Amazing giant planet of a dwarf star discovered

This comparison with the Earth and the Sun illustrates the astonishing mass ratio of the newly discovered planet to its parent star. © Penn State University

It contradicts previous assumptions about planet formation: astronomers have discovered a surprisingly massive planet in a close orbit around a very small star. The exoplanet has at least 13 times the mass of Earth and orbits a cool dwarf star nine times smaller than the Sun in just 3.7 days. According to model simulations, the amount of matter in the circumstellar disk from which this planet emerged would have been much larger than has previously been observed in low-mass stars.

The rule normally applies in star systems: the larger the stellar mother, the more massive its planetary children are. Because both the star and the planets form from a common supply of starting material: After the star is formed through the accumulation of gas and dust, its circumstellar disk ultimately becomes the reservoir for the formation of its planets. The mass of the material is in a typical relationship to that of the star. Therefore, the planets of very small stars usually have relatively low masses. Previous observation data from protoplanetary disks around the so-called ultra-cool dwarf stars also fit this. Against this background, the current discovery by an international team of astronomers seems so surprising.

On the trail of tiny stellar planets

The astronomers led by Guðmundur Stefánsson from Princeton University specialize in searching for planets around ultra-cool dwarf stars: To do this, they used the so-called “Habitable Zone Planet Finder” – an astronomical spectrograph installed on the Hobby-Eberly Telescope at the McDonald Observatory in Texas . Scientists may be targeting life-friendly small planets in orbit around the dwarf stars. To make liquid water possible, they have to orbit very close to their stars so that they get enough of the weak radiation.

This small distance, combined with the low mass of the ultra-cool stars, leads to a signal that the Habitable Zone Planet Finder can detect: the planet's gravitational pull influences the star's movement, which is reflected in subtle changes in its light spectrum. Measuring the so-called radial velocity also enables conclusions to be drawn about the orbital period and the mass of the planet. In the current case, the team of scientists was focusing on the ultra-cool dwarf star called LHS 3154. It has about nine times less mass than our sun.

A serious surprise

As shown by data from the Habitable Zone Planet Finder, a planet actually orbits it in just 3.7 days. But it is not a small rocky planet: the calculation results for its mass showed it to be at least 13 times that of Earth. The planet LHS 3154b is therefore almost as heavy as our Neptune. This ratio of star to planetary mass is twice as high as anything previously known. “We did not expect to discover such a massive planet around such a low-mass star,” says co-author Suvrath Mahadevan from Pennsylvania State University in University Park.

According to the authors, it is difficult to explain the existence of this planetary giant based on current assumptions about planet formation. This also emerges from their model simulations of the possible formation mechanisms in the LHS 3154 system: The amount of material from which LHS 3154b emerged would therefore have to have been at least ten times larger than has previously been found in protoplanetary disks of low-mass stars.

“So according to expectations, there wouldn't actually have been enough solid mass to form this planet,” says Mahadevan. “But it's out there - so we may now need to re-examine our understanding of how planets and stars form. Once again it becomes clear how little we actually know about the universe,” the scientist concluded.

Source: Pennsylvania State University, specialist article: Science, doi: 10.1126/science.abo0233

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