Research redefines lower limit for planet size habitability

Research redefines lower limit for planet size habitability
In this artist’s concept, the moon Ganymede orbits the giant planet Jupiter. A saline ocean under the moon’s icy crust best explains shifting in the auroral belts measured by the Hubble telescope. Astronomers have long wondered whether Jupiter’s moons would be habitable if radiation from the sun increased. Credit: NASA/ESA

In The Little Prince, the classic novella by Antoine de Saint-Exupéry, the titular prince lives on a house-sized asteroid so small that he can watch the sunset any time of day by moving his chair a few steps.

Of course, in real life, celestial objects that small can't support life because they don't have enough gravity to maintain an atmosphere. But how small is too small for habitability?

In a recent paper, Harvard University researchers described a new, lower size limit for planets to maintain surface liquid for long periods of time, extending the so-called habitable zone or "Goldilocks zone" for small, low-gravity planets. This research expands the search area for life in the universe and sheds light on the important process of atmospheric evolution on small planets.

The research was published in the Astrophysical Journal.

"When people think about the inner and outer edges of the habitable zone, they tend to only think about it spatially, meaning how close the planet is to the star," said Constantin Arnscheidt '18, first author of the paper. "But actually, there are many other variables to habitability, including mass. Setting a lower bound for habitability in terms of planet size gives us an important constraint in our ongoing hunt for habitable exoplanets and exomoons."

Generally, planets are considered habitable if they can maintain surface liquid water (as opposed to frozen water) long enough to allow for the evolution of life, conservatively about 1 billion years. Astronomers hunt for these habitable planets within specific distances of certain types of stars—stars that are smaller, cooler and lower mass than our sun have a habitable zone much closer than larger, hotter stars.

The inner edge of the habitable zone is defined by how close a planet can be to a star before a runaway greenhouse effect leads to the evaporation of all surface water. But, as Arnscheidt and his colleagues demonstrated, this definition doesn't hold for small, low-gravity planets.

Research redefines lower limit for planet size habitability
This illustration shows the lower bound for habitability in terms of planet mass. If an object is smaller than 2.7 percent the mass of Earth, its atmosphere will escape before it ever has the chance to develop surface liquid water. Credit: Harvard SEAS

The runaway greenhouse effect occurs when the atmosphere absorbs more heat that it can radiate back out into space, preventing the planet from cooling and eventually leading to unstoppable warming that finally turns its oceans turn to steam.

However, something important happens when planets decrease in size: As they warm, their atmospheres expand outward, becoming larger and larger relative to the size of the planet. These large atmospheres increase both the absorption and radiation of heat, allowing the planet to better maintain a stable temperature. The researchers found that atmospheric expansion prevents low-gravity planets from experiencing a runaway greenhouse effect, allowing them to maintain surface liquid water while orbiting in closer proximity to their stars.

When planets get too small, however, they lose their atmospheres altogether and the liquid surface water either freezes or vaporizes. The researchers demonstrated that there is a critical size below which a planet can never be habitable, meaning the is bounded not only in space, but also in planet size.

The researchers found that the critical size is about 2.7 percent the mass of Earth. If an object is smaller than 2.7 percent the mass of Earth, its atmosphere will escape before it ever has the chance to develop surface liquid water, similar to what happens to comets today. To put that into context, the moon is 1.2 percent of Earth mass and Mercury is 5.53 percent.

The researchers were also able to estimate the habitable zones of these around certain stars. Two scenarios were modeled for two different types of : a G-type star like our own sun and an M-type star modeled after a red dwarf in the constellation Leo.

The researchers solved another long-standing mystery in our own solar system. Astronomers have long wondered whether Jupiter's icy moons Europa, Ganymede, and Callisto would be habitable if radiation from the sun increased. Based on this research, these moons are too small to maintain surface liquid water, even if they were closer to the sun.

"Low-mass water worlds are a fascinating possibility in the search for life, and this paper shows just how different their behavior is likely to be compared to that of Earth-like ," said Robin Wordsworth, associate professor of environmental science and engineering at SEAS and senior author of the study. "Once observations for this class of objects become possible, it's going to be exciting to try to test these predictions directly."


Explore further

Habitable type planets found around nearby small mass star

More information: Constantin W. Arnscheidt et al. Atmospheric Evolution on Low-gravity Waterworlds, The Astrophysical Journal (2019). DOI: 10.3847/1538-4357/ab2bf2
Journal information: Astrophysical Journal

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Sep 11, 2019
One must wonder however what happens when small planets have their oceans protected with a global shell of ice or rock or other material.

Sep 11, 2019
Chemosynthetic life could easily thrive on low mass icy moons as long as there is enough energy available to drive their ecosystem. Tidal forces could keep the worlds habitable for billions of years with impacts and volcanism both spreading life from one to another around the same gas giant.

Sep 11, 2019
One must wonder however what happens when small planets have their oceans protected with a global shell of ice or rock or other material.
Ice significantly increases a planetary body's albedo, so a planet (or dwarf planet) fully covered in ice reflects most of the starlight into space. As a result its temperature plummets, which in turn ensures the ice never melts, which reflects more light, which drops the temperature further etc, at first in a positive feedback (self-amplifying ice coverage) and later on, when it reaches equilibrium, in a negative (stable ice coverage) feedback fashion.

A rocky body will have a much lower albedo, particularly if the surface is rather dark, thus will have a much higher temperature at the same distance from the star. Its subterranean ocean might or might not affect the atmosphere. Whether a planetary body is icy or rocky though, what really matters in terms of it being able to hold an atmosphere is its mass.

Sep 11, 2019
I agree that "...actually, there are many other variables to habitability, including mass." I would add UV hab zone, which for stars larger than our sun is farther out, not coinciding with water zone; and sun smaller means UV zone is inside water zone.
Conceivably habitable planets orbiting M- and K-dwarf stars (88 percent of all stars) will possess atmospheric carbon monoxide levels that will be lethal for aerobic complex life.
https://arxiv.org...4720.pdf
Then there is planetary electric field. The discovery of a strong atmospheric electric field on Venus has serious implications for the possible habitability of exoplanets. It implies that any planet with an atmosphere thicker than 1% of Earth's and any planet that is closer to its star than about 90% of Earth's distance from the sun will very likely possess an atmospheric electric field strong enough to completely dry out the planet. (Have citation but will not fit). There is even a photosynth. zone.


Sep 12, 2019
Neat work.

@Sahstar: "Whether a planetary body is icy or rocky though, what really matters in terms of it being able to hold an atmosphere is its mass."

I think Parsec may have been thinking of subice/subcrust habitability, which as we can see in the case of Enceladus applies for even smaller ice moons. That is, if they are given standard composition and some tidal effects (so is also depending on distance as this smaller size habitability).

@Educator: What a "problem list"!

- UV is not a problem for ocean (or subcrust) life.
- CO2 was not a problem for Earth life.
- Dunno about Venus "electric field" - loss of water is usually proposed due to hothouse heating and then UV dissociation with hydrogen loss. All large bodies are uncharged by necessity of charge balance; I was on a seminar when an astrophysicist entertained us with showing Earth magnetic dynamo produced something like 80 C of charge on Earth in *total*.

Sep 12, 2019
So here is an article that claims Venus has all of 10 instead of < 2 V of an ambipolar like polarization field in its atmosphere that can accelerate ions [ https://agupubs.o...GL068327 ]. Ambipolar like means plasma like, a sea of charged ions and electrons they lost but uncharged when seen from a distance (as there are equally many positive and negative charges). An ionosphere is ambipolar like.

Now, what they say is that they don't know why the field is stronger (but stronger UV dissociation can have something to do with that) and it is only a supplement to what happened to Venus (since hothouse was the primary problem).

Sep 13, 2019
So here is an article that claims Venus has all of 10 instead of < 2 V of an ambipolar like polarization field in its atmosphere that can accelerate ions [ https://agupubs.o...GL068327 ]. Ambipolar like means plasma like, a sea of charged ions and electrons they lost but uncharged when seen from a distance (as there are equally many positive and negative charges). An ionosphere is ambipolar like.

Now, what they say is that they don't know why the field is stronger (but stronger UV dissociation can have something to do with that) and it is only a supplement to what happened to Venus (since hothouse was the primary problem).


Yes, this is just an ambipolar field. These occur where there becomes charge separation, usually because electrons are more easily accelerated out of a region than ions. The separation causes an ambipolar field to be set up, which will retard the electrons and accelerate ions to maintain quasi-neutrality.

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