New exoplanets catalog showcases diverse, exotic worlds

Separate studies this week explain how some exoplanets appear ‘puffy’

Artist rendition of exoplanet orbiting a star
Artist’s rendition of TOI-1824, a star system with an exoplanet that’s unusually dense for its size. It is one of the densest "sub-Neptunes" profiled in the TESS-Keck Survey catalog. (Credit: W. M. Keck Observatory/Adam Makarenko)

Our understanding of exoplanets, those strange worlds that orbit stars beyond our solar system, is now broader and deeper thanks to separate studies published this week featuring the work of researchers at UC Santa Cruz.

The first study, published in today’s edition of the Astrophysical Journal Supplement, catalogs 126 confirmed and candidate exoplanets discovered with the National Aeronautics and Space Administration (NASA) Transiting Exoplanet Survey Satellite (TESS) in collaboration with W. M. Keck Observatory, on Maunakea, Hawaiʻi.

A second set of studies, published on May 20 in Nature, explains why some exoplanets look “puffy,” using data collected with NASA’s James Webb Space Telescope, combined with prior observations from NASA’s Hubble Space Telescope.

Pick your exoplanet

The latest installment of the TESS-Keck Survey includes over 9,200 radial velocity observations, which shed light on a fascinating mix of planet types beyond our solar system, from rare worlds with extreme environments to ones that could possibly support life.

Artist’s rendition of the variety of exoplanets featured in the new NASA TESS-Keck Survey Mass Catalog. (Credit: W. M. Keck Observatory/Adam Makarenko)

Artist’s rendition of the variety of exoplanets featured in the new NASA TESS-Keck Survey Mass Catalog. (Credit: W. M. Keck Observatory/Adam Makarenko)

“The results that have come from the TESS-Keck Survey represent the single largest contribution to understanding the physical nature and system architectures of new planets TESS has discovered,” said Alex Polanski, lead author of the first study and a graduate physics and astronomy student at the University of Kansas. “Catalogs like this help astronomers place individual worlds in context with the rest of the exoplanet population.”

Polanski and a global team of astronomers who comprise the TESS-Keck Survey research consortium spent three years developing the catalog. The team also obtained an additional 4,261 radial velocity (RV) observations with the UC Observatories’ Automated Planet Finder at Lick Observatory. With the combined total of RV measurements, they were able to measure the masses of 120 confirmed planets, plus six candidate planets.

Their study catalogs exoplanets by size, categorizing them in groups related to the radius of planets in our solar system. One of the densest “sub-Neptune” planets in the TESS-Keck Survey catalog, called TOI-1824b, is the subject of another TESS-Keck Survey paper led by UC Santa Cruz graduate Sarah Lange (Merrill ‘23, astrophysics).

“At nearly 19 times the mass of Earth, but only 2.6 times the size of our home planet, TOI-1824b is an exoplanet oddity,” said co-author Joseph Murphy, a Ph.D candidate in astronomy and astrophysics at UC Santa Cruz who, along with professor and director of the UC Santa Cruz Astrobiology Initiative, Natalie Batalha, advised Lange on the project. “Planets similar in size typically have a mass between roughly six and 12 times the mass of Earth.”

One explanation for why TOI-1824b is so massive yet appears much smaller than usual is that it could have an Earth-like core surrounded by an unusually thin, hydrogen-dominated atmosphere. Another possibility is the planet has a water-rich core beneath a steam atmosphere. 

“This superdense sub-Neptune may be the massive cousin of water worlds, which are small planets thought to have high H2O content purported to exist around red dwarf stars,” Murphy said.

Red dwarfs, or M dwarf stars, are the most common star type in the Milky Way galaxy. They make ideal targets in the search for habitable worlds because M dwarfs are cooler than the Sun; this allows for liquid water to exist on planets orbiting closer to them, therefore making these systems easier to study.

Other researchers who contributed to this study include Nicholas Scarsdale, a Ph.D. candidate in astronomy and astrophysics at UC Santa Cruz, and Paul Dalba, a former UC Santa Cruz Heising-Simons 51 Pegasi b Postdoctoral Fellow.

A deeper dive

UC Santa Cruz researchers also made important contributions to parallel studies published on May 20 in Nature, both on the inflated exoplanet WASP-107b in the constellation Virgo. Two independent teams, led by researchers at Johns Hopkins University and Arizona State University, concluded that the planet's interior must be significantly hotter—and its core much more massive—than previously estimated.

The unexpectedly high temperature ​​is thought to be a result of tidal heating caused by the planet’s slightly non-circular orbit stretching it like silly putty. The teams say this can explain how planets like WASP-107b can be so puffy, possibly solving a long-standing mystery in exoplanet science.

Artist's concept of the warm gas-giant WASP-107b in the constellation Virgo. (Credit: NASA, ESA, CSA, Ralf Crawford/STScI)

Artist's concept of the warm gas-giant WASP-107b in the constellation Virgo. (Credit: NASA, ESA, CSA, Ralf Crawford/STScI)

At more than three-quarters the volume of Jupiter but less than one-tenth the mass, the “warm Neptune” exoplanet WASP-107b is one of the least dense planets known. While puffy planets are not uncommon, most are hotter and more massive, and therefore easier to explain.

"The effect of tides is to squeeze a planet's shape, and the friction from squeezing can warm up a planet,” said UC Santa Cruz Professor Jonathan Fortney, chair of the astronomy and astrophysics department. “Most dramatically in the solar system, tidal heating causes volcanoes spewing magma all over the surface of Jupiter's moon Io. Here we have an exoplanet example where tides are so important that they heat up the planet's thick atmosphere, which actually changes the chemistry of the atmosphere we see."

Fortney and UC Santa Cruz graduate students Sagnick Mukherjee and Yao Tang contributed to the ASU-led study on the detailed reconnaissance of WASP-107b. Although the planet has an orbital distance of just 5 million miles—one-seventh the distance between Mercury and the sun—it doesn’t receive enough energy from its star to be so inflated. But, with the distance between the star and planet changing continuously over the 5.7-day orbit, the gravitational pull is also changing, stretching the planet and heating it up.

Researchers had previously proposed that tidal heating could be the cause of WASP-107b’s puffiness. But until the Webb results were in, there was no evidence. The planet’s giant radius, extended atmosphere, and orbit made it ideal for transmission spectroscopy, a method used to identify the various gasses in an exoplanet atmosphere based on how they affect starlight.