Is there life 'out there?' How we'll search for traces of life on nearby exoplanets
We know worlds are common. We need to know if they host life
The discovery of thousands of exoplanets by the Kepler space telescope and other instruments has made it clear that planets are common in the universe, and Earth-like planets likely exist in nearby star systems. These planets could potentially host life.
Now scientists are on a quest to search for life on Earth-like worlds, and they're close to developing the techniques and technology that will get us there.
"I think that we have an unprecedented opportunity to find life not just in this century and not just in this lifetime but in my career," said Shawn Domagal-Goldman of the Planetary Systems Lab at NASA Goddard Space Flight Center.
"We have plans now for telescopes that we can build in the next 20 to 30 years that will be capable of imaging planets around other stars and searching them for signs of life. And I think we can do it, and I think we will."
The subtle science of detecting life
Detecting life — microbial, plant or simple animal life — on exoplanets requires a multi-step approach. First, we need to figure out which of the thousands of planets we've discovered might possibly host life.
Since Earth is the only planet we know that has life, our best guess is that habitable exoplanets need to be a lot like Earth.
They should be similar in size to Earth with a rocky surface, and they need to be the right distance from their host star — not too hot and not too cold — so that liquid water, which we think is essential to life, can exist. This is known as the "habitable zone," which in Earth's solar system occupies the zone beyond Venus and out little past Mars.
Abel Mendez, a planetary astrobiologist from the University of Puerto Rico at Arecibo, studies the habitability of solar and extrasolar planets, and found close to 50 potentially habitable exoplanets out of the four-thousand-plus exoplanets that astronomers have discovered so far.
They are the prime targets for future observations and exploration. One particularly attractive candidate is the TRAPPIST-1 system.
It has seven planets spread out in the different regions of the star system. Four are inside the habitable zone, and three are outside of it. This makes for a great research laboratory for scientists studying habitability.
"We're able to just look at one system and study planets that are not supposed to be habitable but also planets that might be habitable." said Mendez.
Once we've identified a promising exoplanet to search for life, we need to know what signs to look for. Researchers have been giving deep consideration to what might be considered a strong signs of life or 'biosignatures' if it were detected by a telescope.
On Earth, they're gases like oxygen and methane produced by biological processes and emitted into the atmosphere. Aliens studying Earth through a powerful telescope would be able to detect them by taking a picture of Earth and analyzing its light spectrum to see what gases are in the atmosphere.
According to Goldman, good biosignatures must be something that life makes, and something that non-biological processes can't replicate.
Two exoplanet imaging missions have been proposed in recent years to look for life on Earth-sized exoplanets in the habitable zones of their stars. One is called the Habitable Exoplanet Imaging Mission or HabEx. The other is the Large UV Optical Infrared Surveyor or LUVOIR.
"HabEx would let us see maybe 10 or 12 habitable zone rocky planets and then we have a larger, more ambitious mission called LUVOIR that would be able to see anywhere from 30 to 60 rocky planets in a stellar habitable zone," said Karl Stapelfeldt, chief scientist in the NASA Exoplanet Exploration Program.
A major challenge that NASA engineers face in designing these instruments is imaging exoplanets next to the intense glare of their host star.
To solve this problem, NASA is developing devices that can act like a car's sun visor to block the light of the star and enable the telescope to see the small, dim exoplanets.
The coronagraph is a small instrument that sits inside a telescope that blocks the glare of stars. It's built to see planets with one billion to one contrast, enough to spot Jupiter-sized exoplanets.
The starshade takes this idea to the next level. It's actually a separate spacecraft that flies between the telescope and the star to block out the star light.
A starshade would be more than 50 metres across and have the appearance of a large sunflower. It would be launched folded up and then unfolded in space.
"It's just the right mathematical shape to block out the light of the star and let the light of the planet sneak through," explained Stapelfeldt.
In addition to starlight blockers, we'd also need powerful telescopes to capture images of planets in other solar systems.
They'll be much bigger than current systems like the Hubble Space telescope, to collect the faint light of exoplanets that are light years away, and we won't have them for a while.
The next two space telescopes due to launch are the James Webb Space Telescope, due in 2021, and the WFIRST telescope set for the mid-2020s.
These are powerful, general purpose instruments, not planet-hunters. But they're theoretically capable of imaging Jupiter-sized exoplanets around nearby stars. In fact, the WFIRST telescope will carry a coronagraph into space to test its ability to block star light.
The real planet hunters capable of detecting life on Earth-sized, potentially habitable exoplanets, will be the HabEx and LOUVOIR telescopes. Stapelfeldt thinks they could fly in the 2030s if approved by the Decadal Survey Committee that decides on NASA missions.
"It's very important to understand what planets are out there, and what are the chances of those planets having life." said Mendez. "Studying [exoplanets] will tell us what our importance and relevance is regarding life in the universe."