The recent discovery of two new planets, dubbed Kepler-438b and Kepler-442b, proves that small habitable-zone planets exist – something we did not know only five years ago.
The question of whether alien worlds like our own exist around other stars has captured the public imagination since antiquity. Could there be life on other planets, and what might it look like? Philosophers have speculated about these ideas for centuries.
In the last 20 years, we have learned that extrasolar planets (planets orbiting other stars) do indeed exist, and more than 1,000 have now been cataloged. Until relatively recently, however, most of the ones we discovered were not solid like Earth, but were instead much larger balls of gas more akin to Jupiter and Saturn, with no surface where one could imagine life taking hold.
Since its launch in 2009, NASA’s Kepler mission has used its superior sensitivity and privileged location high above the disruptive effects of Earth’s atmosphere to find many more exoplanets, and to demonstrate that smaller, rocky ones like Earth are in fact much more common than larger ones.
Using Kepler, we discovered Kepler-438b and Kepler-442b, which orbit red stars cooler and smaller than the Sun. Both planets are similar in size to Earth and are in the habitable zone of their respective stars (the area around the star in which the temperature is just right for water on a planet to be in a liquid state).
Liquid water is generally considered essential for life as we know it. Move the planet closer to the star, and the water boils off. Move it farther away, and it freezes solid. While we don’t know whether there is any water on these planets, much less life, at least we know that their temperature is right. Meanwhile, their small size means they are almost certainly rocky like Earth and have a solid surface where water could gather in the form of oceans.
Biologists believe that another important condition for habitability is the presence of a breathable atmosphere. Detecting an atmosphere around such small planets is much more difficult, though, especially given that Kepler-438b and Kepler-442b orbit very faint and distant stars that are hundreds of light-years away.
Plans are underway to construct the next generation of large ground-based telescopes, and to launch the successor to the Hubble Space Telescope (called the James Webb Space Telescope), but even with these powerful new facilities, the Kepler stars are so faint that the chances of spotting the atmospheres of these two planets are slim.
Although this may seem discouraging for our prospects of ever finding evidence of extraterrestrial life, the fact that we’ve been able to discover these Earth-like planets around the faint and faraway stars in the small patch of sky that Kepler studied suggests that there should be more of them orbiting nearby stars that are brighter. And brighter stars give us better odds for detecting an atmosphere.
So, now that we know that small habitable-zone planets around brighter stars are likely out there, all we need to do is find them.
This is precisely the goal of NASA’s upcoming Transiting Exoplanet Survey Satellite (TESS), which will scan the entire sky looking for small habitable-zone planets around all the bright nearby stars. These will be the perfect targets to point at with the upcoming suite of large telescopes from the ground and from space, equipped with sophisticated new instruments.
Once we find rocky planets with atmospheres, the last and most important step is to search for biomarkers, or fingerprints of life. We’ll look for signatures of gases that here on Earth are associated with biological activity, such as oxygen (produced by plants through photosynthesis), methane (emitted by microbes), and ozone (from the breakup of oxygen molecules). This is an even more difficult task, but one that astronomers are keen to take on in the next five to 10 years.