger ones do. In other words the smaller planet cools faster. It's liquid core would become solid sooner and no more magnetosphere it would end up like Mars. Although if a planet that is closer to its parent star would that keep its core liquid longer because of the residual heat from the star?
3) Obviously I didn't read the paper (yet) but did it take into consideration other universal solvents like liquid methane?

[SweedRaver]

Quick correction at 3:10. You're saying that the minimum mass is 0.0268 Earth masses, or 5 times Mars, but you're showing Mars being 0.107 masses of Earth. I think you mean to say around a quarter the mass of Mars.

[Evolved_Skeptic]

Sorry, to clarify:

Do you mean that - under the *Solar* conditions that you described - *The Jovian Moons* cannot sustain _liquid water_ on their surfaces *_At All_*, or should _water_ ever heat up & _liquefy_ on their surface, it will not be able to last for over *A Billion Years* (+)..?

Even if the _liquid water_ only lasted for *A Hundred Thousand Years* - hell - even if it only lasted for *Ten Thousand Years*, that could still become an extremely useful colony world for a myriad of _Future Human Species_ ...


(+) The minimum time that it's estimated would be required for any *Life* to develop on it's own.

~ ~ ~

I'm also intrigued about the *mass* of these smaller worlds, as well as their *size* . A planet with proportionately more heavy metals than silicates would have a fairly *small diameter*, perhaps half that of *Earth*, but almost as much *mass* . (This is a complete guess, as I don't know sufficient math to calculate this precisely.)

Are there methods of calculating the *mass* of *Earth* scale (or smaller) *Exoplanets* ..?

How about making educated guesses about their composition, depending upon their location in *The Galaxy* ..? You would think that the proportions of denser substances (iron, lead, gold, etc.) would increase the closer a planet is to *The Galactic Core*, and conversely lighter materials (such as silicates & oxides, etc.) would increase out towards *The Rim* .

[TimBee]

I seem to remember calculating the escape velocity from a planet, determined by the mass, and then determining if the various types of gases in the atmosphere had enough energy to escape.

[cheapskatecoins]

I have to disagree with that paper, i've never heard of a correlation between mass and liquid water. So long as there is enough atmospheric pressure, liquid water is stable, and Titan, which is smaller than their limit, has an atmosphere with almost twice the pressure of Earth's, which means theoretically if it warmed up enough, it could hold liquid water no problem.

[pipertripp]

To hang onto an atmosphere there are a couple of basic things to keep in mind. The surface gravity is important, the mass of the particles in the atmosphere (a diatomic nigtrogen), and the temperature at the body. To keep the gas for very long periods, the escape velocity of a gas particle must be at least 10 times the root mean square velocity of a particle in the gas.


Vesc = sqrt(2GM/R) where M is the mass of the planet and R is its radium. G is Newton's universal gravitation constant.
Vrms = sqrt(3kT/m) where k is Bolzmann's constant, T is temperature, and m is the mass of a gas particle.

If you run the numbers on these, you quickly see why the big moons can't hang onto gases when the temps get into the habitable range. Their surface gravities are too small and the gas particle masses are also too small and so the Vrms gets to the point where many particles near the edges of the atmosphere attain escape velocity and fly off never to return.

the 10x factor takes into account the fact that there is a long tail of velocities greater than Vrms (take a peek at Maxwell-Boltzmann distribution of velocities in a gas for a given temp). The probability of attaining such velocities is quite low, so as long was the Vesc is high enough, a body can hang onto its atmosphere for billions of years.

That's the astro 101 explanation anyways. I'm sure that papers goes into some really great detail on this.

[scottrose]

What would require more energy? Travel to alpha Centauri or terraform Mars? It seems Mars is the future.

[robchissy]

take a drink every time he says "in other words"

[AlmostEthical]

Hi Anton. Always a fan. Just wondering if you plan on a video about the Milky Way's possible/probably habitable zones. Cheers.

[michaelmcgowan]

I understand that Venus is on the edge of the habitable zone -- but probably a bit too close to the sun. Even so, it appears that Venus was habitable until some 700 million years ago when a runaway greenhouse effect occurred. As you explained in a video recently, it could have had liquid water for a few billion years. It seems that we might encounter some planets that might not be permanently habitable, but may be able to sustain the right condition for a billion years or so. That certainly is enough time to develop some forms of life, if earth is a guide.