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Protecting Planets from Their Stars

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Protecting Planets from Their Stars VIDOTTO: EXOPLANS ET AND THE ST ELLAR WIND he continuous flow of matter that escapes out of the solar gravitational well is Tknown as the solar wind. As the material flows out of the Sun, it is accelerated to between 400 and 800 km s–1. Through this continuous flow of material, the Sun loses more than one million tonnes every second. But this is just a tiny fraction, corresponding to 2 × 10 –14 of a solar mass that is lost every year. However, the solar wind does not only involve particles, it also carries the solar magnetic field lines. The magnetized solar wind permeates the entire solar system, having an effect on any body encountered on its way. The meeting of the solar wind and a solar system planet can result in complex interactions that depend on the Downloaded from characteristics of both the local solar wind and the planet. Factors such as whether the planet is magnetized or whether it has an atmosphere can play different roles in this interaction. Planets that are weakly magnetized or not http://astrogeo.oxfordjournals.org/ magnetized, such as Mars, can have their atmospheres exposed to the impact of the solar wind. The solar wind may then remove the atmosphere of the planet by sputtering pro- cesses. Evidence suggests that Mars once had a thicker atmosphere, but most of it may have disappeared due to interaction with the young Sun’s wind, believed to have been more intense in the distant past. In the case of a magnetized planet such as the at St Andrews University Library on September 23, 2014 Earth, the planet’s field acts as a large obstacle for the solar wind: the flow of particles pro- duced by the Sun cannot penetrate all the way to the surface of our planet, but ends up being 1: Artist’s view of the interaction between the solar wind and the Earth’s protective deflected around the Earth’s magnetic field lines magnetic field. (SOHO [ESA & NASA]) (see figure 1). Because of the speed of the flow, the impact of the solar wind in the magneto- sphere of the Earth produces a bow shock that surrounds the dayside magnetosphere of our planet (the side towards the Sun). Protecting planets The formation of bow shocks is not the only signature of the interaction between the mag- netized solar wind and a magnetized planet. Mediated by magnetic reconnection events, from their stars energetic electrons are released in the system. Some of these electrons spiral along planetary magnetic field lines, giving rise to cyclotron Aline Vidotto explores how planets interact with the stellar wind and emission at radio wavelengths via a process how this evidence might help us find and characterize exoplanets. called cyclotron maser instability. Because this kind of inter action is regulated by magnetic hosting star and its surrounding planetary sys- wind–obstacle interaction (Zarka 2007). This reconnections, it can only exist between mag- tem. In fact, planetary interactions could even tight linear correlation is also known as the netized bodies. be more intense, as is certainly the case for stars radiometric Bode’s law. Because the pressure of that harbour more powerful stellar winds, or the solar wind is larger at smaller distances from Exoplanetary interactions with wind that have planets orbiting at close proximity the Sun, this correlation led scientists to propose Here, three (related) processes resulting from (<0.05 au, also called close-in planets). that, if a planet were orbiting at close distance the interaction between the solar wind and a In the case of planetary radio emission, to a star identical to the Sun with the same type planet were outlined, namely: the formation although the specific details of the inter actions of stellar wind, this planet could produce radio of a bow shock, atmospheric erosion (in non- that generate these emissions are yet to be elu- emissions 103–105 times more intense than Jupi- or weakly magnetized planets) and auroral cidated, it has been recognized that the amount ter (Zarka 2007, Jardine and Cameron 2008). radio emission (in magnetized planets). There of energy released at radio wavelengths by the The detection of such intense auroral radio is no reason to doubt that similar interactions giant planets in the solar system correlates signatures from exoplanets would be a direct occur between the stellar wind of an exoplanet- tightly to the energy dissipated in the solar planet-detection method, as opposed to the A&G • February 2013 • Vol. 54 1.25 VIDOTTO: EXOPLANS ET AND THE ST ELLAR WIND VIDOTTO: EXOPLANS ET AND THE ST ELLAR WIND (a) (b) Downloaded from (c) (d) http://astrogeo.oxfordjournals.org/ at St Andrews University Library on September 23, 2014 2: The lowest energy state of the coronal magnetic field oft Boo as seen at different observing epochs. Colours denote the surface radial magnetic field and the solid line represents the neutral line (when the stellar magnetic field changes polarity). widely used indirect methods of radial veloc- etary magnetic field. Therefore, planets with migration (Lovelace et al. 2008). In the solar ity measurements or transit events. Moreover, magnetic field strengths of a few G, for exam- system, this process is negligible, but could the detection of exoplanetary radio emission ple, would emit at a frequency that could not be important in particular circumstances of would also demonstrate that the planet has a be observed from the ground either due to the stars that harbour strong magnetic fields and magnetic field. Earth’s ionospheric cut-off, or because it does dense winds (Vidotto et al. 2009, 2010b) or for However, despite many attempts, exoplan- not correspond to the operating frequencies of synchronizing stellar rotation with the orbital etary radio emission has not yet been detected. available instruments. In that regard, the low- motion of planets during the pre-main-sequence One of the reasons for the lack of success is operating frequency of LOFAR (currently under phase (Lanza 2010). thought to be the beamed nature of the elec- commission), jointly with its high sensitivity at Independently of the process involved, it is tron–cyclotron maser instability. Because the this low-frequency range, makes it an instru- worth noting that, in order to study the inter- emission occurs over a small solid angle, it ment that has great potential to detect radio action of the planets with the local environment would have to be directed towards the Earth emission from exoplanets. in which they are immersed, a key step is to to be detected. Poor instrumental sensitivity Note that different properties of star–planet understand the magnetic coronae and winds of would also explain the lack of detection of radio systems can also give rise to physical inter- the host stars. emission from exoplanets. Another reason for actions that are absent or negligible in the solar the failure to find exoplanets this way may be system. For instance, it has been suggested that Stellar magnetic fields because of a frequency mismatch: the emission the winds of young Sun-like stars could change Although we seem to comprehend reasonably process is thought to occur at cyclotron frequen- the orbital angular momentum of planets by well the properties of the solar wind (especially cies, which depend on the intensity of the plan- the action of dragging forces, causing planetary because we are immersed in it), it is much more 1.26 A&G • February 2013 • Vol. 54 VIDOTTO: EXOPLANS ET AND THE ST ELLAR WIND 3: Final configuration one order of magnitude smaller (about 2 years of the steady-state as opposed to 22 years for the solar magnetic magnetic field of cycle). The polarity reversals in t Boo seem to t Boo for June 2006. occur roughly every year, switching from a Colours denote the negative poloidal field near the visible pole in stellar wind velocity June 2006 (the intensity of the surface field is on the equatorial plane (xy plane). colour-coded in figure 2a) to a positive poloidal Stellar rotation axis field in June 2007 (figure 2b), and then back is along positive z. again to a negative polarity in July 2008 (fig- ure 2d; Catala et al. 2007, Donati et al. 2008c, Fares et al. 2009). The nature of such a short magnetic cycle in t Boo remains an open question. Surface differ- ential rotation is thought to play an important role in the solar cycle. The fact that t Boo pre- sents a much higher level of surface differential Downloaded from rotation than that of the Sun may be responsible for its short observed cycle. In addition, t Boo hosts a close-in planet that, due to its proximity to the star, may have been able to synchronize, through tidal interactions, the rotation of the http://astrogeo.oxfordjournals.org/ shallow convective envelope of the host F-type difficult to probe and identify the properties of 2006a, 2008a; Morin et al. 2008, 2010) and star with the planetary orbital motion. This the winds of other stars. Even if we concentrate high-mass stars (Donati et al. 2006b). Donati presumed synchronization may have enhanced on a subsample of stars with similar masses to and Landstreet (2009) present a recent overview the shear at the tachocline, which may have our Sun, differences in coronal temperatures, of the survey. influenced the magnetic cycle of the star (Fares stellar rotation rates, magnetic field intensities, Although some objects host fields that can et al. 2009). etc, imply different stellar wind properties. resemble the large-scale solar field, there are Because the stellar winds of cool stars are Because of that, theoretical and numerical mod- also fascinating differences.
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