The planet was “too massive for its star,” surprising astronomers

Astronomers surprised to find planet ‘too massive for its star’
Red dwarf stars, which are smaller and less bright than suns, are the most prevalent star type in our Milky Way galaxy. Essentially, it was believed that these stars are too small to support planets significantly bigger than Earth.

Astronomers are reevaluating their theories about how planets form around red dwarf stars, which are common in nature, after discovering a planet with a mass at least 13 times that of Earth orbiting near one that has only 11 percent of the mass of the sun. Compared to Earth and the sun, the mass ratio of this planet and its star is over a hundred times larger.

One of the leaders of the study, Penn State astronomer Suvrath Mahadevan, stated, “We discovered a planet that is too massive for its star,” The findings were published this week in the journal Science.

Situated approximately 50 light years away from Earth, the star known as LHS 3154 is quite near to us. The distance that light travels in a year is 5.9 trillion miles (9.5 trillion km), or one light year.

This star is roughly a thousand times brighter than the sun.
As far as astronomers go, “it is barely a star,” according to lead author and Princeton University astronomer Guðmundur Stefánsson. “It has a mass just above the cutoff of supporting hydrogen fusion to be considered a star.”

The planet, known as LHS 3154 b, completes one orbit of its star every 3.7 days at a distance equal to roughly 2.3% of Earth’s orbit around the sun. It is even closer to the sun than Mercury, the innermost planet in our solar system.
The planet might resemble Neptune, the smallest of the four gas planets in our solar system, in terms of size and makeup. The diameter of Neptune is approximately four times that of Earth. Although the researchers’ technique for examining the planet prevented them from measuring its diameter, they believe it to be roughly three or four times that of Earth.

Neptune is a solid-core planet with a mantle primarily composed of water and slushy ammonia, and a dynamic atmosphere primarily made up of hydrogen and helium. Stefánsson stated that it is unlikely to support life due to its proximity to its star and likely Neptune-like composition.
When dense clusters of interstellar gas and dust collapse due to their own gravitational pull, stars are created. After a star is born at the center of a cloud like this, leftover material swirls around it, feeding the star’s growth and frequently leading to the formation of planets.

Why then wouldn’t a red dwarf be able to support a planet the size of the one that has just been discovered?

“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 disk that is also low mass. Such a disk should not be heavy enough to birth the planet we discovered,” according to Mahadevan.
“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 stated.

The planet’s gravitational pull during its orbit caused a slight wobble in the host star, which led the researchers to identify LHS 3154 b. They made use of an instrument on the Hobby-Eberly Telescope at the University of Texas’ McDonald Observatory called the Habitable Zone Planet Finder (HPF), which was constructed by a team under the direction of Mahadevan.

Its goal was to locate planets with the potential for liquid water on their surfaces—a necessary component for life—and that orbit relatively cool stars.
“As we build new instruments, and as our measurement precision increases, we see the universe in new, unexpected ways,” Mahadevan stated. “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.”

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