Stellar activity can mimic misaligned exoplanets
2016 September 20

Illustration of the Rossiter-McLaughlin effect. Due to the rotation of the star, the light from its left side is blushifted and the light from its right side is redshifted. When a planet starts to transit the star, it blocks part of the blueshifted light, and when the transit is coming to an end, it blocks part of the redshifted light. Credit: Ricardo Cardoso Reis(IA/UPorto).(Left) Artist's impression of a photometric (top) and spectroscopic (bottom) observation of a transiting exoplanet, with a trajectory that takes it on a path to occult a stellar spot. (Right) Simulated light curve (top) and of radial velocity variation (bottom) of a planetary transit (red), and anomaly curves due to the occultaion of, respectively, a stellar spot (blue) and a plage (green). Credit: Artist's impression: Ricardo Cardoso Reis (IA/UPorto) Plots: Oshaugh et al. 2016

The occultation of stellar active regions during the planetary transit1 can lead to inaccurate estimates of the characteristics of these exoplanets, especially the spin-orbit tilt angle. This was the conclusion of simulations made by a team2 of astronomers from the Instituto de Astrofísica e Ciências do Espaço (IA2) in Portugal, and Institute of Astrophysics of Georg-August University of Göttingen, in Germany.

The Rossiter-Mclaughlin(RM) effect has been used to measure the spin-orbit tilt angle in exoplanets, a parameter which can provide crucial information about the planetary formation and evolution processes, and even help to discriminate between different planetary migration models.

Mahmoudreza Oshagh, currently working at the Institute of Astrophysics of Georg-August-University of Göttingen, but developed the work in this paper5 as a post-doc at IA, commented: “Our results showed that the aligned transiting exoplanets are the ones that can be easily misinterpreted as misaligned owing to the stellar activity. Moreover, our study could provide a viable explanation for the few cases in the literature that obtained conflicting spin-orbit angles, for instance the case of exoplanet WASP-19b”.

This study shows that the inaccurate estimation on the spin-orbit angle can reach up to 30 degrees, particularly for the edge-on, aligned, and small transiting planets. The simulations also show that observations in the near infrared are less affected by stellar activity. Mahmoudreza Oshagh mentioned: "These results demonstrate that measurement of the RM effect should be part of the main objective of upcoming near Infrared spectrographs, such as SPIRou, NIRPS instruments with which IA researchers are strongly involved." 

Nuno Cardoso Santos (IA e Universidade do Porto) commented: “These study is very important for the analysis of high precision data coming for a whole new generation of instruments, including the
high resolution spectrograph

IA’s team is right now at the VLT (Paranal observatory, Chile), installing a part if this instrument.

High resolution images at:


1.     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 be in line with the observers’ line of sight. 

2.     The team is: M. Oshagh (Institut für Astrophysik, Georg-August-University of Göttingen  / Instituto de Astrofísica e Ciências do Espaço), S. Dreizler (Institut für Astrophysik, Georg-August-Universität), N. C. Santos (Instituto de Astrofísica e Ciências do Espaço & Dep. de Física e Astronomia da Faculdade de Ciências, Universidade do Porto), P. Figueira (Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto) and A. Reiners (Institut für Astrophysik, Georg-August-Universität).

3.     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). IA’s activity is funded by national and international funds, including Fundação para a Ciência e a Tecnologia (UID/FIS/04434/2013), POPH/FSE and FEDER through COMPETE 2020.

4.     The Rossiter-McLaughlin effect is a radial velocity anomaly occurring while the target planet is transiting its parent star. Because of the stellar rotation, one side of the projected disk of the star is moving towards the observer, and has a blue Doppler shift, whereas the other is moving away, and is redshifted. Because of this, when a planet crosses the stellar disk on the plane of the sky, at each individual moment it blocks light with a different averaged velocity.

5.     The article “Can stellar activity make a planet seem misaligned?” was published in the journal Astronomy & Astrophysics (Vol. 593, A25 August 2016, DOI: 10.1051/0004-6361/201628728).

6.     ESPRESSO (Echelle SPectrogaph for Rocky Exoplanet and Stable Spectroscopic Observations) will be a high resolution spectrograph, to be installed at the VLT telescope (ESO). Its main objective is the detection of Earth-like planets, able to sustain life. To do that, it will be able to detect velocity variations of about 0,3 km/h. Another of its goals will be stability tests of the fundamental constants of the Universe. The consortium responsible for the development and construction of ESPRESSO consists of academic and scientific institutions from Portugal, Italy, Switzerland and Spain, was well as members of the European Southern Observatory (ESO).  The Portuguese partners are IA (which leads the national participation), and the Faculty of Sciences of the University of Lisbon.


Science Communication Group:

Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa Fundação para a Ciência e a Tecnologia