Giant exoplanet found around red dwarf star challenges the 'core accretion model' of planet formation

The planet probably formed from an unstable disc around the young star, which broke up into clumps; This contrasts with how the majority of massive planets are believed to form, where a planet grows slowly as gas falls onto a solid core


                            Giant exoplanet found around red dwarf star challenges the 'core accretion model' of planet formation

Astronomers have discovered a giant Jupiter-like exoplanet in an unlikely location – orbiting a small red dwarf star. The discovery has sparked excitement among astronomers as such a large planet around such a small star challenges current theories of planet formation, says the research team.

The newly identified gas giant, named GJ 5312b, is nearly half as massive as Jupiter. It is very large given the small host star - which is little more than a tenth of the mass of the sun. The gas giant exoplanet GJ 5312b exhibited an eccentric 204-day orbit around the star, discovered the team. The astronomers have also found evidence suggesting the presence of another candidate planet in the system - which means that there could possibly be one more gas giant planet orbiting the nearby red dwarf star GJ 3512.

The planetary system around the nearby red dwarf GJ 3512, is located at approximately 30 light-years from us. The discovery was made by scientists from CARMENES consortium, led by Dr. Juan Carlos Morales, a researcher from the Institute of Space Studies of Catalonia (IEEC) at the Institute of Space Sciences (ICE, CSIC).

According to the astronomers, the planet probably formed from an unstable disc around the young star, which broke up into clumps. This contrasts with how the majority of massive planets are believed to form - wherein a planet grows slowly as gas falls onto a solid core.

"This exoplanet discovered is very surprising. CARMENES is a spectroscopic survey looking for exoplanets around M-dwarf stars. These are stars with a mass much lower than the sun. The statistics of exoplanets found till now seem to indicate that low-mass stars typically host small planets like Earth or mini-Neptunes. The most accepted model of planet formation, core accretion model, also points towards this direction. But here, we demonstrate the contrary, that is, we have found a gas giant planet, orbiting a very low-mass star. For comparison, while the Sun-to-Jupiter mass ratio is about 1050, the mass ratio between GJ 3512 and its planet is only around 250," Dr. Morales told MEA WorldWide (MEAWW).

M-type red dwarfs are among the smallest and coolest stars, but by far the most common type of star in the Milky Way. However, despite their ubiquity, only about 10% of the nearly 4,000 exoplanets discovered to date orbit these low-mass stars. 

According to Dr. Morales, the critical point of this discovery is that, for the first time, "we have accurately characterized an exoplanet that cannot be explained by the core accretion formation model." In other words, this exoplanet proves that the “gravitational instability model” may play a role in the formation of giant planets. The findings have been published in the journal Science. 

"I find it fascinating how a single anomalous observation has the potential to produce a paradigm shift in our thinking, in something as essential as the formation of planets and, therefore, in the big picture of how our own Solar System came into existence," says Dr. Morales.

Comparison of GJ 3512 to the solar system and other nearby red-dwarf planetary systems. Planets around a solar-mass stars can grow until they start accreting gas and become giant planets such as Jupiter, in a few millions of years. But astronomers thought that small stars such as Proxima, TRAPPIST-1, Teegardern’s star and GJ 3512, could not form Jupiter mass planets. (Guillem Anglada-Escude - IEEC, SpaceEngine.org)

Planet formation models should be able to explain how planetary systems come into existence around stars like our sun, but also around smaller stars. Until now, the so-called "core accretion model" for planet formation was considered sufficient to explain Jupiter and Saturn in the solar system, and many other gas giant planets discovered around other stars.

Morales further explains the model used to explain the formation of exoplanets - core (or peeble) accretion model - considers that the dust of the disk piles up forming rocky cores. When these rocky cores reach masses about 10 times those of Earth, they start slowly accreting gas from the disk growing in mass and size until the disk is dissipated. 

In the case of solar-like stars, Morales told MEAWW, the disk is massive enough to allow giant planets to be formed. But for low-mass stars, disks are less massive and the planets do not have time to grow to the size and mass of gas giants. Hence, the presence of a gas giant around a low-mass star indicates that either the original disk was anomalously massive or that the core-accretion scenario does not apply in this case. 

"This model cannot explain GJ 3512b because the star is very small, and so, its protoplanetary disk might also be less massive. Alternatively, we consider the model of gravitational instability, in which the planet is directly formed by the collapse of an unstable region of the disk. This process is much faster, and more massive planets (like a gas giant as Jupiter) can be formed," Morales told MEAWW.

To discover the planets, the astronomers used the Doppler technique, which monitors the back-and-forth motion of a star when it is orbited by one or more planets. The star, however, almost did not make it into the list of observational targets.

"CARMENES was built to find planets around the smallest stars, but we also wanted them as bright as possible. Initially, this star was not included in our observation list because it was too faint. We then realized we didn't have enough small stars in the sample and we added a few, at the very last minute. We were lucky to do so because otherwise, we would have never made this discovery," says Ignasi Ribas, CARMENES project scientist and Director of IEEC. 

After a few initial observations, the star target caught the attention of scientists and triggered further monitoring. "The star was showing a rather strange behavior very early on. Its velocity was changing very rapidly, and consistently in both wavelength channels of the instrument, indicating the presence of a massive companion, an anomalous feature for a red dwarf," explains Morales.

The signal of the planet was clearly detected with both the visible and infrared arms of the CARMENES spectrograph at the Calar Alto Observatory. This makes it the first exoplanet unambiguously discovered by a new-generation infrared high-resolution spectrometer.

Domes of different telescopes at the Calar Alto Observatory. CARMENES is installed on the 3.5-m telescope inside the dome at the right of the image.
(Pedro Amado/Marco Azzaro - IAA/CSIC)

The research team says that GJ 3512 is almost identical to Proxima Centauri and only a bit more massive than Teegarden's star and TRAPPIST-1, which all host terrestrial planets in temperate orbits, but no gas giants. 

"It is becoming the norm to expect small planets around these small stars, so we initially thought this large motion had to be caused by another star in a very long orbital period. We kept observing it, but on low priority. To our surprise, the motion started to repeat again in the next season, indicating that it was actually produced by a planet. At that point, GJ 3512 finally made it to the top priority list," says Dr. Morales.

For this discovery, the CARMENES consortium used, among others, IEEC’s 80-cm diameter Joan Oró Telescope (TJO) at the Montsec Observatory and the facilities at Observatorio de Sierra Nevada (IAA, CSIC).

The research team says the Joan Oro Telescope played a significant role in the discovery, allowing the astronomers to derive the "rotational period of the system at 87 days," which is an important step to confirm that the signal is a planet and not stellar activity, as well as to estimate the age of the system. 

The 3.5-m telescope at the Calar Alto observatory where the CARMENES spectrograph is installed. (Pedro Amado/Marco Azzaro - IAA/CSIC)

According to the researchers, this planet is on an eccentric orbit, “which is the smoking gun of a past event” indicating the presence of another massive planet that was ejected from the system in a chaotic interaction with the current planet, “adding a wandering planet in the galactic void.” 

Researchers from IEEC, the Max Planck Institute for Astronomy (MPIA) and other CARMENES institutes established a collaboration with the planet formation groups at Lund Observatory in Sweden and Bern University in Switzerland, to study plausible formation scenarios for this system.

“After running multiple simulations and long discussions among the different groups to try to explain the system, we concluded that our most up-to-date models could never allow the formation of even one massive planet, let alone two,'' says Alexander Mustill, a senior Research Fellow at Lund Observatory.

But there is a possible alternative planet formation scenario that could save the day, says the team. “The “disk-instability model” advocates that some or maybe all gas giant planets can directly form from the gravitational self-accumulation of gas and dust instead of requiring a “seed” core. While this scenario is plausible, it has been mostly ignored so far because it fails to explain other trends observed for the population of gas giant planets. This new CARMENES discovery is bound to change this,” the findings state.

CARMENES near-infrared arm being mounted in the instrument room at the Calar Alto Observatory. The spectrograph detector must be inside a cryogenic vacuum chamber with stabilized pressure and temperature conditions.
(Pedro Amado/Marco Azzaro - IAA/CSIC)

The CARMENES consortium will continue to monitor the star to confirm the existence of a second, possibly a Neptune-like object, in a longer orbital period. Besides, the scientists have not discarded the presence of "temperate terrestrial planets" orbiting GJ 3512. More data, says the team, will tell if it turns out to be a small scale Solar System.

In a related perspective, Greg Laughlin from Yale University in New Haven, writes: "A freshly discovered exoplanet is, by itself, no longer particularly note­worthy. But one that challenges current theories of planet forma­tion can animate astronomers.”

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