Exploring the Diversity of Exoplanets

Exploring the diversity of exoplanets Dr Suzanne Aigrain Cormack lecture 2012 Putting the Earth in context Earthrise seen by Apollo 8 crew, December 1968 © NASA Planets of the solar system Sizes are to scale, distances are not asteroid Gaspra asteroid Gaspra Halley’s comet (since 240 BCE) asteroid Gaspra Halley’s comet (since 240 BCE) Nebular theory Swedenborg, Kant, Laplace (1700’s) 100 000 000 000 stars in the Milky Way 100 000 000 000 galaxies in the Universe Star nurseries... planet nurseries Star nurseries... planet nurseries The discovery of Uranus William & Caroline Herschel (1781) The discovery of Uranus William & Caroline Herschel (1781) The discovery of Uranus William & Caroline Herschel (1781) The discovery of Neptune Orbit of Uranus estimated from a century of observations The discovery of Neptune Orbit of Uranus estimated from a century of observations Existence of a new planet predicted by Adams & Le Verrier (1845) The discovery of Neptune Orbit of Uranus estimated from a century of observations Existence of a new planet predicted by Adams & Le Verrier (1845) Neptune observed at predicted position the same year by Galle Finding exoplanets I. direct imaging HR 8799 (Marois et al. 2008) Finding exoplanets I. direct imaging HR 8799 (Marois et al. 2008) Finding exoplanets II. radial velocity Finding exoplanets II. radial velocity dispersing element Finding exoplanets II. radial velocity absorption lines dispersing element Finding exoplanets II. radial velocity 51 Peg b (Mayor & Queloz 1995) Finding exoplanets II. radial velocity 51 Peg b (Mayor & Queloz 1995) hot Jupiters Finding exoplanets III. transits RV + transits = size + mass = density = buk composition Finding exoplanets III. transits HD 209 458 b (Charbonneau et al. 2000) RV + transits = size + mass = density = buk composition Finding exoplanets III. transits HD 209 458 b (Charbonneau et al. 2000) STARE RV + transits = size + mass = density = buk composition >750 exoplanets known today Jupiter Neptune Observational limitations Earth Mass-Radius relation Data from www.inscience.ch/transits (maintained by F. Pont & N. Husnoo) Mass-Radius relation low mass stars Data from www.inscience.ch/transits (maintained by F. Pont & N. Husnoo) Mass-radius relations from Fressin et al. (2007), Seager et al. (2007). brown dwarfs pure H/He Mass-Radius relation low mass stars Data from www.inscience.ch/transits (maintained by F. Pont & N. Husnoo) Mass-radius relations from Fressin et al. (2007), Irradiation Seager et al. (2007). brown dwarfs pure H/He Mass-Radius relation low mass stars Data from www.inscience.ch/transits (maintained by F. Pont & N. Husnoo) Mass-radius relations from Fressin et al. (2007), Irradiation Seager et al. (2007). solid core brown dwarfs pure H/He pure H2O rock Mass-Radius relation hot Jupiters low mass stars Data from www.inscience.ch/transits (maintained by F. Pont & N. Husnoo) Mass-radius relations from Fressin et al. (2007), Irradiation Seager et al. (2007). solid core brown dwarfs pure H/He pure H2O rock Mass-Radius relation hot Jupiters low mass stars Data from www.inscience.ch/transits (maintained by F. Pont & N. Husnoo) Mass-radius relations from Fressin et al. (2007), Irradiation Seager et al. (2007). solid core brown dwarfs pure H/He pure H2O hot Neptunes rock The first transiting hot Neptune: Gliese 436b The first transiting hot Neptune: Gliese 436b Gillon et al. (2007) The first transiting hot Neptune: Gliese 436b Gillon et al. (2007) Hot ice going to spaceTerrestrial planets? CoRoT A CoRoT lightcurve intermediate stellar variability (>1d) residuals (smoothed) The first transiting super-Earth: CoRoT-7b P = 0.8541 d, 0.3 mmag deep The first transiting super-Earth: CoRoT-7b P = 0.8541 d, 0.3 mmag deep Kepler: taking exoplanet discovery to the next level Planet Hunters 1 - a circumbinary planet in a quadruple system Schwamb et al. (2012) A transit closer to home www.super-collider.com Transit of Venus First predicted by Johannes Kepler in 1627 (for 1631) First observed by Jeremiah Horrocks in 1639 Measuring the scale of the solar system First predicted by Johannes Kepler in 1627 (for 1631) First observed by Jeremiah Horrocks in 1639 Observations from widely spaced points on Earth proposed by Edmund Halley (1691, 1716) Measuring the scale of the solar system First predicted by Johannes Kepler in 1627 (for 1631) First observed by Jeremiah Horrocks in 1639 Observations from widely spaced points on Earth proposed by Edmund Halley (1691, 1716) Successful expeditions 1761, 1769 (Captain Cook) The first extra-terrestrial atmosphere Mikhail Lomonosov (1761) 2004 transit © D. Kiselman, Royal Swedish Academy of Sciences Using transits to study exoplanet atmospheres secondary eclipse: emitted or reflected light transit: planet radius phase curve: temperature distribution and dynamics transmission spectra: absorbers in atmosphere Transmission spectroscopy to observer star light planet Effect of clouds to observer star light planet Effect of gases to observer star light planet How do we measure this? How do we measure this? How do we measure this? grism How do we measure this? grism How do we measure this? grism 4 N. P. Gibson et al. Figure 3. Extracted 1D spectra of a typical in-transit (green) and out-of-transit (blue) observation of HD 189733 showing the number of electrons collected per pixel channel. The wavelength decreases with increasing x position. Figure 2. NICMOS image of HD 189733 taken with the G206 grism. The blue line marks the centroids of the first-order spectrum of HD 189733, with the green lines flanking representing the extent of the extraction region. Immediately left is the second- order spectrum, and below is a companion star to HD 189733. The bright band at the right of the detector is a feature of G206 grism spectra, caused by the warm edge of the aperture mask. was used to extract the flux for each wavelength channel, after subtraction of a background value for each pixel. A width of 35 pixels was chosen to minimise the RMS in the white light curve of orbits 2, 4 and 5. This was repeated for 90 pixel columns along the dispersion axis. The extraction regions used are marked in Fig. 2. For the background subtraction, we experimented with various techniques. First, a global background subtraction was used, similarly to S08. Figure 4. Raw ‘white’ light curve of HD 189733 obtained by in- The background was taken to be the average of a large un- tegrating the flux from each spectra over all wavelengths, showing illuminated region above the spectral trace (we tested using the sampling of the transit, alongside its best-fit model. While all different regions). This is not ideal, as previously mentioned orbits are seen to suffer from systematics, orbit one exhibits by the background varies spatially over the detector. As a first far the largest, commonly attributed to spacecraft ‘settling’, and order correction, we instead calculated the background sep- is excluded from subsequent analysis. arately for each pixel column, as the average value of the un-illuminated region above the spectral trace along the column. Again we note that this does not provide a satisfac- curves. A ‘white’ light curve was constructed by integrating tory correction, as the background varies along both x and y. the flux over the entire wavelength range for each image. Both global and wavelength dependent corrections were used This was extended to include 110 pixel columns, which min- for the subsequent analysis. For the remainder of this paper, imises systematics arising from small changes in the position we will display results from data that is not flat-fielded, and of the spectral trace. This is plotted in Fig. 4, and shows the using separate columns for background subtraction, but our sampling of the light curve over the five orbits. Systemat- conclusions remain the same for each case. ics are evident in each orbit, but particularly for orbit one. Extracted spectra from a typical in-transit and out-of- This is commonly found in similar NICMOS data (see e.g. transit observation are shown in Fig. 3, giving approximately Sects. 3.1 and 4.1). It is attributed to spacecraft ‘settling’ 430000 electrons in the brightest pixel channel, and approx- (e.g. S08; Pont et al. 2009), and orbit one is excluded for the imately 120000 electrons in the faintest channel. The fea- remainder of this work. tures in the spectra do not correspond to real stellar fea- The raw light curves are shown in Fig. 5 for each of the tures, but result in variation of the sensitivity of the grism 18 wavelength channels, after normalising by fitting a line with wavelength. Each wavelength channel in the 1D spec- through orbits 2, 4 and 5, and dividing the light curves by tra is then used to construct a time-series, after binning in this function. The light curves are clearly seen to exhibit 5 pixels along the dispersion direction, resulting in 18 light strong time-correlated noise, which needs to be removed if c 2002 RAS, MNRAS 000,1–17 How do we measure this? grism 4 N. P. Gibson4 N. P. et Gibson al. et al. Figure 3. Extracted 1D spectra of a typical in-transit (green) and out-of-transit (blue) observation of HD 189733 showing the number of electrons collected per pixel channel. The wavelength decreases with increasing x position. Figure 2. NICMOS image of HD 189733 taken with the G206 Figure 3. Extracted 1D spectra of a typical in-transit (green) grism. The blue line marks the centroids of the first-order spec- and out-of-transit (blue) observation of HD 189733 showing the trum of HD 189733, with the green lines flanking representing the extent of the extraction region. Immediately left is the second- number of electrons collected per pixel channel.

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