Introduction to Directly Imaged Exoplanets

What is/are Imaged Exoplanets?

Imaged Exoplanets - Measuring the orbits of directly imaged exoplanets requires precise astrometry at the milliarcsec level over long periods of time due to their wide separation to the stars (≳10  au) and long orbital period (≳20  yr). ^[1] Measurements of the transmission spectra, dayside emission, and phase curves of transiting exoplanets, as well as the emission spectrum and light curves of directly imaged exoplanets and brown dwarfs have shown that aerosols are distributed inhomogeneously in exoplanet atmospheres, with aerosol distributions varying significantly with planet equilibrium temperature and gravity. ^[2] These are the most challenging directly imaged exoplanets that have been observed at high spectral resolution to date when considering both their angular separations and flux ratios. ^[3] ExoSpec Project is a NASA Headquarters directed work package that links four different tasks at Goddard space flight center to enable future missions to more efficiently characterize directly imaged exoplanets. ^[4] We measured the near-infrared linear polarization of 20 known directly imaged exoplanets and brown dwarf companions with the high-contrast imager SPHERE-IRDIS at the VLT. ^[5] The aim of this project is to investigate biases (deviation of the median and mode of the posterior from the true values of orbital parameters, and the width and coverage of their credible intervals) in the estimation of orbital parameters of directly imaged exoplanets, particularly their eccentricities, and to define general guidelines to perform better estimations of uncertainty. ^[6] Many exoplanet-focused instruments use a lenslet IFS to make datacubes with spatial and spectral information used to extract spectral information of imaged exoplanets. ^[7] Likewise, it suggests that stable and large scale cloud covers could be ubiquitous in strongly irradiated exoplanets but might be more patchy in low-irradiated or isolated objects like brown dwarfs and directly imaged exoplanets. ^[8] We suggest that directly imaged exoplanets at large orbital radii, where the disk mass criterion is more likely to be satisfied, could have significant obliquities due to the tilt instability of their circumplanetary disks. ^[9] The main reason behind the small number of directly imaged exoplanets is that such observations are extremely challenging. ^[10] Atmospheric characterization of directly imaged exoplanets is currently limited to Giant planets and Mini-Neptunes. ^[11] The Keck Planet Imager and Characterizer (KPIC) is a novel instrument that combines high-contrast imaging with high-resolution spectroscopy to enable high-dispersion coronagraphy (HDC) techniques that allow us to characterize directly imaged exoplanets at a spectral resolution of R~35,000. ^[12] In the near term, it will be used to spectrally characterize known directly imaged exoplanets and low-mass brown dwarf companions visible in the northern hemisphere with a spectral resolution high enough to enable spin and planetary radial velocity measurements as well as Doppler imaging of atmospheric weather phenomena. ^[13] Current and future high-contrast imaging instruments require extreme adaptive optics systems to reach contrasts necessary to directly imaged exoplanets. ^[14] Reflected starlight measurements will open a new path in the characterization of directly imaged exoplanets. ^[15] The orbital eccentricities of directly imaged exoplanets and brown dwarf companions provide clues about their formation and dynamical histories. ^[16] Although HR 8799 e was already known, the interferometry technique could be used to refine the orbits and spectra of directly imaged exoplanets. ^[17] KPIC will enable High Dispersion Coronagraphy (HDC) of directly imaged exoplanets for the first time, providing potentially improved detection significance and spectral characterization capabilities compared to direct imaging. ^[18] In this work, we show how it can be used to derive radial velocity (RV) measurements of directly imaged exoplanets. ^[19]

Directly Imaged Exoplanets

Measuring the orbits of directly imaged exoplanets requires precise astrometry at the milliarcsec level over long periods of time due to their wide separation to the stars (≳10  au) and long orbital period (≳20  yr). ^[1] Measurements of the transmission spectra, dayside emission, and phase curves of transiting exoplanets, as well as the emission spectrum and light curves of directly imaged exoplanets and brown dwarfs have shown that aerosols are distributed inhomogeneously in exoplanet atmospheres, with aerosol distributions varying significantly with planet equilibrium temperature and gravity. ^[2] These are the most challenging directly imaged exoplanets that have been observed at high spectral resolution to date when considering both their angular separations and flux ratios. ^[3] ExoSpec Project is a NASA Headquarters directed work package that links four different tasks at Goddard space flight center to enable future missions to more efficiently characterize directly imaged exoplanets. ^[4] We measured the near-infrared linear polarization of 20 known directly imaged exoplanets and brown dwarf companions with the high-contrast imager SPHERE-IRDIS at the VLT. ^[5] The aim of this project is to investigate biases (deviation of the median and mode of the posterior from the true values of orbital parameters, and the width and coverage of their credible intervals) in the estimation of orbital parameters of directly imaged exoplanets, particularly their eccentricities, and to define general guidelines to perform better estimations of uncertainty. ^[6] Likewise, it suggests that stable and large scale cloud covers could be ubiquitous in strongly irradiated exoplanets but might be more patchy in low-irradiated or isolated objects like brown dwarfs and directly imaged exoplanets. ^[7] We suggest that directly imaged exoplanets at large orbital radii, where the disk mass criterion is more likely to be satisfied, could have significant obliquities due to the tilt instability of their circumplanetary disks. ^[8] The main reason behind the small number of directly imaged exoplanets is that such observations are extremely challenging. ^[9] Atmospheric characterization of directly imaged exoplanets is currently limited to Giant planets and Mini-Neptunes. ^[10] The Keck Planet Imager and Characterizer (KPIC) is a novel instrument that combines high-contrast imaging with high-resolution spectroscopy to enable high-dispersion coronagraphy (HDC) techniques that allow us to characterize directly imaged exoplanets at a spectral resolution of R~35,000. ^[11] In the near term, it will be used to spectrally characterize known directly imaged exoplanets and low-mass brown dwarf companions visible in the northern hemisphere with a spectral resolution high enough to enable spin and planetary radial velocity measurements as well as Doppler imaging of atmospheric weather phenomena. ^[12] Current and future high-contrast imaging instruments require extreme adaptive optics systems to reach contrasts necessary to directly imaged exoplanets. ^[13] Reflected starlight measurements will open a new path in the characterization of directly imaged exoplanets. ^[14] The orbital eccentricities of directly imaged exoplanets and brown dwarf companions provide clues about their formation and dynamical histories. ^[15] Although HR 8799 e was already known, the interferometry technique could be used to refine the orbits and spectra of directly imaged exoplanets. ^[16] KPIC will enable High Dispersion Coronagraphy (HDC) of directly imaged exoplanets for the first time, providing potentially improved detection significance and spectral characterization capabilities compared to direct imaging. ^[17] In this work, we show how it can be used to derive radial velocity (RV) measurements of directly imaged exoplanets. ^[18]