The Mysterious Planet. Giant Exoplanet orbits a Tiny Star

Astronomers have discovered a giant planet around half the size of Jupiter orbiting a tiny star — a configuration that current theories say shouldn’t exist.

Artist’s impression of a Jupiter-like planet with a blueish colour orbiting a cool red dwarf. © CARMENES/RenderArea/J. Bollaín/C. Gallego

A team of Spanish and German astronomers have discovered a giant planet orbiting the tiny red dwarf star GJ 3512 — located 30 light-years from Earth with less than a tenth of the mass of the Sun. The finding has been examined by researchers at the University of Bern, Switzerland, with the aim of discovering how this exoplanet formed — in the process, defying current theoretical models.

Christoph Mordasini is one of the world’s leading experts in the theory of planet formation, a professor at the University of Bern and member of the National Centre of Competence in Research (NCCR) PlanetS. With regards to the mysterious planet — named GJ 3512b — he says: “Around such stars, there should only be planets the size of the Earth or somewhat more massive Super-Earths.

“GJ 3512b, however, is a giant planet with a mass about half as big as Jupiter’s, and thus at least one order of magnitude more massive than the planets predicted by theoretical models for such small stars.”

Like Jupiter, GJ 3512b is a gas-giant — not a super-Earth or a Neptune mass exoplanet that current theories of planet formation predict. The presence of this planet was discovered by observing the star’s orbit and radial velocity — revealing an orbiting planet of 0.46 Jupiter masses in a highly eccentric — flattened — orbit.

Mordasini continues: “Our model of the formation and evolution of planets predicts that around small stars a large number of small planets will be formed.”

Mordasini refers to another well-known planetary system as an example — Trappist-1. This star — which is comparable to GJ 3512 in size — possesses seven planets with masses ranging from approximately equal to Earth to less than the mass of our planet.

In the case of Trappist 1, however, the calculations of the Bern model agree well with the observation — this certainly isn’t the case with the red dwarf

GJ 3512.

“Our model predicts that there should be no giant planets around such stars.”

The mysterious planet was first spotted by CARMENES, a Spanish and German collaboration with the mission of finding exoplanets orbiting smaller stars. To carry out this mission, CARMENES use an infrared spectrograph — a new instrument housed at the Calar Alto Observatory at 2100 m altitude in southern Spain.

The observations made by CARMENES revealed that the red dwarf moved towards and away from Earth at regular intervals — a motion caused by a massive companion.

The exoplanet GJ 3512b was discovered by the CARMENES consortium with an instrument installed at the Calar Alto Observatory in southern Spain. © Pedro Amado/Marco Azzaro — IAA/CSIC

The consortium was so taken aback by their observation that they immediately contacted several research institutes — including the Bern research group in which Mordasini works — to discuss possible scenarios which could explain the formation of a giant exoplanet around such a minuscule star.

A paper detailing CARMENES observations and the University of Bern team’s findings has been published in the journal Science.

Two options present them themselves when considering how this tiny red dwarf star came to be orbited by a massive planet. And both explanations are almost opposite in nature.

The first explanation for such a phenomenon suggests that the mechanism behind the formation of the exoplanet is core accretion. This is when planets are formed by the gradual growth of a small body of material by the acculation of matter.

As the core grows its gravitational influence increases also increasing its ability to collect material. This continues until it is a planetary embryo massive enough to begin accreting an envelope of gas. Astronomers refer to this as a bottom-up process.

That’s creation, but what about a theory of destruction?

Another explanation for the presence of such a massive exoplanet is the mechanism of gravitational collapse — a top-down process. Mordasini explains: “A part of the gas disk in which the planets are formed collapses directly under its own gravitational force.”

This latter explanation isn’t without problems though, as the researcher points out: “Why hasn’t the planet continued to grow and migrate closer to the star in this case?

“You would expect both if the gas disk had enough mass to become unstable under its gravity.”

All this leaves little doubt that if planet formation is to be thoroughly understood, the giant planet GJ 3512 b and its star GJ 3512 are crucial subjects for future study.

Mordasini concurs, concluding: “ The planet GJ 3512b is, therefore, an important discovery that should improve our understanding of how planets form around such stars.”