Giant exoplanet found around tiny star shouldn't even exist, astronomers say
Posted September 26, 2019 2:02 p.m. EDT
CNN — Astronomers have unexpectedly discovered a gas giant Jupiter-esque exoplanet that's orbiting a dwarf star, located about 30 light-years from our sun.
The planet, known as GJ 3512 b, completes one orbit around the star every seven months. That makes this a giant planet with a long-distance orbit around a very small star -- something that shouldn't even exist given the current theories about planet formation, according to a new study.
The planet has a mass about half that of Jupiter. The red dwarf star has a mass about 12% of our sun. For comparison, the sun is 1,050 times heavier than Jupiter, the researcher said.
The current understanding for planet formation follows a specific order. Stars form from clouds of collapsing gas and dust. Then, a protoplanetary disk of leftover gas and dust surrounds the star. Planets form from this disk, using the leftover gas and dust that created the star to pull together solid material for planets. Gravity helps create an atmosphere from the gas.
For gas giants, small particles help form an icy, rocky core. It grows, achieving a mass between 10 and 15 times that of Earth, then accumulates hydrogen and helium gas -- the two most abundant elements in the universe. The abundance of gas creates a gas giant.
Because the star has such a low mass, it shouldn't have had a whole lot of leftover material to form planets -- especially not enough to form a gas giant.
Things get more confusing from here because there's evidence of a second planet, also in a long orbit around the star. And the elliptical orbit of the first planet suggests that a potential third planet, similar in mass, gravitationally influenced it. The third planet was likely kicked out of this system.
So in addition to this one giant planet, there must have been enough material in the star's disk to create three planets.
How is this possible? That's what scientists wanted to find out. Their study was published Thursday in the journal Science.
Multiple approaches to researching this phenomenon led them to one conclusion: gravitational disk collapse. This means that the planet formed in a gravitationally unstable disk of gas and dust around the dwarf star when it was young. And it formed without a solid core, which contradicts other models of planet formation.
At a certain point in distance from the star, the disk contains incredibly cold temperatures where the material just collapses under its own weight, forming a planet.
Then, the young planet likely migrated away from the star because of gravitational interactions with other planets in the system.
"Until now, the only planets whose formation was compatible with disk instabilities were a handful of young, hot and very massive planets far away from their host stars," said Hubert Klahr, study author at the Max Planck Institute for Astronomy. "With GJ 3512 b, we now have an extraordinary candidate for a planet that could have emerged from the instability of a disk around a star with very little mass. This find prompts us to review our models."
Astronomers detected the planet using the radial velocity method with the CARMENES spectrograph. The radial velocity method is based on gravity and the Doppler effect, in which light increases or decreases in frequency as a source and observed object move toward or away from each other.
Stars don't remain completely still when they are orbited by planets; they move in small circles as a response to the pull of gravity from the planets. These movements change the light wavelength of the star, going between red and blue depending on the location of the planet. Tracing the shifts can help astronomers find planets.
"Red dwarf stars like GJ 3512 show very active behavior and generate signals similar to those of planets," explains Diana Kossakowski (MPIA), who was instrumental in the evaluation and the analysis of the data. "The infrared spectra were then important to confirm that what we found is indeed a planet."