Discovery of Planet Too Big for Its Sun Throws Off Models of Solar System Formation

AUSTIN, TX – The discovery of a planet that is far too massive for its sun is calling into question what was previously understood about the formation of planets and their solar systems. 

In a paper November 30, 2023, in the journal Science, a team of researchers from The University of Texas at Austin, Penn State University, and other science institutions worldwide join in reporting the discovery of a planet more than 13 times as massive as Earth orbiting the “ultracool” star LHS 3154, which itself is nine times less massive than the Sun. The mass ratio of the newly found planet with its host star is more than 100 times higher than that of Earth and the Sun.

The finding reveals the most massive known planet in a close orbit around an ultracool dwarf star, the least massive and coldest stars in the universe. The discovery goes against what current theories would predict for planet formation around small stars and marks the first time a planet with such high mass has been spotted orbiting such a low-mass star. 

“Nature is a lot cleverer than we are!” said William Cochran, research professor at The University of Texas at Austin and co-author on the paper. “Planet formation can take place in a lot of circumstance we had not necessarily expected.”
 
“This discovery really drives home the point of just how little we know about the Universe,” added Suvrath Mahadevan, Verne M. Willaman Professor of Astronomy and Astrophysics at Penn State and another co-author on the paper. “We wouldn’t expect a planet this heavy around such a low-mass star to exist.”

Stars are formed from large clouds of gas and dust. After the star is formed, the gas and dust remain as disks of material orbiting the newborn star, which can eventually develop into planets.

“The planet-forming disk around the low-mass star LHS 3154 is not expected to have enough solid mass to make this planet,” Mahadevan said, “but it’s out there, so now we need to reexamine our understanding of how planets and stars form.”
 
Cutting-Edge Instrumentation Is Key in Search for Exoplanets
The researchers spotted the oversized planet, named LHS 3154b, using an astronomical spectrograph on the Hobby-Eberly Telescope at The University of Texas at Austin’s McDonald Observatory. The instrument, called the Habitable Zone Planet Finder or HPF, was built at Penn State by a team of scientists led by Mahadevan. It was designed to detect planets orbiting the coolest stars outside our solar system with the potential for having liquid water on their surfaces, a key ingredient for life. 

While such planets are very difficult to detect around stars like our Sun, the low temperature of ultracool stars means that planets capable of having liquid water on their surface are much closer to their star relative to Earth and the Sun. This shorter distance between these planets and their stars, combined with the low mass of the ultracool stars results in a detectable signal announcing the presence of the planet. 

“Think about it like the star is a campfire. The more the fire cools down, the closer you’ll need to get to that fire to stay warm,” Mahadevan explained. “The same is true for planets. If the star is colder, then a planet will need to be closer to that star if it is going to be warm enough to contain liquid water. If a planet has a close enough orbit to its ultracool star, we can detect it by seeing a very subtle change in the color of the star’s spectra or light as it is tugged on by an orbiting planet.” 
 
“The trick is not detecting planets of this mass,” said Cochran, “But doing so around such a low mass star. As you go down in stellar mass, the total brightness of the star drops precipitously. And most of the light it gives off comes out in the infrared region of the spectrum.” The HPF provides some of the highest precision measurements to date of such infrared signals from nearby stars. 

“Making the discovery with HPF was extra special, as it is a new instrument that we designed, developed, and built from the ground-up for the purpose of looking at the uncharted planet population around the lowest mass stars” said Guðmundur Stefánsson, NASA Sagan Fellow in Astrophysics at Princeton University and lead author on the paper, who helped develop HPF and worked on the study as a graduate student at Penn State. “Now we are reaping the rewards, learning new and unexpected aspects of this exciting population of planets orbiting some of the most nearby stars.”

The instrument has already yielded critical information in the discovery and confirmation of new planets, Stefánsson explained, but the discovery of the planet LHS 3154b exceeded all expectations. 

Discovery Challenges Theories of Planet Formation
“Based on current survey work with the HPF and other instruments, an object like the one we discovered is likely extremely rare, so detecting it has been really exciting,” said Megan Delamer, astronomy graduate student at Penn State and co-author on the paper.
 
In the case of the massive planet discovered orbiting the star LHS 3154, the heavy planetary core inferred by the team’s measurements would require a larger amount of solid material in the planet-forming disk than current models would predict, Delamer explained. 

“Our current theories of planet formation have trouble accounting for what we’re seeing,” she said. 
 
The finding also raises questions about prior understandings of the formation of stars, as the dust-mass and dust-to-gas ratio of the disk surrounding stars like LHS 3154, when they were young and newly formed, would need to be ten times higher than what was observed in order to form a planet as massive as the one the team discovered. 
 
“What we have discovered provides an extreme test case for all existing planet formation theories,” Mahadevan said. “This is exactly what we built HPF to do, to discover how the most common stars in our galaxy form planets – and to find those planets.”

Acknowledgements
Penn State authors on the paper are Suvrath Mahadevan, Eric Ford, Brianna Zawadzki, Fred Hearty, Andrea Lin, Lawrence Ramsey, and Jason Wright. The University of Texas at Austin authors are Brendan Bowler, William Cochran, Michael Endl, Gary Hill, and Gregory Zeimann. Other authors on the paper are Joshua Winn of Princeton University, Yamila Miguel of the University of Leiden, Paul Robertson and Rae Holcomb of the University of California Irvine, Shubham Kanodia of the Carnegie Institution for Science, Caleb Cañas of the NASA Goddard Space Flight Center, Joe Ninan of India’s Tata Institute of Fundamental Research, Ryan Terrien of Carleton College, Chad Bender of The University of Arizona, Scott Diddams, Connor Fredrick and Andrew Metcalf of the University of Colorado, Samuel Halverson of California Institute of Technology’s Jet Propulsion Laboratory, Andrew Monson of the University of Arizona, Arpita Roy of Johns Hopkins University, and Christian Schwab of Australia ‘s Macquarie University.
 
The work was funded by the Center for Exoplanets and Habitable Worlds at Penn State, the Pennsylvania Space Grant Consortium, the National Aeronautics and Space Administration, the National Science Foundation and the Heising-Simons Foundation.

The Hobby-Eberly Telescope (HET) is a joint project of The University of Texas at Austin, Pennsylvania State University, Ludwig-Maximilians-Universitaet Muenchen, and Georg-August Universitaet Goettingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly.

The HET Habitable-Zone Planet Finder team is supported by grants from the National Science Foundation, the NASA Astrobiology Institute, and the Heising-Simons Foundation.

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