Our Planetary Universe
Yesterday, the Kepler mission announced that it has found over 1,200 new planets, including 54 in the ‘habitable zone’. Hal Hodson caught up with Sara Seager, a prominent member of the Kepler team who, among other things, looks for the traces of life in the atmospheres of exoplanets.
The Charles River rises near Hopkington, Massachusetts and meanders North East through parks and reserves before spreading and slowing as it flows under University Bridge. It passes Harvard and MIT, then slides out by the airport, into Boston Harbour and the Atlantic Ocean.
Sara Seager’s office sits 16 floors above the river. January has stopped the waters, turning them silver – a thick band running along the spine of the city. Even the dark globs in the river’s warmer middle are starting to freeze. Sara shows me this vista from the office where she studies things farther afield – exoplanets.
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Exoplanets don’t orbit the same star as Earth. We don’t know very much about them and we haven’t named them after Roman gods (or any gods). We’ve never seen any of their moons. In fact, for the most part we can’t ‘see’ them at all, inferring their existence through observable changes in the light from the star they orbit. The nearest one we know about is 10 trillion kilometres away, orbiting a star not too different from the Sun – Epsilon Eridani.
Astronomers find exoplanets in a number of ways: they can watch the parent star for tiny jiggles caused by the gravitational pull of a planet; they can try and look directly at the planet (very hard at 10 trillion kilometres) or they can watch for the parent star to dim slightly as a planet passes in front of it, a technique called transiting.
Io, one of Jupiter's moons, transits across the face of the gas giant. We could never see a transiting exoplanet in such detail.
Sara has dark hair and no glasses. She’s very friendly, gets to the point and speaks excitedly – getting so engaged with her subject that it’s hard to imagine any bored teenagers slouching as she talks about biosignatures and super-Earths.
The wobbles of Peg 51 which show the presence of its planet.
She is the Ellen Swallow Richards Professor of Planetary Science at MIT and has been at the forefront of exoplanet science almost as long as the field has existed, ever since her thesis advisor, Dimitar Sasselov, pushed a risky project her way – a project about a new planet.
In 1995 Michael Mayor and Didier Queloz announced their discovery of that new planet – 51 Pegasi b – whose massive gravitational field tugs at its star as it orbits, causing a slight stellar wobble. Mayor and Queloz found this wobble in the photons that dribbled into their telescope. The stream of light showed a periodic variation in energy representing the motion of the star in space. Photons which were emitted while the star was being pulled towards Earth had a shorter, squashed wavelength and so a greater energy; those which reached the telescope from the retreating star, a longer wavelength and lower energy. The team’s findings were confirmed a week later.
Sara remembers it: “I was in graduate school and had just finished a Masters degree. The exoplanets were just being discovered. 51-Peg, four day orbital period.”
She rattles that last sentence out like it’s her date of birth. Professionally, it sort of was.
“This was a paradigm shift. Big planets like Jupiter which are supposed to have 12 year orbits. All of a sudden here’s one with a four day orbit. A lot of people couldn’t handle that.”
Dimitar Sasselov thought Sara could handle it and suggested she study the implications of placing a Jupiter-like-planet 100 times closer to its star than Jupiter is to the Sun. What would happen to its atmosphere under such intense radiative heating? Sara took the project and, with the publication of her 126 page doctoral thesis in 1999, her first step down a path she would soon make her own.
These days Sara divides her work in two:
“One is theory and computation. I’ve got a research group of about seven or eight people there. We try to work on exoplanet atmospheres, interiors and biosignatures. We use real data and we make big computer models.
“I have another group in space instrumentation. There we’re trying to build a new kind of very small telescope and send up a whole fleet of them to look for Earth sized planets. We want one small telescope per eligible star.”
Light constantly picks up information about the medium it passes through, either by losing bits or gaining bits (wavelengths) as it travels. By looking at which bits are missing from light which has passed through an exoplanet atmosphere in comparison to light straight from the star, Sara can start to piece together the composition of that atmosphere . When the planet disappears behind its star, she can even look at what goes missing from the starlight and figure out what the planet itself is contributing to the light we are receiving on Earth. In this way, the planet’s temperature can be estimated.
Specifically, Sara tries to identify gases in exoplanet atmospheres which might be by-products of life. She says that its difficult to break free of terracentrism, to look for all possible chemical signatures which might indicate life on another planet. She collaborates with William Bains on this, a British biochemist. It’s a brand new field and there are great discoveries to come, especially once analysis starts on the atmospheres of ‘habitable zone’ planets just discovered by the Kepler mission.
Just an outstanding interview with a scientist that regularly asks, "Is There Life Out There?" The answer, of course, is yes, but the great moment will come when this learned lady makes that discovery herself. This is not a fantasy story, the is solid science talking about fantastic possibilities in our galaxy and beyond.
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