When considering planets to look for extraterrestrial life, astronomers are mostly drawn to what is familiar. The best candidates for habitable planets are considered to be the most Earth-like: small, rocky, with breathable atmospheres and a mild warmth from their stars.
But since several planets have been discovered outside the solar system, astronomers debated the usefulness of these definition schemes. Some planets in the so-called habitable zone, where temperatures are right for liquid water, are probably not good for life at all. Others outside that designated area may be perfect for hosting life forms.
Now, two studies propose to revise the concept of “habitable zone” to take into account more planets that astronomers may encounter in the cosmos. A new definition brings more planets in the habitable fold; the other takes out some.
“Both articles focus on questioning the classic idea of the habitable zone. We should extend the range of places we observe so that we don’t miss the habitable planets, ”says Penn State University astronomer Noah Tuchow.
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Some neglected planets may be much larger than Earth as well potentially not receiving starlight at all. Cambridge University astrophysicist Nikku Madhusudhan and colleagues propose a new category of possibly habitable planets, which could be found at any distance from any type of star.
These hypothetical planets have a global ocean of liquid water immersed in a dense hydrogen-rich atmosphere. Madhusudhan calls them planets “Hycean”, for “hydrogen” and “ocean”. They could be up to 2.6 times larger than Earth and up to 10 times more massive, according to Madhusudhan and colleagues on Aug.25 in the Astrophysical Journal.
That dense atmosphere could keep temperatures right for liquid water too with minimal one star input, while the ocean could protect anything alive from overwhelming atmospheric pressure.
“We want to expand beyond our previously established paradigm on Earth-like planets,” says Madhusudhan. “Everything we have learned so far about exoplanets is extremely varied. Why limit ourselves when it comes to life? “