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Planet found too big for its parent star

The discovery of a planet far too large for its sun defies models about the formation of solar systems and planets. In a paper in Science, researchers, including Yamila Miguel of Leiden Observatory, report the discovery of a planet more than 13 times heavier than Earth orbiting the ultracool dwarf star LHS 3154, which is nine times less massive than the Sun. The mass ratio of the newly found planet and its parent star is more than 100 times that of Earth and the Sun.

This is the heaviest planet orbiting an ultracool dwarf star ever found. Dwarf stars are the lightest and coldest stars in the universe. The discovery goes against what current theories predict about planet formation around small stars, and it is the first time a planet with such a large mass around a star with such a low mass has been observed. Stars are formed from large clouds of gas and dust. After the star is formed, the gas and dust remain as disks of material that orbit the newborn star and can eventually grow into planets.

Updating understanding of how planets and stars are formed

"We could never have imagined that the planet-forming disk around the bright star LHS 3154 has enough solid mass to make this planet," says Yamila Miguel. "But it's there, so now we have to update our understanding of how planets and stars are formed."

Instrument must detect planets on which possible life is possible

The researchers discovered the oversized planet (LHS 3154b) using a spectrograph built at Penn State (USA). The instrument, the Habitable Zone Planet Finder (HPF), is designed to detect planets orbiting the coolest stars outside our solar system that may have liquid water on their surface, a key ingredient for the emergence of life. The HPF is mounted on the Hobby-Eberly Telescope at the McDonald Observatory in Texas, USA.

Although such planets around stars like our sun are very difficult to find, they are easier to find around ultracold stars. If the star is colder, then a planet must be closer to that star to get enough heat for liquid water to occur. If the planet is close enough to its ultracool star, it can be found by a very subtle change in the color of the star's spectrum or light.

Larger amount of solid material in heavy core than current predictions

The heavy core of the newly found planet orbiting the star LHS 3154 would require a larger amount of solid material in the planet-forming disk than current models predict. The discovery also raises questions about previous understanding of star formation, because the dust-mass to dust-gas ratio of the disk that surrounded stars such as LHS 3154 when they were young would have had to have been 10 times higher than what has been observed to form such a heavy planet.

"We hope to find more objects like this in the future and eventually learn more about planet and star formation," Miguel concludes.

This article appeared as a press release on the website of de Nederlandse Onderzoekschool voor Astronomie (NOVA)

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