‘Unusually massive’ – Astronomers have discovered a planet that shouldn’t exist

Researchers at Penn State have discovered an unusually massive planet, LHS 3154b, orbiting an ultracool dwarf star. This discovery, which contradicts current theories, prompts a re-evaluation of star and planet formation processes.

The discovery of a planet so massive relative to its sun calls into question what was previously understood about the formation of planets and their solar systems, according to Penn State researchers.

In a research published on November 30 in the journal SciencesResearchers have announced the discovery of a planet more than 13 times the mass of Earth orbiting the “ultra-cool” star LHS 3154, which itself is nine times less massive than the Sun. The ratio of the mass of the newly discovered planet to its host star is more than 100 times higher than that of the Earth and the Sun.

This video is an artistic representation of the newly discovered system, LHS 3154, which contains a planet much larger relative to its sun than current models predict. Credit: Abigail Hope Minnich

Challenging current theories

The discovery reveals the most massive planet known in close orbit around an ultracool dwarf star, the lowest mass and coldest star in the universe. This discovery goes against what current theories predict about planet formation around small stars, and represents the first time a planet with such a high mass has been observed orbiting a low-mass star.

“This discovery really drives home how little we know about the universe,” said Suvrath Mahadevan, a professor of astronomy and astrophysics at Penn State and co-author of the paper. “We did not expect a planet this massive around a low-mass star.”

Artistic rendering of a collective comparison between LHS 3154, our Earth, and our Sun. Credit: Pennsylvania State University

Formation of stars and planets

He explained that stars are formed from large clouds of gas and dust. After a star forms, gas and dust remain as disks of material orbiting the newborn star, which can eventually evolve into planets.

“The planet-forming disk around the low-mass star LHS 3154 is not expected to have enough solid mass to form this planet,” Mahadevan said. “But it exists, so now we need to reconsider our understanding of how planets and stars form.”

Penn State researchers Suvrath Mahadevan and Megan Delamere explain the discovery of a massive planet orbiting a small star. Credit: Pennsylvania State University

Detection using HPF

The researchers observed the massive planet, called LHS 3154b, using an astronomical spectrograph built in Pennsylvania by a team of scientists led by Mahadevan. The tool, called Habitable Zone Planet Finder, or HPF, is designed to discover planets orbiting the coldest stars outside our solar system with the potential for liquid water — a key ingredient for life — on their surfaces.

Discovering planets around ultracool stars

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

“Think of it like a star is a campfire. The cooler the fire gets, the closer you are to that fire to stay warm,” Mahadevan said. “The same is true for the planets. If the star is cooler, the planet will need to be closer to that star if it is to be warm enough to contain liquid water. If a planet has an orbit close enough to its ultracool star, we can detect it by seeing a very slight change in the color or light of the star’s spectra as it is dragged by an orbiting planet.

Artist’s rendering of the possible view from LHS 3154b toward its low-mass host star. Given its large mass, LHS 3154b likely has a Neptune-like composition. Credit: Penn State

Importance of HPF

Located at the Hobby-Eberly Telescope at McDonald Observatory in Texas, the HPF provides some of the highest resolution measurements to date of infrared signals from nearby stars.

“Making the discovery with the HPF was very special, as it is a new instrument that we designed, developed and built from the ground up for the purpose of looking at unknown planetary populations around lower-mass stars,” said Gumundur Stefansson. NASA Sagan Fellow in Astrophysics Princeton University and the paper’s lead author, who helped develop the HPF and worked on the study as a graduate student at Penn State. “We are now reaping the rewards, learning new and unexpected aspects of this exciting group of planets orbiting some nearby stars.”

The tool has already yielded important information in Discovery and confirmation Stefansson explained that there are new planets, but the discovery of the planet LHS 3154b exceeded all expectations.

Reconsidering planet formation theories

“Based on existing survey work with HPF and other instruments, an object like the one we discovered is likely extremely rare, so its discovery was really exciting,” said Megan Delamere, an astronomy graduate student at Penn State and co-author of the research project. paper. “Our current theories about planetary formation have difficulty explaining what we see.”

Delamere explained that in the case of the discovered massive planet orbiting the star LHS 3154, the core of the heavy planet inferred by the team’s measurements would require a greater amount of solid material in the planet’s disk than current models predict. The discovery also raises questions about prior understanding of star formation, as the ratio of mass and dust to gas in the circumstellar disk like LHS 3154 — when it was young and newly forming — must have been ten times higher. From what was observed in order to form a massive planet like the one discovered by the team.

“What we discovered provides an extreme test case for all existing planet formation theories,” Mahadevan said. “This is exactly what we built the HPF to do, to discover how the most common stars in our galaxy form planets — and to find those planets.”

Reference: “A Neptune-Mass Exoplanet “In close orbit around a low-mass star defies models of formation” by Gumundur Stefansson, Suvrath Mahadevan, Yamila Miguel, Paul Robertson, Megan Delamere, Shubham Kanodia, Caleb I. Kanias, Joshua N. Wynn, Joe B. Neenan, Ryan C. Therrien, Ray Holcomb, Eric B. Ford, Brianna Zawadzki, Brendan B. Bowler, Chad F. Bender, William D. Cochran, Scott Diddams, Michael Endell, Connor Frederick, Samuel Halverson, Fred Harty, Gary J. Hill, Andrea S. J. Lane, Andrew J. Metcalfe, Andrew Munson, Lawrence Ramsay, Arpita Roy, Christian Schwab, Jason T. Wright and Gregory Zeman, November 30, 2023, Sciences.
doi: 10.1126/science.abo0233

Other Penn State authors on the paper are Eric Ford, Brianna Zawadzki, Fred Harty, Andrea Lin, Lawrence Ramsay, and Jason Wright. Other authors on this paper are Joshua Wen of Princeton University, Yamila Miguel of Leiden University, Paul Robertson of the University of California, Irvine, Ray Holcomb of the University of California, and Shubham Kanodia of the University of California. Carnegie Institution for ScienceCaleb Kanias of NASA’s Goddard Space Flight Center, Joe Neenan of India’s Tata Fundamental Research Institute, Ryan Therrien of Carleton College, Brendan Bowler, William Cochran, Michael Endel, and Gary Hill of the University of Texas at Austin, and Chad Bender of the University of Texas at Austin. University of Arizona, Scott Diddams, Connor Frederick and Andrew Metcalf of the University of Colorado, Samuel Halvorson of the Jet Propulsion Laboratory of the California Institute of Technology, Andrew Munson of the University of Arizona, Arpita Roy of Johns Hopkins University, Christian Schwab of Macquarie University in Australia and Gregory Zeeman of the Hobby Telescope. Eberly at the University of Texas at Austin.

This work was funded by the Center for Exoplanets and Habitable Worlds at Penn State, the Space Grant Consortium of Pennsylvania, the National Aeronautics and Space Administration, the National Science Foundation, and the Hysing-Simons Foundation.