Direct Imaging

When the (flat) wavefronts from a star pass through the Earth’s atmosphere they be- come distorted due to the turbulence. This distortion blurs the point spread function (PSF) of the star and hence the measured full width at half maximum of the PSF would be greater than in the diffraction limit. This makes it impossible to detect faint companions to stars without some form of correction for the atmospheric distortion (seeing). The problem is particularly acute for planets because they are so faint com- pared to their host star. For example, Jupiter is of order a billionth the brightness of the Sun (Haswell 2010). A technique that has been developed to account for this distortion is adaptive optics. By projecting a laser into the atmosphere, the sodium atoms are excited and fluoresce. This mimics the light of a star passing through the Earth’s atmosphere. The telescope then observes this and the adaptive optics measures the wavefronts of the laser. A segmented mirror deforms to the opposite shape of the distorted wavefronts which produces a corrected wavefront and hence a cleaner image. This combined with using a coronagraph can be used to image planets. A coronagraph covers up the majority of the star so that the telescope’s detectors are not blinded by the light from the star. Currently this method is sensitive to young planets far from their star. The typical distance at which planets can be detected is ₁₀100 (Guyon et al.

2014). It has been seen from work by (Marois et al. 2008) that planets with a contrast ratio of 10−5 (12 mags) relative to their host star are detectable. Giant planets will be very hot immediately after they form but will cool very quickly (Figure 1.14, Brandt et al. 2014). The temperature of the planet at a given age and its cooling rate is de- pendent on the mass of the planet (Baraffe et al. 2003). The heat is generated through the conversion of gravitational potential energy caused by the accretion of gas from the protoplanetary disk. Coronagraphs have also been used to image planets from space. An example of a planet seen using a space based telescope is Fomalhaut b. Figure 1.15 (Kalas et al. 2008) shows the direct detection of Fomalhaut b through the use of a coronagraph. Two images were taken of the system in 2004 and 2006, during which time the planet is seen to move (bottom right hand box).

Figure 1.14: The change in luminosity and temperature of planets (solid lines) and brown dwarfs (dashed lines) of given masses with time (in years) (Baraffe et al. 2003). The red dashed line shows the 1300K limit where the model used to determine these cooling rates is valid for extrasolar giant planets. Usually a model that includes dust is generally used for planets with higher temperatures than this limit, however, it has been shown that the dusty model is negligibly different to the model plotted here and hence it is valid to use the non-dusty model shown here for exoplanets that have temperature greater than 1300K (Chabrier et al. 2000)

Figure 1.15: Image of the debris disk around Fomalhaut b taken using a coronagraph on the Hubble Space Telescope (Kalas et al. 2008). The box in the bottom right hand corner shows the movement of the planet from images taken in 2004 and 2006. This shows how a coronagraph can be used to find exoplanets.

Instruments that have a coronagraph that can be used simultaneously with a spectrograph will be able to do direct spectroscopy of these exoplanets. In conjunction with adaptive optics this will give very high quality spectra of their atmospheres. With real spectra over the 1 − 10 µm range much better fits to model SEDs will be possible (as there will be fewer free degrees of freedom).

Current instruments that are in operation looking for planets using direct imaging are the Gemini Planet Imager (GPI) (Graham et al. 2007; Macintosh et al. 2006) and Spectro-Polarmetric High-contrast Exoplanet REsearch (SPHERE) (Beuzit et al. 2008). GPI will use adaptive optics to correct for the distortion of star light caused by the Earth’s atmosphere enabling it to produce high-resolution images of planets in orbit around nearby stars. GPI is sensitive to planets more massive than 6 Jupiter masses orbiting 10 or more AU away from their host star at an age of less than 2Gyr (Graham et al. 2007). It is during this time that the planets are bright because they are still hot from formation. GPI is also equipped with an integral field unit, which allows spectra to be taken of objects it is observing and hence it is able to get spectra of exoplanet

atmospheres. An example of a system that has the atmospheres planets analysed using this method is HR 8799 (Ingraham et al. 2014). SPHERE will be used to detect both reflected and thermal radiation of planets orbiting their host stars between 1 and 100 AU (Beuzit et al. 2008). The reflected light can be detected because when star light is reflected off a planets atmosphere it is polarised and the light detected directly from the star is not. The thermal radiation will be detected through IR photometry. Using both these techniques allows both young planets (bright in the IR) and older, cooler planets (visible in polarised light) to be detected.

EPICS: The Exoplanet Imaging Camera and Spectrograph for the European Extremely Large Telescope (E-ELT) will be able to image hot young giant planets and possibly smaller terrestrial worlds in the habitable zone. It will have sensitivity to planets > a few AU from their host star (Kasper et al. 2010). Currently atmospheres of exoplanets can only be studied through transmission spectroscopy and secondary eclipse photometry (see later sections), EPICS will be able to do direct spectroscopy of these bodies which will lead to unprecedented characterisation of their atmospheres. It may even be possible to look for biosignatures in the atmospheres of the terrestrial bodies it could image.