Cecilia Lee

A team of astronomers, led by three Yale scientists, has developed a new technique to determine the presence of exoplanets by observing the surface of stars.

Debra Fischer, professor of astronomy at Yale, Rachael Roettenbacher, a 51 Pegasi b Postdoctoral Fellow, and Sam Cabot GRD ’24, a graduate student, are all part of the team of astronomers who searched for exoplanets near stars. The results of their research will be published in the Astronomical Journal, which recently accepted their paper detailing the use of EXPRES, a spectrograph built at Yale by Fischer and her team to infer the presence of exoplanets based on the physical properties of stars.

“We never ‘see’ the exoplanets — we infer their presence by the effect they have on the star — which we can see because it emits light,” Fischer wrote in an email to the News. “Flows and features of the surface of stars also cause the measured velocities of stars to change — they add ‘noise’ to our planet-detecting data. To understand this ‘noise’ effect, we need to map the surfaces of stars — then we have a hope of modeling that noise away and detecting Earth-like planets.”

According to Cabot, the main goal was to find exoplanets by looking for their effects on neighboring stars. For example, as a planet orbits its star, the star will move slightly, and this small movement can be detected by EXPRES. Cabot and Roettenbacher both said that this movement, commonly referred to as the “wobble” of a planet, is called a radial velocity shift.

Cabot added that stars have dark spots similar to those on the Sun. According to Roettenbacher, these starspots get carried around the star as it rotates. When astronomers observe the star, they can see the starspots come in and out of view as the star rotates on its axis. This observation also mimics the wobble caused by a planet orbiting its star.

Astronomers need to determine whether the measured wobble is due to an exoplanet orbiting the star or simply the rotation of the star. To do so, the astronomers take a picture of the star’s surface and calculate the expected wobble due to rotation. They then subtract this expected rotation from the actual measured rotation to find the wobble caused by a potential orbiting planet.

“So our method takes an image of the star’s surface and models what its signal — radial velocity contribution — is.” Roettenbacher wrote. “From there, we can remove that signal from our observed — radial velocity — data that was collected at the same time as the data used to make the surface image. We then look to see if there is evidence of a planet in the data with the surface contribution removed.”

Roettenbacher added that the star used in the journal article, Epsilon Eridani, did not have evidence of an orbiting planet since the surface contribution accounted for that star’s wobble. She also said that there were three important tools necessary for the completion of this project.

The first tool is the EXtreme PREcision Spectrograph, or EXPRES, instrument. This instrument was built by Fischer and her team at Yale in order to find analogs to Earth who orbit around a star that resembles the Sun. The term “extreme precision” refers to the spectrograph’s ability to detect radial velocity shifts, or wobbles, of about 30 cm/s. For this project, EXPRES measured the wobble of Epsilon Eridani.

The second tool is the Transiting Exoplanet Survey Satellite, or TESS, which provided the astronomers with the data needed to construct the images of stars. TESS observes the sky for approximately 27 days and collects photons from stars through a method called photometry. TESS allowed scientists to visualize the changes in brightness of Epsilon Eridani as its starspots came in and out of view.

Finally, the last tool is an interferometer called the Center for High Angular Resolution Astronomy, or CHARA, Array. Interferometers essentially combine separate telescopes into one larger telescope. In this project, the astronomers used CHARA to see far-away stars as disks in the same way they can observe the Sun and the Moon. This was also the first time interferometry was used to see evidence of a starspot on the surface of Epsilon Eridani, according to Roettenbacher.

“Our images of eps Eri come from two techniques,” Cabot wrote in an email to the News. “In one, we measured the total amount of light emitted by the star over time, and determined the locations and sizes of spots that matched the patterns we saw. In the other, we used interferometry, where spaced out telescopes together act as a single large telescope, and let us actually see detail on the surface of the star.”

According to Roettenbacher, observing the surface of a star is much easier than observing the surface of an exoplanet. Cabot added that by measuring the signals emitted by a star and correcting for signals emitted by the starspots, astronomers can search for smaller planets similar to Earth. Some of these small planets are hidden by the activity of their star. This new method’s goal is to reveal the existence of these small, once-hidden exoplanets and, as such, find planets analogous to Earth.

Epsilon Eridani is the third closest star visible to the naked eye.

Selin Nalbantoglu covers breakthrough research for SciTech. She is a first-year in Saybrook College.