It contradicts previous assumptions about planet formation: astronomers have discovered a surprisingly massive planet in close orbit around a very small star. This exoplanet has a mass of at least 13 times that 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 disk surrounding the star from which this planet emerged would be much greater than previously observed in low-mass stars.
The rule usually applies in stellar systems: the larger the parent star, the more massive its planetary children. Because both stars and planets form from a common source of raw materials: After a star forms through the accumulation of gas and dust, its circumstellar disk eventually becomes the reservoir for the formation of its planets. The mass of matter has a typical relationship to the mass of a star. Therefore, planets with very small stars usually have relatively low masses. Previous observational data from protoplanetary disks on so-called ultracool dwarf stars also fits this. Against this background, the current discovery made by an international team of astronomers seems very surprising.
On the trail of small stellar planets
Astronomers led by Jomundur Stefansson of Princeton University specialize in searching for planets orbiting ultracool dwarf stars: to do this, they used the so-called “habitable zone planet finder” – an astronomical spectrograph mounted on the Hobby-Eberly telescope at the McDonald Observatory in Texas. Scientists may target small, life-friendly planets in orbit around dwarf stars. To make liquid water possible, they would have to orbit close to their stars so they would receive enough weak radiation.
This small distance, combined with the low mass of ultracool stars, results in a signal that a planet detector in the habitable zone can detect: the planet’s gravity affects the star’s motion, which is reflected in subtle changes in its light spectrum. Measuring the so-called radial velocity also makes it possible to draw conclusions about the orbital period and the mass of the planet. In the current case, the team of scientists was focusing on an ultra-cool dwarf star called LHS 3154. Its mass is about nine times less than that of our Sun.
As shown by data from the Habitable Zone Planet Finder, the planet actually orbits in just 3.7 days. But it is not a small rocky planet: calculation results show that its mass is at least 13 times the mass of the Earth. Thus, LHS 3154b is almost as heavy as our planet Neptune. This ratio of star mass to planet mass is twice anything previously known. “We did not expect to discover such a massive planet around a low-mass star,” says co-author Suvrath Mahadevan of Pennsylvania State University in University Park.
According to the authors, it is difficult to explain the existence of this giant planet based on current assumptions about the planet’s composition. This also appears from model simulations of possible formation mechanisms in the LHS 3154 system: thus, the amount of matter from which LHS 3154b emerged must be at least ten times greater than previously found in low-mass protoplanetary disks. stars.
“So, according to expectations, there actually wasn’t enough solid mass to form this planet,” Mahadevan says. “But it is 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 know about the universe.”
Source: Pennsylvania State University, specialized article: Science, doi: 10.1126/science.abo0233
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