When it comes to hunting down signs of life on distant planets astronomers have typically played it safe. They’ve looked for Earth-sized planets, with Earth-like surface temperatures, and Earth-like atmospheres.
But now, a team of researchers from the University of Cambridge have identified a new class of habitable planets that could change the game completely. Dubbed ‘Hycean’ planets – meaning hot, ocean-covered planets with hydrogen-rich atmospheres – the newly identified exoplanets are far greater in number and easier to spot than Earth-like planets.
And specifically hunting for Hycean planets could lead to us discovering biosignatures of life outside our Solar System within the next two or three years, they say.
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“Hycean planets open a whole new avenue in our search for life elsewhere,” said study leader Dr Nikku Madhusudhan from Cambridge’s Institute of Astronomy.
“Essentially, when we’ve been looking for these various molecular signatures, we have been focusing on planets similar to Earth, which is a reasonable place to start. But we think Hycean planets offer a better chance of finding several trace biosignatures.”
Hycean planets can be up to 2.6 times larger than Earth and have atmospheric temperatures as high as 200ºC. However, their oceanic conditions could be similar to Earth’s and so could potentially harbour microbial life. A significant proportion of the exoplanets discovered so far fall into this category.
The larger sizes, higher surface temperatures and hydrogen-rich atmospheres of Hycean planets also make their atmospheric signatures much easier to detect than Earth-like planets.
When looking for signs of life on other planets astronomers look at so-called biosignatures such as oxygen, ozone, methane and nitrous oxide, which are all present on Earth. There are also a number of other biomarkers, such as methyl chloride and dimethyl sulphide, that could suggest the existence of life on planets with hydrogen-rich atmospheres where oxygen and ozone may not be as abundant.
“A biosignature detection would transform our understanding of life in the Universe,” said Madhusudhan. “We need to be open about where we expect to find life and what form that life could take, as nature continues to surprise us in often unimaginable ways.”
The team has identified a number of Hycean planets between 35 and 150 light-years away that they hope will be prime targets for the next generation of space telescopes, such as the James Webb Space Telescope (JWST), which is due to be launched later this year.
What does it mean if an exoplanet is ‘habitable’?
All forms of life that we know of depend on one critical component: liquid water. So, in the search for life, astronomers focus on planets where liquid water could exist, which they call ‘habitable’.
Every star has a ‘habitable zone’, also called the ‘Goldilocks zone’, where it is not too hot and not too cold. A planet in the habitable zone gets the right amount of energy from the star to support liquid water. Any closer in to the star and water would boil, and any further out and it would freeze.
However, this doesn’t guarantee that liquid water would exist on a planet in the habitable zone. The planet’s atmosphere could be too thick, raising the temperature even higher. And even if liquid water does exist on the planet, habitable doesn’t mean inhabited.
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