# How big can a planet get, with respect to its star for it to sustain life? [closed]

Earth is 108 times smaller than the sun in terms of its diameter while Jupiter is 10 times. Assuming Jupiter was located such that it could sustain life, theoretically how big a planet can get with respect to its star in order for it sustain life?

• Please note that Earth is a rocky planet as are Mercury, Venus and Mars. Jupiter on the other hand is a planet made mostly out of gas like Saturn. It is usually assumed that a rocky planet has better odds for hosting life, until proven otherwise by the nature. In the realm of exo-planets, we find Jupiter-like planets near their host stars, as a result of planetary migration. But this is not a criterion that allows life to emmerge as we know it. A life-hosting planet needs to be at the right distance from the host star for water to stay liquid and have enough organics and heat to sparkle life. Commented Jul 13, 2021 at 9:46
• I’m voting to close this question because it is overly speculative. Commented Jul 13, 2021 at 10:51
• I’m voting to close this question because I expect a bit of research effort before posting a question. Commented Jul 14, 2021 at 7:26

There is indeed a limit to size of planets for holding life. For planet habitability, the radius of planet should range between 0.5 and 2.5 Earth radii. (there is a list of exoplanets that are more likely to have a rocky composition and maintain surface liquid water (i.e. 0.5 < RP ≤ 1.5RE or 0.1 < MP ≤ 5ME). Let's look at the limits of the planet size and what happens if the limits are crossed:

## Higher Limit

[...] the size of the planet places an important constraint on this process. Bigger planets will have more gravity and this determines both the atmospheric pressure and the pressure at the bottom of the oceans. If this is too large, ice will form at the bottom of the oceans preventing liquid water from interacting with the silicate material that makes up the ocean floor. And when that happens, the carbon cycle immediately shuts down leaving the planet at the mercy of any temperature changes.

Alibert goes on to calculate the radius of an Earth-like planet above which a carbon cycle cannot operate. The critical threshold turns out to be about twice the radius of Earth (although this depends on a variety of factors such as the mass, density and make up of the planet). Alibert’s claim is that although there is no guarantee that planets in the habitable zone that are less than twice the radius of Earth will actually be habitable, those that are bigger than this threshold are not habitable.

https://medium.com/the-physics-arxiv-blog/how-big-can-a-habitable-planet-be-ed24d7148ef3

## Lower Limit

If a low-mass planet is too small, it won’t have enough gravity, and the atmosphere will be stripped away, and the water will either be stripped away with it, or frozen on the surface. That means the prospects for life are dim. The researchers say there is a critical lower limit for a planet to be habitable. That means that not only is there a band of proximity to the star that determines a planet’s habitability, there’s a size limit.

That critical size, according to Arnscheidt and the other authors of the study, is 2.7 percent the mass of Earth. They say that any smaller than that, and the planet simply won’t be able to hold onto its atmosphere and water long enough for life to appear. For context, the Moon is 1.2 percent of Earth’s mass, and Mercury is 5.53 percent.

https://www.universetoday.com/143731/planet-sizes-matter-for-habitability-too/

There are other parameters like orbit/rotation, geology, surface temperature, magnetosphere, is it located in so called "goldilocks" or habitable zone? etc. that also determine if the exoplanet is suitable for harboring life and when all parameters are taken together then the supposed data of "appropriate planet size for holding life" could change.

• There is one planet in the entire universe that is known to support life as we know it. It is ludicrous to think that that one planet is the only planet in the universe that supports life. But what about life as we do not know it? There are multiple science fiction stories about life in Jupiter. Commented Jul 13, 2021 at 11:43
• @DavidHammen I am not thinking there is only one planet in the whole universe that can support life. I am just pointing out the parameters and the constraints of a planet to support life. If an exoplanet have similar constraints to Earth along with other constraints not discussed like orbit/rotation, geology and other factors, it is likely to harbor life. See planet habitability index: universetoday.com/tag/planetary-habitability-index Commented Jul 13, 2021 at 11:52
• The moon Titan, which has an atmosphere, methane seas, and a possibility for exotic life, has 0.4 Earth radii and 0.022 Earth masses, both less than the values provided. You may want to make clearer that only planets/moons at habitable distance are meant, and only life as is known. The lower limit also depends on the planet's magnetosphere. If it's strong enough, any atmosphere can't be stripped away that easily.
– John
Commented Jul 13, 2021 at 12:00
• @John I have to ask whether stripping off a world's atmosphere by the solarwind is the major component of atmospheric loss, or whether escape from the outer atmosphere due to insufficient escape velocity is the major component of atmosphereic loss. In the latter case no magnetosphere could be capable of retaining an atmosphere on a world with insufficient escape velocity. Commented Jul 13, 2021 at 16:27
• @M.A.Golding The escape velocity limit is about 2.3 mi/s (3.7 km/s) for a CO2-atmosphere at habitable temperatures. But the upper answer talks about sizes and masses. A dense 0.02 Earth mass planet can have an escape velocity at 2.3 mi/s or more.
– John
Commented Jul 13, 2021 at 17:51

Relative sizes don't matter for life, what matters is the correct distance for the correct temperature so that liquid water can exist on a planet's surface. If the Sun became a white dwarf it would be about as big as Venus or Earth. As for how big solid planets (or ocean planets) can get, the biggest known super-Earth is Kepler-10c at 2.35 Earth radii and 7 Earth masses (by comparison Uranus has 14.5 and Neptune 17.1 Earth masses).

Edit: User sno found an even larger planet at 3.36 Earth radii, see his answer.

• Kepler-10c was later found to have a much lower mass about 7 Earth masses and is not a large terrestrial planet but a mini-Neptune. See Wikipedia
– sno
Commented Jul 13, 2021 at 10:55
• @sno I corrected the value but it reportedly is a super-Earth.
– John
Commented Jul 13, 2021 at 11:08

The Wikipedia article https://en.wikipedia.org/wiki/Mega-Earth lists Kepler-277c as being a mega-earth with a radius of 3.36 Earth radii and a mass of 64.2 Earth masses. It is too close to its star for liquid water but a similarly sized planet could have liquid water if it was further from its star.