By Rim Fares, Osservatorio Astrofisico di Catania, Italy
I am sure that you are already convinced of this statement if you have followed the previous contributions to this blog. But let’s make sure things are clear: isn’t everything relative? Magnetic fields do trick planet hunters, but their study gives us insights into stars and planetary environments, which help us better understand exoplanetary worlds.
So why do we worry about magnetic fields if we want to detect Earth-like planets?
They can mimic a planet’s signature and make our goal of detecting Earth-like planets much tougher.
In cool stars (don’t be mislead by this name; these stars still have surface temperatures up to 7,000 K), magnetic fields manifest on the stellar surface in different ways. They can emerge in the form of dark spots. These regions have strong magnetic fields (up to a few thousand Gauss in the solar case). As Xavier Dumusque explained in his contribution, the effect of these spots can be greater than the effect of an Earth-like planet on the radial velocity variations of a star.
This is not everything; stars have large-scale magnetic fields too. While spots are small areas on the surface relative to the size of the star (but can be the size of a planet!), large-scale fields are distributed on the scale of the star itself. The star can, for example, act as a huge dipole. This dipolar field contributes to the large-scale field.
As Elodie Hebrard and Rakesh Yadav told us in their contribution, stars have magnetic personalities. Extensive studies of large-scale fields show that stars with similar masses and rotation periods seem to have magnetic fields that share similar properties (field strength, configuration). To detect very small planets, we need to understand the contribution of the large-scale field to the radial velocity variations of a star. But to make things complicated, stars can have magnetic cycles. Which means that both small-scale (e.g. spots’ emergence) and large-scale fields vary with time.
Cristina Rodriguez told us in her contribution that stars beat by waves’ propagation inside the stars. These beats can mimic planetary signals. Over a cycle, the magnetic field changes, and so does the music of the star.
But magnetic fields are not as evil as you think. Have you ever dreamt of watching an aurora, wondered about the ice age, the safety of astronauts? Well, there is a magnetic field contribution to all that. They play an important role from the birth of a star to its death (they affect stellar rotation, stellar wind, mass loss of the star, …). They contribute to the interactions between stars and their surrounding planets. Magnetic fields of planets protect them from stellar winds and are probably important for habitability.
While it is a tough job to detect Earth-like planets, it is much easier to detect massive planets that are very close to their stars (because they produce a much stronger effect on the radial velocity of the star, or on the light-curve when they are transiting). These planets are called hot-Jupiters, because like Jupiter they are very massive, but more than 50 times closer than Jupiter is to the Sun, so they are pretty hot. Their discovery was a big surprise to the community, because we do not have such planets in our own solar system (the massive gaseous planets are the furthest from the Sun). It raised questions about how they form, and how they migrate from further distances to very close to the star. At these distances, being massive, these planets interact with their stars through tidal interactions (like the Earth-Moon case, and its effect on the oceans’ tides). The large-scale magnetic field of the star and the stellar wind also interact with these planets (remember the auroras I talked about? They result from the interaction between the solar wind and our atmosphere, and the amazing colours they produce are due to the molecules we have in the upper atmosphere).
These interactions affect the planet but they might also affect the star. In fact, some observations suggest there are spots on the stellar surface that follow the planet on its orbit, instead of rotating with the star. When we observed a sample of stars that have hot-Jupiters, we discovered the first polarity flip of the magnetic field for a star other than the Sun. This star is Tau Bootes, a star visible to the naked eye in the Bootes constellation.
Imagine this star being a huge dipole (Figure 2), the north-south poles flip every year. This discovery was very surprising because the Sun flips polarity every 11 years. Tau Bootes’ flips are very fast compared to the Sun. When we discovered that, we though that it might be due to the tidal interactions with its very massive planet. This star and the planet are synchronised, which means that the star rotates with the same period of the planet on its orbit. This system was the only one observed with such a characteristic, and it was the only one for which a fast magnetic flip is detected. Coincidence or causality? We are still puzzled about that. We did observe polarity flips in other stars more recently, but for now Tau Bootes is the only star observed with regular flips over a long time period.
Back to the planet—imagine it close to the star, and bombarded by the stellar wind. It can interact with this wind and produce signatures at different wavelength (that’s cool because if you observe the same system with different instruments, you can have the whole picture of the physics that is happening up there). It can for example produce radio emission and signatures in the UV that can be due to a bow shock formation, or to the evaporation of the atmosphere.
We know that these hot-Jupiters are not habitable, so why do we care? Because they are easy to observe and characterize, but also because they give us hints to our future studies on habitable planets. Actually, Earth-like planets we are trying to detect in the habitable zone around cool red dwarfs are subject to a more aggressive environment than that of our own Earth. Red dwarfs can have strong magnetic field, and frequent ejections of particles toward the planet. The planet needs to be protected from the stellar wind in order to keep its atmosphere and be habitable. Consider the example of a volcano. Here in Catania where I currently work, we have an amazing view of Mount ETNA, the highest active volcano in Europe. The soil is fertile and farmers are happy. But the volcano is active, it can erupt and lava flows can destroy villages (luckily though, the flows are usually slow). The stellar ejections can have similar effect as volcanoes on earth, the exoplanet needs to get a protection from them: a planetary magnetic field (that will help deviate the stellar particles from hitting the planet). For now, we cannot detect planetary magnetic fields directly, we have to use indirect techniques, for example detecting a bow shock in the ultra-violet. If we study the stellar magnetic field and wind, and detect these bow shock (or radio emissions), we can calculate the magnetic field of the planet. Currently, we are trying to detect these effects on hot-Jupiters. This will shed light on the strength of the planetary magnetic fields.
The coming years will see many space and ground based instruments, and we are all excited about getting new data and making new discoveries. Among these instruments, SPIROU-–a spectropolarimeter for the study of Earth-like planets in the habitable zones—will help us study the magnetic field while having very good radial velocity precision. SPIROU also has another goal: helping to understand planetary formation and migration by observing young stars and the hot-Jupiters that are formed around them. Exciting times ahead!
About the author. Rim Fares is a research fellow at the Osservatorio Astrofisico di Catania, Sicily, since February 2015. Her main research interests are cool stars’ magnetism, the interactions between exoplanets and their cool stars, and exoplanets’ environment. Before moving to Italy, she was a research fellow at the University of St Andrews, Scotland. She has a PhD from Paul Sabatier University, Toulouse, France.