Sara Seager's Search for Distant Habitable Worlds

Reporting on the #SciWri15 #ExoplanetScience session.

Like a 21st-century Spock, Dr. Sara Seager seeks out new worlds and civilizations. With continually improving telescopes, she persistently and passionately pursues her grand quest: to search throughout our galaxy for habitable planets, a few of which might even resemble the Earth.

Seager, an accomplished professor of planetary science and astrophysics at MIT, gave an engaging presentation at the 2015 Science Writers meeting. She spoke clearly and intensely about her research and the exciting future of planetary exploration.

She and her research group have made important breakthroughs while characterizing newly discovered planets beyond our solar system, known as exoplanets, using the NASA Kepler space telescope. With powerful next-generation observatories, she also looks forward to the next frontier, where her ongoing mission could come to fruition.

Hunting for planets

Seager employs an array of planet-hunting methods. She focuses on "transiting" planets, which briefly reduce the detected starlight emanating from the suns behind them. Utilizing such techniques, observations with Kepler transformed the fledgling field of exoplanet research, revealing that many stars have planets in their systems. However, stars glow brightly and vastly outshine the tiny faint planets orbiting them, making it exceedingly difficult to identify each planetary candidate, even when they are only tens of light-years away.

Astronomers have an easier time finding massive gas giant planets, like Jupiter and Saturn, which are much bigger than their Earth-like peers. For this reason, Seager and her colleagues spent many years studying the structure and evolution of gas giants, gas dwarfs, and other inhospitable planets.

Smaller planets--between the sizes of Earth and Neptune--are much more common than massive ones, argues Seager. The challenge is to carefully pick them out and determine whether they might be of the habitable variety.

In most exoplanet work, astronomers consider only certain planets as potentially life-friendly. Their orbit, atmosphere, surface and climate all must be just right, falling within narrow ranges of parameters. A successful search requires a daunting understanding of biology, chemistry, and geology, as well as astronomy and physics.

"Goldilocks" zone

For alien life to have a chance of forming on a planet, that world must be not too close and not too far from its sun (or suns; see Kepler-16b 'travel poster' below). It must be neither too hot nor too cold, and its atmosphere must have the right mixture of "biosignature" gases like oxygen and water vapor. To spot such elusive objects, Seager searches for planets lying within this "Goldilocks" zone (see figure at right).

For stars like our sun, the Goldilocks zone includes small planets orbiting their stars from a distance between Venus and Mars's orbits. Those two planets are viewed as optimistic boundaries of the habitable range, but Venus suffered from a runaway greenhouse effect while chilly Mars lost most of its atmosphere. Earth happens to reside at a happy medium.

Astronomers used Kepler to discover more than 1,000 planets, with many more candidates under scrutiny. Based on these data, they inferred that our galaxy could be teeming with billions of planets. Of the newly identified planets, as many as ten Earth-like ones—rocky worlds in habitable zones—have been found. But only Kepler-452b, a possibly rocky "super-Earth" (or "sub-Neptune") discovered this summer, orbits a star like our sun. It generated excitement and even speculation as a possible home of extraterrestrials.

Seager argues that the traditional concept of habitable zone is too rigid and should be expanded. "Exoplanets are diverse, covering nearly all masses, sizes and orbits possible," she says. What scientists mean by habitable should be more inclusive, or they risk missing outlier planets that nonetheless could be conducive to life. Accounting for habitability varying depending on the type of star or planet alleviates the situation.

Although Kepler has completed its mission, astronomers now look forward to upcoming telescopes such as Hubble's successor, James Webb Space Telescope (JWST), and the Transiting Exoplanet Survey Satellite. JWST's sensitive instruments will provide more detailed follow-up observations of selected planets, providing "the first true characterization of exoplanetary atmospheres," according to Nikole Lewis, astronomer at Space Telescope Science Institute and former Sagan Postdoctoral Fellow with Seager. By probing the biosignature gases, these new telescopes will take exoplanet research to the next level.

The Next Generation

The prospect of locating another planet hosting life is an exciting possibility. Even if it takes decades, Seager adds, "we want to make sure that we have the capability so that the next generation of scientists could find it."

exoplanets appear to be more diverse than the current generation of astronomers studying them

Unfortunately, exoplanets appear to be more diverse than the current generation of astronomers studying them, though that situation is beginning to change. Seager points out that the relatively young field has many women in it, possibly more than other fields in astronomy. "We're making progress especially in terms of women," says Lewis. "But we still have a very long way to go for other underrepresented groups."

Nonetheless, considering the startling sexual harassment scandal involving Geoff Marcy, former University of California, Berkeley professor and exoplanet authority, Seager and Lewis agree that much more needs to be done to work toward gender equality in astronomy [Ghorayshi 2015]. American Astronomical Society president, Meg Urry, writes: "The solicitations, slights and stress that are all too familiar to many women astronomers don't need to be an everyday part of life for the next generation" [Urry 2015].

References or Relevant Literature

A. Ghorayshi (2015) Famous Berkeley Astronomer Violated Sexual Harassment Policies Over Many Years, BuzzFeed News, October 9

S. Seager (2013) Exoplanet Habitability, Science, 340: 577-581

S. Seager (2014) The future of spectroscopic life detection on exoplanets, Proc. Natl. Acad. Sci. USA, 111:12634–12640

S. Seager & W. Bains (2015) The search for signs of life on exoplanets at the interface of chemistry and planetary science, Science Advances, 1:2

M. Urry (2015) How to End Sexual Harassment in Astronomy, Scientific American, October 14

The citation for this article is:
R. Skibba (2015) Sara Seager's Search for Distant Habitable Worlds. DiverseScholar 6:5

Ramin Skibba is a science communication student at the University of California, Santa Cruz. As a physics and astronomy researcher and lecturer, he previously worked in San Diego, Tucson, southwest Germany, and Pittsburgh, where he earned his PhD. He has written for the Committee on the Status of Women in Astronomy and elsewhere about gender and racial inequality at all levels of the pipeline and about the hidden effects of unconscious bias. As a half-Iranian, he has also written about the struggles of Iranian students and scientists. Dr. Skibba attended the ScienceWriters 2015 conference on a DiverseScholar NASW Diversity Travel Fellowship. We thank mentor Matthew R. Francis, PhD for editing this article. Any opinions expressed in this article are solely those of the author.

Originally published 14-Dec-2015