Lighting up exoplanets

Ever since the discovery of the first exoplanet, 51 Pegasi b, hidden within the well-known Pegasus constellation in 1995, the burgeoning field of exoplanetary astronomy has taken off. Since then, the field has seen an explosion of related scientific specialties, and scientists have been able to detect more than 3,500 worlds beyond our solar system. People had always dreamed about the secrets other worlds held, now researchers empowered by advances in technology are able to find and study them.

As the 50th anniversary of the first Apollo moon landing approaches, excitement about planetary bodies has been heightened. The growing field of planetary astronomy studies celestial objects both within and beyond our solar system, bridging planetary science and astronomy.

This year, the Heising-Simons Foundation has granted six researchers from around the U.S. 51 Pegasi b fellowships to support the search for this knowledge, and two of those six will conduct their studies at MIT. Clara Sousa-Silva and Benjamin Rackham were selected as some of the most promising astronomers to pursue “innovative independent research ideas, take risks, and tackle challenging questions what will accelerate the field.” They will be conducting their work within MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).

Hosting life

“I want to study every molecule on every possible exoplanet atmosphere, and then try to recognize under what conditions certain molecules can signify life,” Sousa-Silva says.

Sousa-Silva is a quantum astrochemist and an EAPS postdoc with a desire to understand the mechanisms of the universe, particularly exoplanet habitability — with a focus on the molecules that compose their atmospheres. As a theorist, Sousa-Silva simulates the ways individual molecules interact with light so that they can be identified anywhere in the galaxy. She combines quantum physics and computer calculations to create molecular “fingerprints” that allow scientists to detect remote gases on exoplanet atmospheres, in particular gases that are associated with life.

Sousa-Silva is currently examining molecules that are associated with anaerobic biology in order to detect life on planets without oxygen. Her work has culminated in a new method, Rapid Approximate Spectral Calculations for ALL (RASCALL), that generates simulations of molecules. RASCALL can help scientists make rapid decisions about whether to follow up promising observational data with more detailed study.

“What excites me most is the idea of combining supremely fundamental science with really inspirational, out-of-this-world questions about what life looks like elsewhere,” says Sousa-Silva.

During her fellowship, Sousa-Silva will expand her publicly available database of spectra to ultimately include sixteen thousand molecules, and use those data to completely and accurately interpret exoplanet atmospheres. With these, she’ll be able to rank promising biosignatures by their spectroscopic potential, not limited to Earth-like molecules. The goal is to have the tools to identify any given alien biosphere. She points out that, not only is molecular behavior a fundamental and universal truth, and as such worth knowing, but understanding which molecules are on an exoplanet, and their role within it, is the most promising avenue for detecting life in the galaxy.

To understand the role of molecules in space, Sousa-Silva’s research will straddle astronomy/astrobiology and spectroscopy. She’ll continue to work with the biosignatures group led by Sara Seager, EAPS' Class of 1941 Professor of Planetary Sciences with an appointment in the Department of Physics. She will also work with other MIT atmospheric chemists, and the access to exoplanet data and specialists will provide Sousa-Silva with unparalleled resources. Additionally, collaborations with other groups will help ensure her spectra are of the highest quality and available to scientists: HITRAN at Harvard University, ExoMol at University College London, the Burgasser group at the University of California at San Diego, and McKemmish’s team at the University of New South Wales.

“The literal astronomical scale of the problems that need to be solved to understand exoplanets is daunting, but very much where I feel at home,” says Sousa-Silva. “As a 51 Pegasi b fellow, I want to bridge the gap between spectroscopy and astronomy, and by doing so create a computational chemistry toolkit to identify life on an exoplanet.”

Dimming stars

Benjamin Rackham is motivated by a similar question — “Are we alone?” — and uses light to study exoplanets and their stars, but in a different way. As a postdoc at the University of Arizona’s Steward Observatory, Rackham examines transmission spectra from transiting exoplanets.

“It’s phenomenal to watch the brightness of a star dim and increase again, telling you that a planet is passing in front of the star. It’s exciting to witness a secret in the sky that’s been going on for billions of years but has taken the invention of telescopes to discover,” Rackham says.

Light that arrives at Earth from extrasolar systems contains a lot of information, both from the stars and their associated transiting exoplanets. Encoded, is data on the planet’s size, the composition of its atmosphere, the type of light emitted from the host star, distance to its star, among other things. But it’s not so straight forward; some signals can mimic or mask others, making them difficult to discern.

One example is the transit light source effect, which happens when a star’s photosphere is not uniform and shows blemishes like starspots due to magnetic activity. This causes the emitted light to vary before, during and after a transit, making it difficult for researchers to understand what the full spectrum of the star is, and if light variations during a transit are caused by a feature of the planet’s atmosphere or originating from the star itself.

Rackham works to disentangle these signals. As an experimentalist, he aims to pilot and refine techniques that lead to more robust observations on the true nature of such exoplanets. Systematically investigating this phenomenon on a diverse range of stars, Rackham’s research has revealed that spots on small, cool stars (red dwarfs), which tend to be more active than Sun-like stars, can produce spectral features that give the appearance of water vapor and other atmospheric traits on transiting planets.

“This is problematic for astrobiology because these are the same types of stars that we would like to focus on to study small, temperate, rocky worlds that might host life,” Rackham says.

In his fellowship at MIT working with EAPS Assistant Professor Julien de Wit, Rackham will address this problem of stars imprinting spectral features on transit depths by refining scientists’ understanding of the properties of active stars that host exoplanets. To do this, he will conduct a series of observational studies of exoplanet host stars and ultimately produce an open-source tool that untangles stellar and planetary signals, taking into account transit observations alongside high-resolution stellar spectra and long-term brightness monitoring data. Using datasets from space missions like Kepler and TESS, he will measure the properties of starspots and how they affect transmission spectra.

This tool will be essential to studying the smallest transiting exoplanets with the James Webb Space Telescope and, in the long run, searching for atmospheric biosignatures with transits. In order to decompose these spectra, Rackham will collect highly-detailed observations and evolution of the brightness of host stars over time using the SPECULOOS telescope network, and the Magellan Telescopes located at the Las Campanas Observatory in Chile. In addition, his efforts to characterize the star TRAPPIST-1 in detail will support the study of its seven Earth-sized planets. Altogether, these contributions will be critical to catalog exoplanet atmospheres in the search of life elsewhere in the universe.

“With the TESS mission, it’s a particularly great time for exoplanets at MIT,” says Rackham. “I’m really excited to be in the thick of the action now and, in the future, to be part of the team that seeks to find life elsewhere in the universe!”

Other postdocs and their hosting institutions who have also received fellowships are Juliette Becker, who is joining Caltech; Cheng Li, who is joining the University of California at Berkeley; Jessica Spake, who is joining Caltech; and Xinting Yu, who is joining the University of California at Santa Cruz.

The Heising-Simons Foundation is a family foundation based in Los Altos and San Francisco, California. The foundation works with its many partners to advance sustainable solutions in climate and clean energy, enable groundbreaking research in science, enhance the education of our youngest learners, and support human rights for all people. The fellowship not only provides up to $375,000 of support for independent research over three years, but also access to professional networks and mentorship, as well as the time and space to establish distinction and leadership in the field.

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