Exoplanet Atmosphere Measurements from Direct Imaging

Abstract

In the last decade, about a dozen giant exoplanets have been directly imaged in the IR as companions to young stars. With photometry and spectroscopy of these planets in hand from new extreme coronagraphic instruments such as SPHERE at VLT and GPI at Gemini, we are beginning to characterize and classify the atmospheres of these objects. Initially, it was assumed that young planets would be similar to field brown dwarfs, more massive objects that nonetheless share similar effective temperatures and compositions. Surprisingly, young planets appear considerably redder than field brown dwarfs, likely a result of their low surface gravities and indicating much different atmospheric structures. Preliminarily, young free-floating planets appear to be as or more variable than field brown dwarfs, due to rotational modulation of inhomogeneous surface features. Eventually, such inhomogeneity will allow the top of atmosphere structure of these objects to be mapped via Doppler imaging on extremely large telescopes. Direct imaging spectroscopy of giant exoplanets now is a prelude for the study of habitable zone planets. Eventual direct imaging spectroscopy of a large sample of habitable zone planets with future telescopes such as LUVOIR will be necessary to identify multiple biosignatures and establish habitability for Earth-mass exoplanets in the habitable zones of nearby stars. arXiv:1807.05136v1 [astro-ph.EP] 13 Jul 2018 1.6 R Super Earth Exoplanet in the Habitable Zone of a G2 Star. AJ150:56, DOI 10.1088/ 0004-6256/150/2/56, 1507.06723 Joergens V, Bonnefoy M, Liu Y et al (2013) OTS 44: Disk and accretion at the planetary border.

Aerosols in Exoplanet Atmospheres

“…Importantly, these observations, along with those of young, low gravity, isolated objects (e.g. Metchev et al, 2015;Gizis et al, 2015;Liu et al, 2016;Faherty et al, 2016;B. A. Biller et al, 2018;Vos et al, 2019Vos et al, , 2020, show that lower gravity objects tend to be more variable and possess redder near-infrared colors (higher J-K) compared to higher gravity objects of the same effective temperature ( Figure 5).…”

“…Near-infrared spectroscopy of directly imaged companions has shown that, like isolated brown dwarfs, aerosols are common in their atmospheres and that the distribution of aerosols appears to be heterogeneous, with some objects exhibiting temporal variability in photometry and spectra (Marois et al, 2008;Currie et al, 2011;Skemer et al, 2012;Marley et al, 2012;Bonnefoy et al, 2013;Skemer et al, 2014;Ingraham et al, 2014;Macintosh et al, 2015;Zhou et al, 2016;Bonnefoy et al, 2016;Rajan et al, 2017;Samland et al, 2017;Delorme et al, 2017;Greenbaum et al, 2018;Müller et al, 2018;B. A. Biller & Bonnefoy, 2018;Manjavacas et al, 2019;Lew et al, 2020;Bowler et al, 2020;Zhou et al, 2020;J.…”

“…Near‐infrared spectroscopy of directly imaged companions has shown that, like isolated brown dwarfs, aerosols are common in their atmospheres and that the distribution of aerosols appears to be heterogeneous, with some objects exhibiting temporal variability in photometry and spectra (B. A. Biller & Bonnefoy, 2018; Bonnefoy et al., 2013, 2016; Bowler et al., 2020; Currie et al., 2011; Delorme et al., 2017; Greenbaum et al., 2018; Ingraham et al., 2014; Lew et al., 2020; Macintosh et al., 2015; Manjavacas et al., 2019; Marley et al., 2012; Marois et al., 2008; Müller et al., 2018; Rajan et al., 2017; Samland et al., 2017; Skemer et al., 2012, 2014; J. J. Wang et al., 2020; Zhou et al., 2016, 2020). Trends in wavelength‐dependent variability with spectral type are also seen among companions, where L‐type objects (those with spectra similar to L dwarfs) tend to have gray or linear dependence while L‐T transition objects show lower variability amplitudes in absorption features (Manjavacas et al., 2018; Miles‐Páez et al., 2019; Zhou et al., 2018).…”

Scientific discovery with the James Webb Space Telescope

“…Direct imaging observations of exoplanets are a complementary technique to study planetary atmospheres, and, like secondary eclipse and phase curve measurements, probe deeper into the planetary atmospheres than tranmission spectroscopy. Such data can achieve higher S/N of interesting fea- tures [169,170]. As described in Section 3, Webb's instruments include coronagraphy modes at several wavelengths ranging from 1.7 to 22.75 microns.…”

Reference Star Differential Imaging of Close-in Companions and Circumstellar Disks with the NIRC2 Vortex Coronagraph at the W. M. Keck Observatory

“…Adaptive optics (AO) imaging surveys determine crucial occurrence statistics for giant exoplanet populations with masses 1 M Jup at orbital separations >10 au and provide unique insight into their formation and migration histories (Bowler 2016;Meshkat et al 2017). Furthermore, resolving low-mass companions from their host stars reduces measurement noise allowing for indepth spectral characterization, which yields a wealth of information about the physical properties of their atmospheres (Biller & Bonnefoy 2017). AO observations also allow for the detailed mapping of scattered light from dust in circumstellar disks providing context for planet formation theories.…”