By Will Dunham
WASHINGTON (Reuters) – The most common type of star in our Milky Way galaxy is the red dwarf, which is significantly smaller and less luminous than our sun. It was previously believed that these stars were unable to host planets much larger than Earth.
However, the discovery of a planet at least 13 times the mass of Earth orbiting very close to a red dwarf that is only 11% of the sun’s mass has led astronomers to reevaluate the prevailing theories on planetary formation around these stars. The mass ratio of this planet with its star is more than 100 times greater than that of Earth and the sun.
“We discovered a planet that is too massive for its star,” said Penn State astronomer Suvrath Mahadevan, one of the leaders of the study published this week in the journal Science.
The star, known as LHS 3154, is relatively close to Earth, located about 50 light years away. For context, a light year is the distance light travels in a year, equivalent to 5.9 trillion miles (9.5 trillion km).
“It is barely a star,” said Princeton University astronomer Guðmundur Stefánsson, the study’s lead author. “It has a mass just above the cutoff of supporting hydrogen fusion to be considered a star.”
The planet, LHS 3154 b, orbits at about 2.3% of Earth’s orbital distance from the sun, circling its star every 3.7 days. It is much closer to its star than Mercury is to the sun in our solar system.
The planet may be similar in size and composition to Neptune, the smallest of our solar system’s gas planets. Neptune’s diameter is about four times that of Earth. Although the researchers were unable to measure the diameter of LHS 3154 b, they suspect it is about three to four times the size of Earth.
Due to its probable Neptune-like composition and its proximity to its star, it is unlikely that the planet could support life, according to Stefánsson.
Stars form when dense clouds of interstellar gas and dust collapse under their own gravitational pull. Eventually, a star is born at the center of such a cloud, with leftover material forming a swirling disk around it that often gives rise to planets.
So why were red dwarfs previously thought to be unable to host a planet the size of the newly discovered one?
“The planet-forming disk around stars is only a small fraction of the stellar mass and is expected to scale with that mass. So a very low mass star should have a low mass disk, which should not be heavy enough to birth the planet we discovered,” Mahadevan said.
“This planet raises questions of how planets form around the lowest mass stars, because such stars were previously thought to primarily only be able to form small terrestrial planets similar in mass to Earth,” Stefánsson said.
The researchers detected LHS 3154 b by observing a subtle wobble in the host star caused by the planet’s gravitational effects during its orbit. They used an instrument called the Habitable Zone Planet Finder (HPF), built by a team led by Mahadevan, on the Hobby-Eberly Telescope at the University of Texas’ McDonald Observatory.
“As we build new instruments, and as our measurement precision increases, we see the universe in new, unexpected ways,” Mahadevan said. “We built HPF to detect terrestrial planets around these cool stars. This discovery is another in the constant stream of surprises showing how much we still have to learn about planets and planet formation.”
(Reporting by Will Dunham, Editing by Rosalba O’Brien)
