This artist’s view shows the hot Jupiter exoplanet 51 Pegasi b, orbits a star about 50 light-years from Earth in the constellation of Pegasus (The Winged Horse). This was the first exoplanet around a normal star to be found in 1995. Twenty years later this object was also the first exoplanet to be be directly detected spectroscopically in visible light.Selected population of planet candidates detected by the Kepler Space Telescope. Their transit depth is displayed here as a function of their orbital period. The size of the marker is relative to the magnitude of the host.Artist's conception of a brown dwarf. Brown dwarfs are more massive and hotter than giant gas planets but lack the mass required to become sizzling stars. Their atmospheres can be similar to the giant planet Jupiter's.

An international team¹ led by Alexandre Santerne from Instituto de Astrofísica e Ciências do Espaço (IA²), made a 5-year radial velocity³ campaign of Kepler’s giant exoplanet candidates, using the SOPHIE⁴ spectrograph (Observatory of Haute-Provence, France), and found that 52,3% were actually eclipsing binaries⁵, while 2,3% were brown dwarfs6.

Santerne (IA & University of Porto), first author of this paper⁷ commented: “It was thought that the reliability of the Kepler exoplanets detection was very good – between 10 and 20% of them were not planets. Our extensive spectroscopic survey, of the largest exoplanets discovered by Kepler, shows that this percentage is much higher, even above 50%. This has strong implications in our understanding of the exoplanet population in the Kepler field”.

One of the team members, Vardan Adibekyan (IA & University of Porto) added: "Detecting and characterizing planets is usually a very subtle and difficult task. In this work, we showed that even big, easy to detect planets are also difficult to deal with. In particular, it was shown that less than half of the detected big transiting planet candidates are actually there. The rest are false positives, due to different kind of astrophysical sources of light or noise."

Giant transiting exoplanets are easily mimicked by false positives, so spectroscopic follow-up observations are needed to establish the planetary nature of the transit detections, and easily reveal blended multiple stellar systems.

Susana Barros (IA & University of Porto), another EXOEarths team member said: “Kepler found a large number of transiting planets down to the size of the Earth. However radial velocity follow-up of the candidates, which is one of the expertise’s of IA’s Origin and Evolution of Stars and Planets group, is crucial to understanding those planetary systems.”

The research, which ran between July 2010 and July 2015, started with all 8826 objects on the list of Kepler objects of interest (KOI). The sample number was progressively reduced to 129 KOIs on 125 target stars, by removing already known false positives, stars too faint to be observed by SOPHIE, and candidates with orbits of more than 400 days, to insure that at least 3 transits could be observed.

Santerne also thought that: “After 20 years of exploring planets as big as Jupiter around other suns, we still have a lot of questions left open. For instance, we don't understand what is the physical mechanism that forms Jupiter-like planets with orbital periods as little as a few days. It is like if our annual rotation around the Sun would last only a few days - imagine your age! We also don't understand why some of these giant planets are so puffy”.

The radius of giant gas planets depends on its atmosphere and interior giant zone, with the irradiation from the star heating its atmosphere, inflating it like a hot air balloon. But the inflation of some giant, highly irradiated planets could not be modelled with reasonable physical processes.

This spectroscopic survey provided mass constrains, which combined with the radius measured by Kepler transits⁸, allowed the calculation of the bulk density of these giant exoplanets. The team also found a hint of connection between the density of these planets and the metallicity of the host stars, but this needs more confirmation.

This research also found that moderately irradiated giant planets are not inflated. Detailed characterization of the internal structure of these planets should shed new light on planet formation and evolution theories.

The results were announced today at the Extreme Solar Systems III conference in Hawaii, celebrating the 20 years of discovery of the first exoplanet around a Sun-like star.

1. The team is: A. Santerne, C. Moutou, M. Tsantaki, F. Bouchy, G. Hébrard, V. Adibekyan, J.-M. Almenara, L. Amard, S. C. C. Barros, I. Boisse, A. S. Bonomo, G. Bruno, B. Courcol, M. Deleuil, O. Demangeon, R. F. Díaz, T. Guillot, M. Havel, G. Montagnier, A. S. Rajpurohit, J. Rey and N. C. Santos.
2. The Instituto de Astrofísica e Ciências do Espaço (Institute of Astrophysics and Space Sciences – IA) is the largest portuguese research unit of space sciences, encompassing most of the field’s national scientific output. It was evaluated as Excellent in the last evaluation from the European Science Foundation (ESF).
3. The Radial Velocity method detects exoplanets measuring tiny variations in the (radial) velocity of the star, due to the motion that an orbiting planet induces in the star. As an example, the speed variation that the Earth imprints in the Sun is of about 10 cm/s (about 0.36 km/h). With this method you can set a minimum value for the planets’ mass.
4. SOPHIE (Spectrographe pour l’Observation des Phénomènes des Intérieurs stellaires et des Exoplanètes, or spectrograph for the observation of stellar interior phenomena and exoplanets) is a high resolution spectrograph, with precision to measure radial velocities of about 2 meters per second. It’s installed in the 1.93 meter telescope of the Observatory of Haute-Provence (France), the same location where, in 1995, Michel Mayor and Didier Queloz detected the first exoplanet orbiting a Sun-like star.
5. An Eclipsing Binary is a binary star system aligned with the observers’ line of sight, which causes the larger star to eclipses the smaller, and the smaller to transit the largers’ disk. This transit can sometimes be mistaken with the transit of a giant exoplanet.
6. A Brown Dwarf, sometimes referred as a “failed star” is a sub-stellar object, without enough mass to fuse hydrogen in its core. They occupy the gap between gas giant planets, and M dwarf (also known as red dwarf) stars. However, the threshold between gas giants and brown dwarfs is still highly debated.
7. The article “SOPHIE velocimetry of Kepler transit candidates XVII. The physical properties of giant exoplanets within 400 days of period” (arXiv:1511.00643) was accepted for publication in Astronomy & Astrophysics.
8. The Transits method measures the dimming of starlight produced when an exoplanet crosses in front of its star (something similar to a “micro-eclipse”). A transit enables for the determination of the planets’ radius only. It’s also a difficult method to use, because it requires that both planet and star be exactly aligned with the observers line of sight.

• Alexandre Santerne
• Susana Barros
• Science Communication Office
  Ricardo Cardoso Reis
  Filipe Pires (Coordination, Porto)
  João Retrê (Coordination, Lisboa)