Planet much bigger than its star surprises astronomers – 12/05/2023 – Science

The most common type of star in the Milky Way is called a red dwarf, much smaller and less luminous than our Sun. These stars — or so it was thought — simply aren’t big enough to support planets much larger than Earth.

The discovery, however, of a planet with at least 13 times the mass of Earth orbiting very closely a red dwarf with just 11% the mass of the Sun has sent astronomers back to square one in the theory of planet formation involving this type of star. . The mass ratio between this planet and its star is more than a hundred times greater than that of the Earth and the Sun.

“We discovered a planet that is too massive for its star,” said astronomer Suvrath Mahadevan, from Pennsylvania State University, one of the leaders of the study published last Thursday (30) in Science.

The star, called LHS 3154, is relatively close to us, about 50 light years from Earth. A light year is the distance that light travels in one year, 9.5 trillion km.

The Sun is about a thousand times more luminous than this star.

“It’s a close call,” said the study’s lead author, Guðmundur Stefánsson, an astronomer at Princeton University. “It has a mass just above the limit to support hydrogen fusion and be considered a star.”

The planet, called 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 than our solar system’s innermost planet, Mercury, is to the Sun.

The planet may resemble Neptune in size and composition, the smallest of the four gas planets in our Solar System. Neptune’s diameter is about four times that of Earth. The method used to study the planet did not allow researchers to measure its diameter, but they suspect it is about 3 to 4 times that of Earth.

Neptune, which does not have a solid surface, has a dynamic atmosphere mainly of hydrogen and helium over a mantle composed mainly of ammonia and mushy water and a solid core. Based on its likely Neptune-like composition and its proximity to its star, it is unlikely to support life, Stefánsson said.

Stars form when dense clumps of interstellar gas and dust collapse under their own gravitational pull. When a star is born in the center of such a cloud, the remaining material forms a rotating disk around it that fuels stellar growth and often gives rise to planets.

So why wouldn’t a red dwarf be able to host a planet the size of the one just described?

“The planet-forming disk around stars is only a small fraction of the stellar mass and is expected to be on scale with that mass. Therefore, a very low-mass star must have a low-mass disk. Such a disk does not it must be heavy enough to give rise to the planet we discovered,” said Mahadevan.

“This planet raises questions about how planets form around lower-mass stars, because it was previously thought that these stars could only form small terrestrial planets similar in mass to Earth,” Stefánsson said.

Researchers discovered LHS 3154 b by detecting a slight 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, at the Hobby-Eberly telescope at the University of Texas McDonald Observatory.

The instrument was designed to find planets that orbit relatively cool stars and have the potential for liquid water on their surfaces, a key factor for life.

“As we build new instruments and increase our measurement precision, we see the universe in new and unexpected ways,” said Mahadevan. “We built the HPF to detect terrestrial planets around these cool stars. This discovery is yet another of the constant surprises that show how much we still have to learn about planets and planetary formation.”