According to Penn State researchers, the discovery of a planet far too huge for its sun calls into doubt what was previously thought about the genesis of planets and solar systems. Researchers describe the finding of a planet more than 13 times the mass of Earth orbiting the “ultracool” star LHS 3154, which is itself nine times the mass of the sun, in a paper published online in the journal Science. The newly discovered planet’s mass ratio to its host star is more than 100 times greater than that of Earth and the sun.
The discovery exposes the largest massive known planet in a tight orbit around an ultracool dwarf star, the universe’s least massive and coldest star. The discovery contradicts current beliefs about planet formation around small stars, and it is the first time a planet with such a large mass has been discovered around such a small star.
“This discovery really drives home the point of just how little we know about the universe,” said Suvrath Mahadevan, the Verne M. Willaman Professor of Astronomy and Astrophysics at Penn State and a co-author on the research. “We wouldn’t expect a planet this heavy around such a low-mass star to exist.”
He explained that 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.
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. Our current theories of planet formation have trouble accounting for what we’re seeing.”
Megan Delamer
“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.”
The huge planet, known as LHS 3154b, was discovered by the researchers using an astronomical spectrograph created at Penn State by a team of scientists led by Mahadevan. The HPF, or Habitable Zone Planet Finder, was designed to find planets orbiting the coldest stars outside our solar system that have the potential for liquid water – a necessary component for life – on their surfaces.
While such planets are extremely difficult to detect in the vicinity of stars like our sun, the low temperature of ultracool stars means that planets with liquid water on their surfaces are far closer to their star than Earth and the sun. According to Mahadevan, the shorter distance between these planets and their stars, along 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 said. “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.”
Located at the Hobby-Eberly Telescope at the McDonald Observatory in Texas, 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.”
According to Stefánsson, the device has already provided vital information in the finding and confirmation of new planets, but the discovery of LHS 3154b exceeded all expectations.
“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, a Penn State astronomy graduate student and article co-author. “Our current theories of planet formation have trouble accounting for what we’re seeing.”
Delamer noted that in the case of the huge planet discovered circling the star LHS 3154, the hefty planetary core predicted by the team’s findings would necessitate a greater amount of solid material in the planet-forming disk than current models would predict. The discovery also calls into question previous theories about star formation, because 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 10 times higher than what was observed in order to form a planet as massive as the one 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.”
