Scientists find enormous planet orbiting small star, shaking up solar system theories

Artistic rendering of the possible view from LHS 3154b towards its low mass host star. Given its large mass, LHS 3154b probably has a Neptune-like composition. Credit: Penn State

Scientists from Penn State University have made a groundbreaking discovery that’s turning heads in the world of astronomy.

In a recent study published in the journal Science, they revealed the existence of a planet so massive compared to its star that it’s challenging everything we thought we knew about how planets and their solar systems form.

The planet, named LHS 3154b, is over 13 times the mass of Earth and orbits a star, LHS 3154, that’s nine times smaller than our sun.

This unusual pairing has a mass ratio more than 100 times greater than that of Earth and the sun, making it the most massive planet ever discovered in a close orbit around one of the universe’s smallest and coldest stars, known as an ultracool dwarf star.

“This discovery really makes us question our understanding of the universe,” said Suvrath Mahadevan, a Penn State professor involved in the study.

He explains that stars form from large clouds of gas and dust, which then leave behind disks of material that can develop into planets.

However, the planet-forming disk around a small star like LHS 3154 isn’t expected to have enough solid mass to create a planet as large as LHS 3154b. “But here it is, existing against our current theories,” Mahadevan adds.

The discovery was made using the Habitable Zone Planet Finder (HPF), an astronomical spectrograph built at Penn State.

The HPF was specifically designed to detect planets orbiting ultracool stars, which could potentially harbor life-supporting liquid water on their surfaces.

Detecting such planets is usually difficult around stars like our sun. However, since ultracool stars are much colder, planets that can support liquid water are closer to their stars. This closeness, combined with the low mass of ultracool stars, creates a detectable signal indicating the presence of a planet.

Mahadevan likens this to being near a campfire. “The cooler the fire, the closer you need to be to stay warm,” he says. “It’s similar for planets. If the star is colder, a planet needs to be closer to have liquid water.

And if a planet orbits close enough to its ultracool star, we can detect it by observing subtle changes in the star’s light color as the planet orbits around it.”

The HPF, located at the Hobby-Eberly Telescope in Texas, provides high-precision measurements of such signals from nearby stars.

“Discovering this planet with HPF is particularly exciting,” says Guðmundur Stefánsson, a NASA Sagan Fellow in Astrophysics at Princeton University and the study’s lead author.

“We built HPF from scratch to explore unknown types of planets around the lowest mass stars, and now we’re learning new and unexpected things about these planets.”

Megan Delamer, a graduate student at Penn State and co-author of the study, points out that finding a planet like LHS 3154b is extremely rare based on current surveys. “Our theories of planet formation are struggling to explain this observation,” she says.

The discovery suggests that the heavy core of the planet LHS 3154b would have required a larger amount of solid material in its formation than current models predict.

It also implies that the dust-mass and dust-to-gas ratio around stars like LHS 3154, when they were young, would need to be ten times higher than observed to form such a massive planet.

“What we’ve found is a real challenge for existing theories of planet formation,” Mahadevan concludes. “Discoveries like this are exactly why we built the HPF, to find out how the most common stars in our galaxy form planets.”