by Vivian Hemmelder |
Unlike the early explorers who sailed vast oceans to reach faraway shores, planetary scientist Sara Seager will never set foot on new lands she may discover. Her goal is to find habitable exoplanets, worlds that are not merely outside our solar system but many light-years away.
In the past years, exciting advancements in astronomy have led to the discovery of more than 2,000 new exoplanets. Their diversity is stunning, featuring landscapes and atmospheres we could never have imagined to exist. These worlds may support life in ways that are not at all “Earth-like.”
This myriad of different worlds has inspired Seager to rethink which conditions may support life and how we can detect life if it looks nothing like what lives on Earth. She is now fashioning instruments for close-up studies of planets that may change scientists’ view of what is habitable.
Seager spoke Oct. 12 to science writers attending CASW’s New Horizons in Science briefings (part of ScienceWriters2015) at the Massachusetts Institute of Technology in Cambridge, Mass., where she is a professor of planetary science and physics. Asked afterwards what drives her to devote her life to finding habitable exoplanets, she said simply: “Exploration.”
Life's diverse building blocks
For Seager’s quest to succeed, she explained, researchers must first know what is required for life to develop. One problem with answering this question is that we have only one sample planetary environment that we can rely on: Earth.
On Earth, life depends on liquid water and carbon, and a temperature range that allows for chemical reactions between these two. In past decades, astronomers focused on finding planets similar in temperature and atmosphere to Earth. The strategy seemed straightforward, but Seager said that lately scientists have increasingly recognized that it might be possible for life to exist outside Earthly parameters.
A planet’s surface temperature relies not only on the warmth of a star and the distance between the planet and the star, but also on the planet’s atmosphere. Certain gases on Earth produce a “greenhouse effect,” warming our planet by preventing heat from escaping into space.
If a newly found planet featured an atmosphere of strong greenhouse gases, it could be located much farther away from its star and remain habitable, according to Seager. Similarly, a planet without a lot of water vapor in its atmosphere could be located closer to its sun and still have temperatures in which Earth-like life can exist.
In the past few years, the controversy around defining the habitable zone has expanded to include broader questions about the basic ingredients for life. Just as one might use many different arrangements of Lego blocks to build a replica of a spaceship, life could sprout from a huge range of different circumstances. “Nature has way more [combinations] than we can think of usually,” Seager said, “Any liquid would be okay, methane and ethane lakes would be okay,” which is why the astronomy world was so excited to find these on Titan, Saturn’s largest moon.
In addition, meeting all requirements for life does not mean that life will flourish, just like having all the right blocks does not automatically mean you will build a spaceship. In her lecture, Seager used Venus and Earth, which both lie in the habitable zone, as examples: “Venus’s surface is hot enough to melt lead, whereas the Earth is such a pleasant place to live!”
So, like Columbus, whose voyages showed Europeans that Earth had no edge, Seager has set out to redefine the habitable zone. Armed with a team of graduate students rather than prisoners, and relying on telescopes rather than sailboats, she has made it her mission to scan dozens of exoplanets for signs of life by looking at their atmospheres.
Certain atmospheric gases—such as dimethyl sulfide, produced by algae in our oceans—can only be formed by life. Others, such as carbon dioxide, can also be produced by volcanic eruptions or other geological processes. However, there are specific proportions of these molecules that cannot result from anything but life. For example, finding both high levels of oxygen and carbon dioxide in a planet’s atmosphere would make Seager and her fellow explorers 95 percent certain life is present, she said.
The planets Seager investigates are tens to hundreds of light years away and can’t usually be seen when their telescope images are overpowered by the light of their star. How, Seager asked, can we see the composition of these planets if we can’t even see the planet itself?
So bright, a telescope's gotta wear shades
Seager’s strategy at the moment is to look at transiting planets—planets crossing in front of their star. Examining the light of the star shining through the planet’s atmosphere, “just like shining a flashlight through a fog,” can reveal the composition of the atmosphere, Seager said.
Unfortunately, not all planets can be studied with this strategy. Planets that are far away from their star have long orbits and thus cross the star very rarely. The chance that scientists will be watching at exactly the right moment is tiny.
To solve this problem, Seager has developed a technique called direct imaging, which looks at planets directly while blocking out the light of the star. Her team has built a “star shade,” a beautiful flower-like apparatus engineered to be flown thousands of miles away from the imaging telescope. By stopping light coming from the star from flooding telescopes, Seager hopes the shade will enable a direct view of planetary atmospheres and thus their composition.
For this, ‘Look how long that tip is!’ she marvels, showing the crowd a picture of one of the petals that indeed has a long and hair-fine tip sticking out proudly like a fencing sword.
That we will find life in space is certain to Seager. Whether it will be found during her lifetime is unclear. Greeting the audience after her lecture, she recalled her son asking what will happen if she doesn’t find extraterrestrial life. “Then you or another child will,” she told him, with determination.