The Planet Hunter

Dr. Kaloyan Penev

Searching for planets that orbit stars outside the solar system is like trying to find a needle in a haystack — when both the needle and the haystack are trillions of miles away.

Nonetheless, scientists around the world have discovered more than 5,000 of these so-called exoplanets in more than 3,600 planetary systems, according to the NASA Exoplanet Archive.

Dr. Kaloyan Penev is one such planet hunter. The assistant professor of physics, who joined The University of Texas at Dallas faculty in 2017, is part of an international research team that has so far found more than 70 exoplanets. With the aid of robotic land-based telescopes and computer software, Penev combs data for small dips in the light of stars, which could indicate the presence of a planet passing, or transiting, in front of its star.

Penev’s research is focused on finding rare planets dubbed hot Jupiters and hot Saturns, which are large and gaseous and reside very close to their parent stars.

“It’s the proverbial needle-in-a-haystack search,” Penev said. “We search for these hot Jupiters and Saturns because they’re planets we can actually detect. About half a percent of all stars have such planets, and about a tenth of those have the correct orientation for us to detect their presence.”

While Penev’s research is not specifically aimed at finding evidence of extraterrestrial life or potentially habitable planets for humanity, his work does shed light on the formation of stars and planets throughout the universe.

“Prior to the discovery of hot Jupiters and Saturns, scientists did not believe these types of planets could form so close to a star,” Penev said. “But apparently they can. It has forced us to re-evaluate how planets form and how they can move around.

“We have a very incomplete picture about how planets form, including our own. We’re also learning various other aspects about how planets and stars interact.”

One way they interact is through gravitational tides — the gravitational pull they have upon each other.

“Tides turn out to be one of the least-understood phenomena in stellar astrophysics,” Penev said. “Our group studies how the object feeling the tides actually responds.”

As a planet orbits around a star, tides stretch and squeeze them. The objects resist this stretching and squeezing, which causes the planet to heat up and its orbit to change.

“We look for signs of these changes and/or heating and try to use them to figure out how much resistance to tides these objects provide,” he said. Penev’s interest in the heavens began as a middle school student in Stara Zagora, Bulgaria.

“My school had an observatory on its roof,” he recalled. “My mom encouraged me to check it out. She had told me about participating in a program when she was a kid where they would chase U.S. satellites across the sky to measure their orbits.”

“Tides turn out to be one of the least-understood phenomena in stellar astrophysics.”

Penev went on to earn his bachelor’s degree in astronomy from the California Institute of Technology and his master’s degree and doctorate from Harvard University, also in astronomy. He was nearing completion of his doctorate when he began charting the next course of his career. As fate would have it, the answer was just down the hall.

“I had a job offer in Switzerland, but that would have meant a potentially very disruptive move for my wife and myself,” he said. “Luckily, shortly before I would have had to either accept or reject that offer, I heard that the Hungarian-made Automated Telescope group was looking for someone to join their team. At the time they were based at Harvard, with the head of the project having an office right next to my PhD advisor. In the end, after a brief chat, they decided to offer me the position.”

In 2011, the entire group moved to Princeton University, where Penev was an associate research scholar before joining the UT Dallas faculty in 2017.

He continues to work with the Hungarian-made Automated Telescope Network-South (HATSouth) Exoplanet Survey, which consists of researchers from Princeton, the Max Planck Institute for Astronomy and the Australian National University. HATSouth is a network of six astrograph telescope systems — distributed over South America, Africa and Australia — designed to detect transiting exoplanets in orbit around relatively bright stars visible from the southern hemisphere. Since the network became operational in 2009, the group has discovered 73 exoplanets.

Data from the collaboration informs other planet-hunting efforts as well, such as NASA’s Transiting Exoplanet Survey Satellite, launched in 2018, and the new James Webb Space Telescope, which was launched in late 2021.

Penev said that when it comes to searching for exoplanets, the biggest challenge is precision.

“You need to be very accurate when measuring the brightness of stars, and there are many factors that can get in the way,” he said. “The atmosphere of the Earth changes how bright things appear; telescopes can do funny things to data — all types of things can happen.”

– Paul Bottoni