CARMENES: Giant exoplanet around a small star challenges our understanding of how planets form

This 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.

The signal of the planet is 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.

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 result will be published in the forthcoming issue of the journal Science.

Astronomers from the CARMENES consortium, led by Juan Carlos Morales, a researcher from the Institute of Space Studies of Catalonia (IEEC) at the Institute of Space Sciences (ICE, CSIC) have discovered one, possibly even two, gas giant planets orbiting the nearby red dwarf star GJ 3512.
 
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,” declares Ignasi Ribas, CARMENES project scientist and Director of IEEC. “We then realised 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”.
 
The 140 observations clearly reveal a motion of the star caused by a massive companion in both the optical and infrared arms of the CARMENES spectrograph. The infrared arm of CARMENES was the core contribution of the Spanish institutes to the consortium and it was built at CSIC's Instituto de Astrofísica de Andalucía. The instrument is in operation since 2016 at the 3.5 meters telescope in the Calar Alto Observatory in Almeria (Spain).
 
“As their name indicates, red dwarfs emit most of their light in the red and near infrared parts of the spectrum. CARMENES was designed to make optimal use of all the light wavelengths where the stars are brighter,” explains Ansgar Reiners, from the Institute for Astrophysics Göttingen, in Germany. “Despite the fact that optical high-resolution and stabilised spectrographs have existed for a while, for example HARPS, near-infrared ones represent a new technology.”
 
With this discovery, CARMENES achieves the first detection of an exoplanet solely using a new-generation near-infrared precision instrument, highlighting again the leading role played by European researchers in the field of exoplanets. An earlier detection of an exoplanet using an infrared spectrometer required the use of several other facilities to confirm it [2].
 
After a few initial observations, this target caught the attention of scientists and triggered further monitoring. “The star was showing a rather strange behaviour 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 Juan Carlos Morales.
 
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,” explains Dr. Morales.
 
“IEEC’s 80-cm diameter Joan Oro Telescope at the Montsec Observatory played a significant role in the discovery, allowing the astronomers to derive the rotational period of the system at 87 days, 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”, declared Enrique Herrero, IEEC researcher.

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 our Solar System, and many other gas giant planets discovered around other stars.
 
The “core-accretion” model assumes that planets form in two phases: at first, rocky cores, the size of a few Earth masses, form a nucleus within the protoplanetary disk and then, when a critical mass is reached, they start to accumulate and retain large amounts of gas until they reach the mass of Jupiter or more.
 
Low-mass stars should have proportionately low-mass disks, so the amount of available material in the disk to form planets is also significantly reduced. The presence of a gas giant around a low-mass star indicates that either the original disk was anomalously massive [3] or that the core-accretion scenario does not apply in this case. Moreover, 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, all of them world leaders in planet formation theory, 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,'' explains Alexander Mustill, a Senior Research Fellow at Lund Observatory.
 
But there is a possible alternative planet formation scenario that could save the day. 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. Although 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.
 
“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,'' declares Juan Carlos Morales.
 
The CARMENES consortium keeps monitoring the star in order 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 will tell if it turns out to be a small scale Solar System.
 
Notes
 
[1] Technically speaking, the first solid exoplanet detection reported using a high resolution infrared spectrometer was done with CSHELL at IRTF, and corresponds to a massive object at the limit between a planet and a brown dwarf (~13 Mjup) in orbit around CI Tau. CARMENES is a new-generation instrument specifically built for exoplanet searches. Since CARMENES started operations, a number of similar instruments came on-line at top world observatories, such as the Subaru Telescope and the Canada-France-Hawaii Telescope, both in Mauna Kea, Hawaii.
 
[2] The existence of such anomalously massive disks is currently not confirmed by observations of star forming regions.

Observatories and Instruments

The CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs) instrument is a high-resolution optical and near infrared spectrograph built in collaboration by various Spanish and German research institutions, and it is operated by the Calar Alto observatory (Spain).

Links
– CARMENES
– Calar Alto Observatory
– IEEC
– Montsec Observatory

More information

This research is presented in a paper entitled "A giant exoplanet orbiting a very-low-mass star challenges planet formation models", by J. C. Morales et al., to appear in the journal Science on 27 September 2019.

The Institute of Space Studies of Catalonia (IEEC  — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of over 20 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centers, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors. 

IEEC is a private non-profit foundation, governed by a Board of Trustees composed of Generalitat de Catalunya and four other institutions that each have a research unit, which together constitute the core of IEEC R&D activity: the University of Barcelona (UB) with the research unit ICCUB — Institute of Cosmos Sciences; the Autonomous University of Barcelona (UAB) with the research unit CERES — Center of Space Studies and Research; the Polytechnic University of Catalonia (UPC) with the research unit CTE — Research Group in Space Sciences and Technologies; the Spanish Research Council (CSIC) with the research unit ICE — Institute of Space Sciences. IEEC is integrated in the CERCA network (Centres de Recerca de Catalunya).

Images

PR_Image_1: Infographic of GJ3512 orbit comparison
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 we thought that small stars such as Proxima, TRAPPIST-1, Teegardern’s star and GJ 3512, could not form Jupiter mass planets.
Credit: Guillem Anglada-Escude – IEEC, using SpaceEngine.org.
Distribution license: Creative Commons Attribution 4.0 International (CC BY 4.0).

PR_Image_2: CARMENES spectrograph
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.
Credit: Pedro Amado/Marco Azzaro – IAA/CSIC.
Distribution license: Creative Commons Attribution 4.0 International (CC BY 4.0).

PR_Image_4: the 3.5-m telescope at the Calar Alto Observatory
Dome of the 3.5-m telescope at the Calar Alto Observatory where the CARMENES spectrograph is installed.
Credit: Pedro Amado/Marco Azzaro – IAA/CSIC.
Distribution license : Creative Commons Attribution 4.0 International (CC BY 4.0).

PR_Image_5: the 3.5-m telescope at the Calar Alto Observatory
3.5-m telescope at the Calar Alto observatory where the CARMENES spectrograph is installed.
Credit: Pedro Amado/Marco Azzaro – IAA/CSIC.
Distribution license: Creative Commons Attribution 4.0 International (CC BY 4.0).

PR_Image_6: artistic impression of GJ 3512b
Artistic impression of the gas giant planet GJ 3512b orbiting its red dwarf host star.
Credit: Guillem Anglada-Escude – IEEC/Science-wave, using SpaceEngine.org.
Distribution license: Creative Commons Attribution 4.0 International (CC BY 4.0).

Videos

PR_Video_1: Artistic representation of the orbits of the planets around GJ 3512
Artistic representation of the orbits of the planets around GJ 3512. The giant planet discussed in the paper is represented in the inner orbit, while the outer one corresponds to the long period signal also found in the data but for which confirmation is still needed.
Credit: Guillem Anglada-Escude – IEEC/Science Wave, using SpaceEngine.org.
Distribution license: Creative Commons Attribution 4.0 International (CC BY 4.0).

PR_Video_2: Planet formation simulation
Simulation using current main-stream models for planet formation around a Sun-like star and a small star like GJ 3512. The current main-stream model (pebble accretion) can form large planets like Jupiter and Saturn around Sun-like stars (in black, 00:23), but it fails to produce planets like GJ 3512 b around small red-dwarfs (red dots, never climb up to become large gas giants, see end of simulation).
Credit: Anders Johanson- Lund Observatory.
Distribution license: Creative Commons Attribution 4.0 International (CC BY 4.0).