Exoplanets
The characterization of exoplanets is
key to our understanding of planets, including those in the Solar
System. POLLUX unique, simultaneous, high-resolution and polarimetric
capabilities in the UV are essential to unveiling the origins of the
huge range of chemical and physical properties found in exoplanetary
atmospheres (e.g., Sing et al. 2016), and to understand the
interaction between planets and their host stars (e.g., Cuntz et al.
2000).
The characteristics of exoplanetary atmospheres
The line and continuum polarization state of starlight that is reflected by a planet depends on the star-planet observer phase angle and is sensitive to the optical properties
of the planetary atmosphere and surface (Fig. 1, left). Atmospheric gases are efficient scatterers at UV wavelengths making UV polarization a unique tool revealing the presence,
coverage, particle size, and composition of aerosols (Rossi & Stam 2018; García Muñoz 2018). Furthermore, irregular temporal variations in polarization could indicate changing
cloud patterns (i.e., weather), constraining heat transportation and distribution (e.g., García Muñoz 2015), while regular temporal variations could reveal planetary rings,
trojans, and/or moons (Berzosa Molina et al. 2018). Because the polarized and unpolarized intensities originate at different altitudes in the exoplanet atmosphere, POLLUX
observations will simultaneously probe two altitude ranges, thus being critical for testing models of exoplanet atmospheres. POLLUX can detect and spectrally resolve UV
polarization signatures for close-in gas giants and brown dwarfs lying several tens of pc away from us, particularly if they orbit stars hotter than the Sun (Fig. 1, right).
Such a sample currently comprises about twenty targets, but several more will be found by Gaia, TESS, and PLATO. POLLUX will thus enable studying single planets in detail and
gaining insights into the large diversity of planetary atmospheres using a sample large enough to be statistically valid.
The polarimetric capabilities of POLLUX will allow us to constrain the composition, optical thickness, and particle size of dust clouds resulting from disintegrating rocky planets, which could be the remnant cores of larger, gaseous, planets. The high spectral resolution of POLLUX, together with the large aperture of LUVOIR that allows to keep exposures short, further enables us to avoid blurring arising from the large orbital velocities of these objects. Kepler and K2 have already discovered such planets (e.g., Rappaport et al. 2012). The recent Dispersed Matter Planet Project already finds systems likely to host analogues and progenitors within 100 pc (Haswell et al. 2019), and TESS and PLATO will undoubtedly find more. These dust clouds provide the unique opportunity to analyze the composition of planetary cores, which is impossible even for Solar System bodies. The high spectral resolution of POLLUX enables using cross-correlation techniques to detect and measure abundances of various key molecules in the atmospheres of nearby low-mass planets. This technique, applied to polarimetric high-resolution spectra, further enables the measurement of cloud coverages (García Muñoz 2018). These observations are not possible from the ground because the terrestrial atmosphere blocks the UV radiation. High-resolution spectropolarimetry is superior to broadband polarimetry to disentangle the planet and stellar signals through the corresponding planet-star Doppler shift. Key molecules presenting significant UV bands and possibly revealing Earth-like habitats are O2, O3, SO2, CH2O, and NO2 (e.g., Schwieterman et al. 2018; Lammer et al. 2018). These molecules provide contextual information on a planet's habitability through their relations with the planet atmospheric composition, energy budget, volcanic activity, and the presence of hydrocarbons and lightning. Similar observations could be done for (young) planets in wide orbits around their stars, yielding for example planetary spin velocities, essential for understanding accretion and hence informing about planet formation, or UV auroral emission (e.g., for Proxima Cen, Barnard's Star b, and a handful of nearby brown dwarfs such as Luhman 16AB), revealing planetary magnetic fields and upper atmospheric composition (e.g. Ribas et al. 2018).
Tidal and magnetic star-planet interactions
Cuntz et al. (2000) theorized that star-planet interactions (SPI), either of gravitational or magnetic
origin, could generate detectable signatures in exoplanetary systems. The repeated expansion
and contraction of stellar tidal bulges produced by a close-in planet can lead to an increased
level of stellar activity (Cuntz et al. 2000), and hence also planetary mass loss (Lanza 2013).
Indeed, enhanced stellar activity has been seen for WASP-43 (Staab et al 2017), a system with
parameters suggesting particularly vigorous tidal interactions. A fraction of the particles released
in the magnetic star-planet reconnection funnels along the stellar magnetic field lines down to
their foot-points on the star. The resulting condensations of material lost from the planet are
expected to produce non-stationary narrow absorbing features across the profiles of stellar UV
emission lines formed in the chromosphere and transition region, such as Mg II h & k, C IV, Si III
and N V (e.g. Lanza 2009, 2014; Fossati et al. 2015a).
Thanks to its high spectral resolution, POLLUX can detect the absorption signatures of the condensations of planetary-lost material for numerous nearby systems already known to host close-in giant planets. This will give the opportunity to observationally constrain the fate of the material that has escaped from the planet and to uniquely characterize the topology of the magnetic fields arising from SPI. Rocky planets orbiting late M dwarfs (Anglada-Escudé et al. 2016; Gillon et al. 2017; Ribas et al. 2018) are possibly subject to significant internal tidal and/or induction heating (e.g., Driscoll & Barnes 2015; Kislyakova et al. 2017, 2018; Barr et al. 2018). These heating mechanisms can be powerful enough to melt planetary mantles leading to strong volcanic activity or even magma oceans. The ejected volcanic material may then form a torus along the planetary orbit, similar to the plasma torus of the Jovian satellite Io. The high spectral resolution of POLLUX enables to reveal these structures through the detection of narrow absorption lines at the position of a variety of stellar emission features (e.g., Kislyakova et al. 2018, their Fig. 6). The observations would uniquely yield knowledge on atmospheres and interior composition of rocky planets orbiting M dwarfs.
The characteristics of exoplanetary atmospheres
The line and continuum polarization state of starlight that is reflected by a planet depends on the star-planet observer phase angle and is sensitive to the optical properties
of the planetary atmosphere and surface (Fig. 1, left). Atmospheric gases are efficient scatterers at UV wavelengths making UV polarization a unique tool revealing the presence,
coverage, particle size, and composition of aerosols (Rossi & Stam 2018; García Muñoz 2018). Furthermore, irregular temporal variations in polarization could indicate changing
cloud patterns (i.e., weather), constraining heat transportation and distribution (e.g., García Muñoz 2015), while regular temporal variations could reveal planetary rings,
trojans, and/or moons (Berzosa Molina et al. 2018). Because the polarized and unpolarized intensities originate at different altitudes in the exoplanet atmosphere, POLLUX
observations will simultaneously probe two altitude ranges, thus being critical for testing models of exoplanet atmospheres. POLLUX can detect and spectrally resolve UV
polarization signatures for close-in gas giants and brown dwarfs lying several tens of pc away from us, particularly if they orbit stars hotter than the Sun (Fig. 1, right).
Such a sample currently comprises about twenty targets, but several more will be found by Gaia, TESS, and PLATO. POLLUX will thus enable studying single planets in detail and
gaining insights into the large diversity of planetary atmospheres using a sample large enough to be statistically valid.The polarimetric capabilities of POLLUX will allow us to constrain the composition, optical thickness, and particle size of dust clouds resulting from disintegrating rocky planets, which could be the remnant cores of larger, gaseous, planets. The high spectral resolution of POLLUX, together with the large aperture of LUVOIR that allows to keep exposures short, further enables us to avoid blurring arising from the large orbital velocities of these objects. Kepler and K2 have already discovered such planets (e.g., Rappaport et al. 2012). The recent Dispersed Matter Planet Project already finds systems likely to host analogues and progenitors within 100 pc (Haswell et al. 2019), and TESS and PLATO will undoubtedly find more. These dust clouds provide the unique opportunity to analyze the composition of planetary cores, which is impossible even for Solar System bodies. The high spectral resolution of POLLUX enables using cross-correlation techniques to detect and measure abundances of various key molecules in the atmospheres of nearby low-mass planets. This technique, applied to polarimetric high-resolution spectra, further enables the measurement of cloud coverages (García Muñoz 2018). These observations are not possible from the ground because the terrestrial atmosphere blocks the UV radiation. High-resolution spectropolarimetry is superior to broadband polarimetry to disentangle the planet and stellar signals through the corresponding planet-star Doppler shift. Key molecules presenting significant UV bands and possibly revealing Earth-like habitats are O2, O3, SO2, CH2O, and NO2 (e.g., Schwieterman et al. 2018; Lammer et al. 2018). These molecules provide contextual information on a planet's habitability through their relations with the planet atmospheric composition, energy budget, volcanic activity, and the presence of hydrocarbons and lightning. Similar observations could be done for (young) planets in wide orbits around their stars, yielding for example planetary spin velocities, essential for understanding accretion and hence informing about planet formation, or UV auroral emission (e.g., for Proxima Cen, Barnard's Star b, and a handful of nearby brown dwarfs such as Luhman 16AB), revealing planetary magnetic fields and upper atmospheric composition (e.g. Ribas et al. 2018).
 |
Figure 1: (1) & (2): schematic of a planetary system in which the unpolarized stellar light becomes polarized through reflection by a planetary atmosphere. Left (3): degree of polarization (in %) as a function of planetary orbital phase, labelled as in panel (1), at 300 nm (from Loic Rossi). The different lines indicate different atmospheric cloud coverages, where zero corresponds to the cloudless condition. |
Tidal and magnetic star-planet interactions
Cuntz et al. (2000) theorized that star-planet interactions (SPI), either of gravitational or magnetic
origin, could generate detectable signatures in exoplanetary systems. The repeated expansion
and contraction of stellar tidal bulges produced by a close-in planet can lead to an increased
level of stellar activity (Cuntz et al. 2000), and hence also planetary mass loss (Lanza 2013).
Indeed, enhanced stellar activity has been seen for WASP-43 (Staab et al 2017), a system with
parameters suggesting particularly vigorous tidal interactions. A fraction of the particles released
in the magnetic star-planet reconnection funnels along the stellar magnetic field lines down to
their foot-points on the star. The resulting condensations of material lost from the planet are
expected to produce non-stationary narrow absorbing features across the profiles of stellar UV
emission lines formed in the chromosphere and transition region, such as Mg II h & k, C IV, Si III
and N V (e.g. Lanza 2009, 2014; Fossati et al. 2015a).Thanks to its high spectral resolution, POLLUX can detect the absorption signatures of the condensations of planetary-lost material for numerous nearby systems already known to host close-in giant planets. This will give the opportunity to observationally constrain the fate of the material that has escaped from the planet and to uniquely characterize the topology of the magnetic fields arising from SPI. Rocky planets orbiting late M dwarfs (Anglada-Escudé et al. 2016; Gillon et al. 2017; Ribas et al. 2018) are possibly subject to significant internal tidal and/or induction heating (e.g., Driscoll & Barnes 2015; Kislyakova et al. 2017, 2018; Barr et al. 2018). These heating mechanisms can be powerful enough to melt planetary mantles leading to strong volcanic activity or even magma oceans. The ejected volcanic material may then form a torus along the planetary orbit, similar to the plasma torus of the Jovian satellite Io. The high spectral resolution of POLLUX enables to reveal these structures through the detection of narrow absorption lines at the position of a variety of stellar emission features (e.g., Kislyakova et al. 2018, their Fig. 6). The observations would uniquely yield knowledge on atmospheres and interior composition of rocky planets orbiting M dwarfs.
Figure 2: Average S/N between 2500 and 4000 Å obtained with 3 hours of POLLUX shutter time as a function of the distance of the target star and size of the telescope primary mirror. Results are for a G2V (Sun-like; solid line), F5V (dashed line), and A0V (dash-dotted line) star. The red and blue dotted horizontal lines indicate the maximum UV polarization signal of respectively a Jupiter-radius and Neptune-radius planet in a two-day orbit around the host star. The black dotted vertical lines indicate the size of the primary mirror for LUVOIR's architectures A and B. |