The discovery of a planet too large relative to its sun throws off models of solar system formation

The discovery of a planet too large relative to its sun throws off models of solar system formation

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

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

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 paper published in the journal SciencesResearchers report the discovery of a planet 13 times more massive than 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.

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.”

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.


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

“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.”

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.

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

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 this 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 NASA’s Gumundur Stefansson. A Sagan Fellow in astrophysics at Princeton University and lead author of the paper, 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.”

Stefansson explained that the instrument has already yielded important information in discovering and confirming new planets, but the discovery of the planet LHS 3154b exceeded all expectations.


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

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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

“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.”

more information:
Guðmundur Stefánsson et al. A Neptune-mass exoplanet in close orbit around an extremely low-mass star challenges formation models. Sciences (2023). doi: 10.1126/science.abo0233. www.science.org/doi/10.1126/science.abo0233

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