Another Earth called a 'certainty'
CNN — Sara Seager is a professor of physics and planetary science at MIT. She works on exoplanets, which orbit stars other than the sun.
Seager considers herself a pioneer and risk taker. She worked on exoplanets before it was considered cool, when people thought the field would go nowhere. Time magazine named Seager one of the 25 most influential in space in 2012, and she recently appeared in a CNN gallery of top women scientists.
CNN Light Years recently chatted with Seager about her work. Here is an edited transcript:
CNN: Why should you be featured on CNN Light Years?
Sara Seager: The field of exoplanet research is among the most compelling of all sciences because many people wonder, "Are we alone in the universe?"
I now have a world-renowned research team in exoplanet characterization. We use extensive computer models that we write from scratch as well as data from the Hubble, Spitzer and Kepler space telescopes to understand what exoplanets are made of, what is in their atmospheres and what makes a planet "habitable." In addition, my research portfolio also includes space engineering, where we are building small satellites for both science and to test new spacecraft technology.
CNN: What is an exoplanet?
Seager: An exoplanet is a planet that orbits a star other than the sun. Hundreds of exoplanets are known, and thousands of more planet candidates have been found. All evidence shows that a very large fraction of stars have at least one planet. Given that we know our Milky Way galaxy has at least 100 billion stars, this implies that the galaxy has at least 100 billion planets. Planets are just simply everywhere.
CNN: Why did you decide to study exoplanets?
Seager: My whole motivation is to find planets like Earth. In particular, the holy grail is to find signs of life by looking for gases produced by life that have accumulated in an exoplanet atmosphere and can be detected and studied by futuristic space telescopes.
CNN: What are transiting exoplanets?
Seager: Transiting exoplanets are those that go in front of the star as seen from Earth. A fortuitous alignment must occur -- planet orbits are actually randomly oriented, that is the orientation with respect to Earth is different for each planet. When the planet goes in front of the star, the star light drops by a tiny amount with a very characteristic signal. The transit lasts from a few to several hours (depending on the planet's orbit).
Transiting exoplanets are so valuable for two reasons. First, we can determine the planet size, and this helps us to understand what kind of planet it is, rocky like the Earth, or with a massive gas envelope like Jupiter. Transiting planets are also valuable because so far, the transiting planet finding technique is the only one ... that can (detect) true Earth analogs (that is an Earth-size planet orbiting a sun-size star).
CNN: Why should we care about exoplanet atmosphere?
Seager: The exoplanet atmosphere acts like a blanket to control the planet surface temperature. Here on Earth, we are worried about parts-per-million increases in the greenhouse gases for warming. That's because of the atmosphere. On exoplanets, we want to know which planets are not too cold or not too hot for life, and the atmosphere is a determining factor. Atmospheres are also important because they are a window into other planet processes. ... Finding water vapor in the atmosphere of a small rocky planet would indicate surface liquid water in the form of oceans. Water is required for all life on Earth, so finding water vapor in a planet atmosphere may indicate that planet is habitable.
CNN: Why is finding signs of life on exoplanets important to scientists?
Seager: As astronomers, we strive to understand our place in the universe. We try to answer questions such as how did our planetary system form and evolve? How did Earth come to be? A main question is did life originate elsewhere, beyond Earth. So finding signs of life on exoplanets would help all of us understand our origins. A more interesting way to phrase it is we are dying to know, "Are we alone?" While we are not necessarily searching for "little green aliens," we are searching for signs of life caused by bacteria or other life forms on exoplanets.
CNN: Do you think there is another planet similar to Earth out there?
Seager: Absolutely yes. There are hundreds of billions of stars in our Milky Way galaxy and upward of 100 billion galaxies in our universe. So the existence of a planet similar to Earth somewhere, is, in my mind a certainty. Less certain is if such a planet is near enough to us that we can find and identify it sometime in the future.
How soon can we find another Earth?
Seager: How soon we can find another Earth depends on how we identify Earth. If we want a true Earth twin, that is a planet with a thin nitrogen atmosphere, a planet with oceans and continents and orbiting a sun-like star, it will be well over a decade before we have the sophisticated space telescope tools to find and identify one.
Our best bet is to expand our definition of Earth to big Earths orbiting small stars. We can use the future James Webb Space Telescope to search for signs (of) habitability and signs of life in the planet atmosphere. If we would be satisfied to just find an Earth-size or Earth-mass planet, that has already been done. But I caution that Venus and Earth appear the same size and mass, yet Venus is inhospitable to life due to a massive greenhouse atmosphere that creates surface temperatures hot enough to melt lead, while Earth has a surface oasis in comparison.
CNN: What equipment, research or theories would scientists need to find planets similar to Earth?
Seager: To find and identify a planet similar to Earth we would need a very sophisticated large space telescope that can block out the light of the host star. This type of telescope is at the limits of what astronomers and engineers can conceive of, and a huge amount of work has been done under an umbrella term called Terrestrial Planet Finder. In terms of theories, we are working hard to understand what kinds of planets could be habitable.
CNN: What are scientists doing to look for super-Earths orbiting small stars?
Seager: At MIT, we are leading a space telescope mission called TESS: Transiting Exoplanet Survey Satellite. TESS will survey hundreds of thousands of stars, including small stars, searching for transiting exoplanets. It has been an exciting few years to develop the mission, and now we are in the final phase of a NASA competition. We will find out in a couple of months if TESS will be selected for launch
CNN: What's your scientific goal, and what's the inspiration behind it?
Seager: My goal is to find other Earths. ... The hope is that exoplanets transiting bright sun-like stars might have their atmospheres accessible to observation. In the exoplanet game, brighter is always better, because more light means a better signal (think of a brighter flashlight that helps to see more).
The real inspiration was that the set of Terrestrial Planet Finder missions got canceled. Terrestrial Planet Finder was a concept for an extremely sophisticated space telescope ... that would be able to take an image of an exoplanet, by getting rid of the glare of the host star. Though the planet would appear only as a "point source" (think single pixel), the huge challenge is that a sun-like star would be 10 billion times brighter than an Earth-like planet. For transits, the star is (only) is 10,000 times brighter. The renowned Kepler Space Telescope uses the transit technique, but the stars are typically thousands of light-years away and relatively faint, not a few to tens of light-years away.
CNN: Tell me about your research that led to the first detection of exoplanet atmospheres.
Seager: First, let me tell you one reason why planetary atmospheres are so important. The gases in a planet atmosphere control the surface temperature, via a "greenhouse effect." Ultimately we want to know if a planet has surface temperatures compatible with liquid water since all life as we know it requires liquid water. So we first have to understand the composition of exoplanetary atmospheres.
Small planet atmospheres are challenging to observe so now we are stuck with or focused on giant planet atmospheres. Giant planets are completely unsuitable for life as we know it because their atmospheres extend toward the interior into an "envelope," where temperatures rapidly increase, and they have no surface as we know it. Even though giant planets are too hot for life, their atmospheres are observable now with current space telescopes and techniques so we study them, also for their own sake.
A newly minted Ph.D., I was awaiting the discovery of a transiting planet, a planet that goes in front of its host star as seen from Earth. I believed such a special configuration could enable studies of exoplanet atmospheres; when the planet is in front of the star, some of the starlight passes through the planet atmosphere and the planet atmosphere is imprinted on the starlight. I wrote a computer code to predict that sodium (an atom in gas form in the so-called "hot Jupiters) was a very strong absorber and should be searched for. While I was writing the code, the first transiting planet ever was discovered. I tried to wrap up the paper ASAP. Then a team used my prediction, got time on the Hubble Space Telescope and observed sodium (albeit at a level smaller than my model predictions). This was an exciting series of events.
CNN: In one of your research articles, you said, "The graduated growth of a constellation might be a new paradigm for space missions." How are you going to demonstrate that?
Seager: My team is working on ExoplanetSat (a space telescope). We hope to send a fleet into low Earth orbit, each pointing to its own star. The brightest stars in the sky are spread all around the sky, hence the use of a fleet of tiny telescopes. We can send up one now, a handful later, and if the engineering and science is right, many more can go up anytime we have enough funding.
CNN: What should we expect from you in the near future?
Seager: I have big plans for the distant future. In the short term, watch for my biosignature gas research and my technology demonstrations on small satellite platforms.
CNN: You've made a lot of new scientific discoveries. Which one are you most proud of and why?
Seager: I am most proud of my early work on exoplanet atmospheres that led to the first detection of an exoplanet atmospheres by the Hubble Space Telescope. I predicted that the gas sodium would be observable in a type of planet that is easily accessible to observations, the so-called hot Jupiters. (These) planets (are) about the size and mass of Jupiter but so close to their host star, several times closer to their star than Mercury is to our sun. I'm proud of my related work that helped to lay down the theoretical foundation for exoplanet atmospheres.
I am proud of laying the basic understanding and approaches to many areas of exoplanet characterization.
CNN: What interesting facts can you share with us about exoplanets' interiors and their internal structure?
Seager: In the deep interiors of some planets, conditions reach such high pressures that matter takes on a different form than in everyday life, such that if you imagine adding mass to a planet it gets smaller overall, not larger.
CNN: What advice would you give to aspiring scientists?
Seager: Success: Find something you love doing and are also great at.
Failure: ... Don't be afraid to fail (but have a backup plan). ...
I'd like to try to reach the people who are struggling, who think they never have a chance to succeed in the way I have.