Stellar Discovery: Terrien ’09 and team of scientists search for habitable planets

8 April 2019

The search for life on other planets took one small step forward this year when a team including assistant professor of physics and astronomy Ryan Terrien ’09 confirmed and observed a recently discovered super-Earth orbiting Barnard’s star—one of our sun’s closest neighbors—during the commissioning of a new tool designed to find habitable planets.

The Penn State-led group’s discovery was detailed in the paper, “Stellar spectroscopy in the near-infrared with a laser frequency comb,” co-written by Terrien and published in the February 2019 issue of Optica. The team built a tool called the Habitable Zone Planet Finder (HPF) to better detect planets orbiting M dwarf stars. The tool repeatedly measures the spectrum of a star—how much light it emits at different wavelengths—over weeks, months and years.

Terrien began working on the Habitable Zone Planet Finder project in 2010 while studying astronomy and astrophysics at Penn State, and furthered his research as a postdoctoral fellow with the National Institute of Standards and Technology’s Optical Measurements Frequency Group. He is now helping with the exoplanet survey, continuing to characterize the stability of the Habitable Planet Finder, which is housed at the University of Texas at Austin’s McDonald Observatory, and observing other stars with the tool.

Terrien and the team of scientists initially set out to find planets around M dwarf stars, which are smaller, less massive and cooler than stars like our sun, but also much more common. Recent observations suggest there are planetary systems around M dwarf stars, Terrien said, however, the stars are faint and emit mostly infrared light instead of visible light, making them harder to discover and study. Therefore, although astronomers think there are many planet-hosting M dwarf stars out there, only a handful have been found.

Studying M dwarf planets allows the scientists to determine whether the patterns observed in other types of stars hold true for planets orbiting M dwarfs, as well. This new knowledge adds important context to the astronomical discoveries of decades past.  

“As a scientific community, we’ve learned a wealth about how planets form and evolve, and have advanced far beyond the understanding we can get from just studying our solar system,” Terrien said. “We’ve observed how planet formation can be impacted by the presence of different elements, how planets can migrate to different places in their systems, and how planets can interact with their host stars, among many other interesting behaviors.”

Terrien and his team are particularly interested in finding planets that might harbor life, hence the name Habitable Zone Planet Finder. They seek to find planets in the “Habitable Zone” around M dwarf stars where conditions might be right to allow for liquid water on the surface. Because M dwarf stars are much less massive than stars like our sun, planets orbiting in their Habitable Zones are closer in and therefore easier to detect.

The Habitable Zone Planet Finder has been used to observe many stars, including the bright and well-studied Barnard’s star. Barnard’s star is one of Earth’s closest neighbor stars, and the Habitable Zone Planet Finder has already been working to detect nearby exoplanets—an exciting population for Terrien to pursue.

“There are many challenges with detecting these types of planets that HPF is helping to address, including how to account for the impacts of stellar activity on M dwarfs, how to make these types of measurements with infrared (rather than visible) detectors, and how to best correct for the effects of Earth’s atmosphere,” Terrien said. “I am particularly excited about what we will learn as we continue to study these challenges.”