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How small is the smallest habitable exoplanet? | space

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  Three planets at different distances from a star with a habitable zone as a green ring.

The habitable zone, as traditionally understood. New discoveries show that rocky planets smaller than Earth can still have liquid water, even if they are somewhere outside the star's main habitable zone. Image via NASA / Astronomy Now.

What makes a planet potentially habitable? Life, as we know, requires liquid water, among other factors. And it makes sense that larger rocky planets, such as Earth, can maintain their liquid water – and their atmosphere – easier than many smaller planets whose gravity is weaker. But now, Harvard scientists have discovered that even many small rocky exoplanets orbiting other stars can still linger on their water, increasing their chances of habitation. This finding extends to the traditional point of view of a star's habitable zone – the zone around a star where temperatures are right to right allowing liquid water to exist.

The new peer-reviewed results were first published in the Astrophysical Journal on August 13, 2019. This new study redefines the lower mass limit for potentially habitable exoplanets. Mass is simply the amount of matter that the body contains. This new definition extends what we can think of as a habitable zone for small, low mass and (since gravity depends on mass) low gravitational exoplanets.

How Small is Too Small? The critical cut-off point seems to be about 2.7 percent of Earth's mass. Any planets that are less massive than them would lose their atmosphere to space before liquid water would form on their surfaces, and any water present would evaporate or freeze. For comparison, the Moon is 1.2 percent of Earth's mass, and Mercury is 5.53 percent.

As astronomer Konstantin Arnsheid explained to the lead author of the article:

When people think of the inner and outer edges of a habitable zone, they tend to think only of that space, which means how close the planet is to a star. But there are actually many other variables for habitation, including mass [a planet’s].

Defining a lower limit for habitation in terms of planet size gives us an important limitation in our continued hunt for habitable exoplanets and exomons.

Robin Wordsworth, co-author of the study, added:

The masses of the aquatic world are a fascinating opportunity in the search for life, and this document shows how different their behavior is from that of planets like Earth. Once observations for this class of objects become possible, it will be exciting to try to test these estimates directly.

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  Order of planets in descending dimensions, showing more and less habitation.

Graph showing the new lower bound for smaller rock exoplanets. According to this new job, many smaller planets – at least larger than just 2.7% of Earth's mass – could hold liquid water (depending on other factors), while planets smaller than this limit would escape from water in space or freeze. Image via Harvard SEAS / Astrobiology Magazine.

According to traditional thinking about habitable zones, if the planet is too close to its star, an escape of the greenhouse effect may occur, causing the planet to lose all its water. This can happen even in the inner edge of a star's habitable zone. Venus is often cited as an example of this process in our own solar system; there may have been an ocean once, but a greenhouse effect escaped, leaving Venus dry as bone and hot enough on its surface to melt lead.

According to the authors, a new study:

… sheds light on the important process of atmospheric evolution of small planets.

In other words, their work suggests that – for small exoplanets that are not also small – something interesting happens as one of these planets, even if it is on the edge of the star's habitable zone, begins to warm due to the greenhouse effect. In a warming atmosphere, in a world of low mass with relatively low gravity, the exoplanet's atmosphere expands outward, becoming larger and larger in size than the planet. This leads to an increase in both the absorption and radiation of the star's heat, which allows the planet to achieve a kind of balance while maintaining a stable temperature. In this way, atmospheric expansion can prevent small planets with low gravity from experiencing the greenhouse effect . Instead, they could maintain their surface liquid water even on the inner edge of the habitable zone, relatively close to their stars.

It is also interesting to note that, according to the researchers, these findings appear to apply to both type G sun-like stars and type M. red dwarf stars.

The smaller, cooler reds dwarfs are the most common stars in our galaxy, so that in itself would increase the chances of finding more habitable worlds.

  Cracked surface of an ice moon.

The icy ocean moon of Jupiter Europe as seen by Galileo in the late 1990s. Researchers wondered if these types of small ice moons could be habitable on the surface if they were closer to the sun, but new research suggests they would be too small. Image via NASA / JPL-Caltech / SETI Institute / Europa Clipper.

The researchers also used their findings to clarify the question of certain moons in our solar system. Scientists wondered if the icy moons of Jupiter Europe, Ganymede and Callisto could become habitable on the surface if they were closer to the sun, especially since they all had underground oceans beneath their icy crust. Although the answer doesn't seem to be because they are too small.

Although there is still a limit to how small a planet can be and can still be inhabited, this new study shows that there may still be many more such worlds – smaller than Earth but habitable, with liquid water – than previously thought. This is good in our search for life beyond our solar system.

  Rocky landscape, jet of water approaching the red sun, two other planets visible in the deep red sky.

The artist's conception of one of the rock worlds in orbit. TRAPPIST-1 red dwarf star, with possible liquid water on the surface. A new study says smaller rocky planets may have a better chance of retaining their water than previously thought. Image via ESO / M. Kornmesser.

Bottom line: A new study finds that small exoplanets are more likely to stay above their water than previously thought, increasing the chances of some of them being habitable.

Source: Low Atmospheric Evolution – Gravity Aquatic Worlds

Via Astrobiology Magazine

  Paul Scott Anderson

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