Working through years' worth of data from an old observational campaign, a team of astronomers have directly imaged a new exoplanet, meaning a planet that exists outside our solar system. New Mexico State University Astronomy Assistant Professor Eric Nielsen and graduate students Anne Peck and William Roberson are co-authors on the paper.

The NMSU team collaborated closely with Jason Wang, assistant professor of physics and astronomy at Northwestern University, lead author Nathalie Jones, a member of Wang's research group, and other members of the Gemini Planet Imager Exoplanet Survey (GPIES) team. Together, they revisited data taken between 2016 and 2019 through the GPIES campaign and combined it with data taken in 2022 from the W. M. Keck Observatory in Hawaii.

Amidst the old data, they found the new planet.

Beyond the excitement of the initial discovery, features of the planet itself make it an intriguing target for future research. It's massive — five times the mass of Jupiter – and around 13 million years old. For comparison, Earth is around 4.5 billion years old.

"By astronomy standards, it is very, very young," Nielsen said. "Planet formation should have been finished by this stage, but we're still looking at a really young system. "But, the planet's orbit could be changing, either now or in the future."

The NMSU team is particularly interested in the system in which the planet orbits. The planet orbits a binary star system, where two stars are orbiting each other. While the exoplanet takes 300 years to complete a full orbit, these two stars are in such close proximity that they complete one orbit in just 18 days. This discovery provides astronomers with a rare opportunity to study planet formation in complex systems.

For Nielsen, part of the excitement is in the many new questions this planet introduces.

"Do you get different types of planets depending on the density of the star formation region? How much does planet formation care if your host star is a binary, or if it's a single star?" Nielsen said. "We only have preliminary orbital information on this new planet. So, is it on a nice circular orbit very happily orbiting the two stars, or is it on a more extreme eccentric orbit?"

In studying the planet's host stars, Nielsen, Peck and Roberson worked out the orbit of the planet around the stars, the orbit of the stars around each other and used models and measurements to calculate other properties of the stars like orbital period and mass.

Even the technique used to find this exoplanet makes it special. Thousands of exoplanets have been found so far, but only a few dozen of those were discovered through direct imaging.

"It's a difficult technique," Nielsen said. "You need the star to be relatively young and relatively nearby. You need the planet to cooperate as well by being massive enough, bright enough and far enough away from the star to be detectable."

In addition to using a tricky method, this planet was already just at the edge of what is detectable. In fact, it went undetected during the initial round of data reduction. It wasn't until what the team called the "Great Reprocessing," which used improved data processing methods, that signs of the planet began to appear.

"The campaign's observations finished just before the pandemic. This is an example of going back to old data and pulling out every little bit of information you can," Nielsen said. "Anne and William together were able to pull out the radial velocities very, very precisely, and that allows for incredibly precise orbits."

Peck, a fifth-year doctoral candidate, led the analysis of spectra of the binary stars, developing a new technique to characterize the two stars and how they were moving in their orbit around each other. Roberson, a third-year graduate student, investigated how accurate this method is at accurately characterizing each star.

"It's always exciting when you get to be a part of the team that found a planet," Peck said. "Every time you find an exoplanet, it helps you put our own solar system in context. There are still only a few dozen directly imaged planets, but every time we add to that data set, it opens up so many questions."

"I'm interested in planets in binary systems, and they can often be harder to study," Roberson said. "Having a second star there just makes things harder. So, anytime we can add a planet going around a binary to the exoplanet catalogue, I think that's a really great data point to have."

Both Peck and Roberson have begun incorporating elements of their work on this paper into other research areas.

"To understand HD 143811 b we developed a new method for studying stars in close binary systems," Peck said. "We're now applying this new tool to other stellar binaries."

The team's paper, "HD 143811 AB b: A directly imaged planet orbiting a spectroscopic binary in Sco-Cen," is available here. The companion paper led by Peck, Roberson and Nielsen detailing the stellar binary hosts is available here.

The full article can be seen at https://newsroom.nmsu.edu/news/nmsu--northwestern-university-astronomers-discover-new-planet-in-old-data/s/2e6aa913-d763-4792-b661-17dd5049d592