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In the vicinity of other worlds

How to observe the moons, the rings and even the magnetism of planets outside the solar system

DrümOur vision of exoplanets, i.e., the planets that orbit stars other than the Sun, is still very vague. Instead of wonderful photographs, for the time being we must settle for deductions concerning their radius and mass and the characteristics of their orbits, worked out indirectly using the two most common methods of detection: the radial velocity technique, whereby one measures how the influence of the planet’s gravity causes its star to oscillate, and the planetary transit method, that records a drop in star brilliance when the planet crosses in front of it. It was thanks to the planetary transit method, for example, that some 2 thousand possible exoplanets have been identified using Nasa’s Kepler space telescope. One of these discoveries, confirmed by observations made with other telescopes, is the planet Kepler 22b, whose radius is only 2.4 times greater than that of Earth. It orbits the habitable zone of a star that is very similar to the Sun. In other words, its distance from the star would allow the existence of liquid water on it. Nobody knows, however, whether Kepler 22b is a huge rocky planet, a super-Earth or a mini-Neptune – a miniature version of the Solar System’s gaseous giants.

Nevertheless, our image of exoplanets should become far richer in future years thanks to the work of the theoretical astrophysicists that are putting forth new ways to observe planetary transit indications of the other properties of such worlds. The astrophysicist Adriana Valio, from Mackenzie Presbyterian University in São Paulo, and her PhD student Luis Ricardo Tusnski, from Inpe (the National Institute of Space Research) were the first to determine what the minimum size of the moons and rings around the extrasolar planets should be to be viable to detect them with the Kepler telescope and with the European Space Agency’s Corot space telescope, which also resorts to planetary transit and has Brazilian researchers on its staff. As for the team coordinated by the Brazilian astrophysicist Aline Vidotto, from the University of St. Andrews in Scotland, it discovered that planetary transit can be used, under certain circumstances, to measure the magnetic field of an exoplanet.

The cutting-edge work of these Brazilians furthers in one way or another the advance of the quest for an exoplanet capable of supporting life such as we know it. Although most of the more than 700 extra-solar planets whose discovery has been confirmed are gaseous giants, as large as or larger than Jupiter, those in the habitable zone of their stars might have sufficiently large rocky moons to retain an atmosphere for billions of years and thus to be home to oceans full of life. “If Kepler 22b were to have a moon the size of Mars, for instance, it would be habitable,” says Adriana. “Another important factor that makes a planet habitable is its magnetic field,” explains Aline. “The field works like a protective shield, keeping the high-energy particles coming from the star from wearing out the atmosphere.”

024-027_Capa_Exoplanetas_191-2Hidden moons
Since 2003, Adriana has been working on a computer model to study how stellar spots – a phenomenon akin to the spots that appear on the surface of the Sun – interfere with the light curve of planetary transit. In 2009, Tusnski, who was then one of her master’s students, decided to adapt the model to simulate the transit of a planet with one moon. Other researchers had previously proposed the idea of identifying moons by means of the disturbance they cause in the planet’s movements, but actually observing this might require monitoring the fluctuation of the brilliance of the star for longer than telescopes usually monitor them. The model of the Brazilians showed that this was unnecessary. If a moon is big enough, an unmistakable signal of its presence should appear on the light curve of planetary transit, in the form of small “steps.”

However, the light curves obtained using Kepler and Corot are not smooth like those in the models, because the brilliance of stars is not constant, fluctuating erratically, among other reasons because of the appearance and disappearance of stellar spots. “The issue is even more complicated because there is some noise in the tool, which leads to uncertainties in the measurements,” explains Tusnski. The “steps” indicating the presence of moons would therefore have to be identified in the midst of the noise generated by this variation. Nonetheless, in an article published in December in Astrophysical Journal, Tusnski and Adriana showed via simulations of these fluctuations that it should be possible to distinguish, in the Corot data, moons 1.3 times larger than the Earth, whereas in the Kepler data there might be evidence of satellites as small as our Moon. Tusnski has already started looking for these indications in the data. “The application of this tool might lead to the discovery of the first natural satellite among exoplanets,” states Othon Winter, a specialist in planetary dynamics from Unesp. “One of the main advantages of this work is how easy it is to improve the model (that is used already) to cover stellar spots and more moons.”

Even though the largest solar system moon, which is Jupiter’s Ganymede, is slightly smaller than half of the Earth, Winter, along with Rita Domingos and Tadashi Yokoyama, both from Unesp as well, calculated, according to an article published in 2006 in the journal Monthly Notices of the Royal Astronomical Society (MNRAS), that exoplanets similar to Jupiter orbiting the habitable zone of stars with the magnitude of the Sun might have satellites the size of Earth or larger. “ There is a rising expectation that detection of the moon will be achieved in the near future, because of the huge volume of data that is awaiting analysis,” says the astronomer Darren Williams, from Pennsylvania State University, who recently also showed that giant gaseous exoplanets might have large moons. “I suspect that most of the planets detected with Kepler have moons and that some of them are larger than Mars.”

Adriana and Tusnski were also the first researchers to determine how the presence of rings around the exoplanets might affect the light curve of planetary transit. Their model indicated that the effect of the rings would be to smooth out the edges of the “well” of the light curve, besides making it deeper. By conducting an analysis similar to the one about moons, they showed that a system of rings such as those of Saturn could be detected by Kepler, whereas the rings would only be visible using Corot if they were at least 50% greater than the Saturn ones.

The next steps for the researchers will be to adjust their model to identify the signal of the rings of exoplanets that are extremely close to their respective stars. In this case, the gravitational pull of the star might twist the rings. According to Tusnski, it should be possible to use this deformation to obtain information about the density of the nuclei of the extoplanets.

Collision arcs
It would also be possible to learn more about the interior of exoplanets if astronomers were able to detect their magnetic fields. Researchers have been looking for signs of such fields using radiotelescopes. The idea is to capture the radio waves emitted by electrically charged particles released by stars, when they are captured by the magnetic fields of planets – it is the same phenomenon that produces the aurora borealis on Earth. However, all searches so far have failed.

Since 2010, Aline and her colleagues Moira Jardine, Christiane Helling, Joe Llama and Kenneth Wood, all of them from the University of Saint Andrews, have published a series of four articles in the journals Astrophysical Journal Letters, MNRAS and MNRAS Letters, detailing a new method that is more indirect but promising for measuring the magnetic fields of exoplanets. In fact, the team states that they have managed to estimate the strength of the magnetic field of the exoplanet Wasp 12b, discovered in 2008 with the Super Wasp telescope, located on La Palma, in the Spanish archipelago of the Canary Islands.

024-027_Capa_Exoplanetas_191-1Almost twice the size of Jupiter, Wasp 12b orbits its star at a distance 16 times smaller than the distance between the Sun and Mercury, going completely around its star every 26 hours, at the astonishing speed of approximately 300 kilometers per second. Observations of planetary transit using the Hubble telescope showed that the light curve of the star starts to drop in the ultraviolet light wavelength before it does so in the visible light wavelength. Aline and her team believe that this effect is caused by the formation of a “collision arc” in front of the planet that arises because it is moving at a speed greater than that of sound within an environment permeated by particles emitted by the star, the so-called stellar wind.

According to the model of the researchers, the stellar wind particles are colliding with the magnetic field of Wasp 12b, forming in front of the latter an arc-shaped region that is transparent to visible light, but opaque when to ultraviolet light. By measuring the difference at the start of transit in the two wave lengths, the team was able to estimate the distance between the planet and the collision arc, and to infer, from this, the strength of the planet’s magnetic field, which should be lower than 24 Gauss, a figure comparable to the field on Jupiter’s poles, which varies between 10 and 14 Gauss and which is four times greater than on Earth.

To orientate further observations of the phenomenon, the team analyzed a series of exoplanets that have already been discovered using planetary transit, verifying data such as distance from the planets and their respective stars and the strength of the stellar winds. “We listed the exoplanets that should be the best candidates to have an observable collision arc,” said Aline. These include several of those that are closest to Earth, discovered by using the Super Wasp and the Corot telescopes.

“Aline and her colleagues are facing a very difficult astrophysical problem,” comments Evgenya Shkolnik, an expert on magnetic interactions between stars and planets, from the Lowell Observatory in Arizona in the United States. “It would be extremely valuable if we could at least measure the magnetic field of some of the planets that are closest to their stars, the so-called hot Jupiters, in order to distinguish their structural differences.”

The Project
Investigation of high energy and plasma astrophysics phenomena: theory, observation, and numerical simulations (nº 2006/50654-3); Modality Thematic Project; Coordinator Elisabete Maria de Gouveia Dal Pino – IAG/USP; Investment R$ 366,429.60 (FAPESP)