Sara Seager: Mapping the Nearest Stars for Habitable Worlds

Sara Seager of MIT (USA) opens her plenary session with a picture of the Mauna Kea Observatories under a night sky. She asks the audience if anyone has been to Mauna Kea and seen the dark night sky and all the spectacular stars out there.

The image serves to remind us that we now know of thousands of planets orbiting stars other than the Sun, we call them exoplanets. "We think now, that almost every star has planets," says Seager. "It turns out, that after people wondering for thousands of years, we now know that our very nearest star to Earth, Proxima Centauri, has an approximately Earth-mass planet around it."

Thousands of exoplanets are known to orbit nearby stars and small rocky planets are established to be common. The ambitious goal of identifying a habitable or inhabited world is within reach. The race to find habitable exoplanets has accelerated with the realization that "big Earths" transiting small stars can be both discovered and characterized with current technology, such that the James Webb Space Telescope has a chance to be the first to provide evidence of biosignature gases.

Exoplanets are diverse with a range of planet size, mass, and orbits. "One point I want you to walk away with is, every point here (see image below) is a planet and each color corresponds to a technique for finding planets," says Seager. "What you can see in this diagram (bottom left) is that there are planets, Earth-size, that are so close to the star. They're 100 times closer to their star than Earth is to our Sun."

Today we use the Hubble Space Telescope to study largely hot giant exoplanet atmospheres. Seager notes this is a busy, kind of cottage industry, there are dozens of people working on this in astronomy, and dozens of planets have been observed. In the near future, we will use the James Webb Space Telescope to push down to small rocky exoplanet atmospheres.

"But there's one kind of important twist here," Seager tells the audience, "and that is that we're not going to be focusing on sun-like stars with the JWST. We'll be looking at small, red dwarf star planets. And that's mostly because the annulus of the atmosphere is so tiny."

Seager take the audience on a virtual trip to a planet orbiting a red dwarf star, "because that's our future for identifying and studying habitable worlds."

To Seager, one of the most interesting thing is that these planets are so close to the star that they'd be tidally locked due to gravitational interaction with the star over millions or tens of millions of years. This would put the planet into its lowest, most stable energy state, showing the same face to the star at all times, just like our moon does to Earth. This means the planet would rotate one time for every time it orbits. What this means is if we could visit this planet,  its star or sun would be at the same place at all times. So you could chose to visit where it's always day, if you're a solar physicist, or where it's always night if you're an astronomer building telescopes.

Transiting exoplanets requires a fortuitous alignment and the fast-track approach is therefore only the first step in a long journey. The next step is sophisticated starlight suppression techniques for large ground-based telescopes now under construction and hopeful future space-based based telescopes to observe small exoplanets directly.

These ideas will lead us down a path to where future generations will implement very large space-based telescopes to search thousands of all types of stars for hundreds of Earths to find signs of life amidst a yet unknown range of planetary environments.

"That's our future," says Seager in conclusion. "Everyone in this room has some role to play, whether it's in detectors or systems engineering; whether you're working on Subaru today or one of the ELTs for the future, it's an effort. We don't know which ones are going to turn up planets that are habitable and which ones of these planets may have life on it."

Sara Seager of the Massachusetts Institute of Technology is a professor of Planetary Science, Physics, Aerospace Engineering, and holds the Class of 1941 Professor Chair. She has pioneered many now foundational research areas of characterizing exoplanets, with present focus on the search for life by way of exoplanet atmospheric "biosignature" gases.

She is PI of the CubeSat ASTERIA; Deputy Science Director of the MIT-led NASA Explorer-class mission TESS; and a lead of the Starshade Rendezvous Mission (a space-based direct imaging exoplanet discovery concept under technology development). Seager was elected to the US National Academy of Sciences in 2015 and is a 2013 MacArthur Fellow.

Related SPIE content:

STAR GAZER: Sara Seager on exoplanets, exploration, and elusive Earth twins
Our fascination with the concept of alien life drives scientists and civilians alike to explore.