Challenges and Opportunities in Constraining the Bulk Properties of Super-Earths with Transmission Spectroscopy

Transcription

1 Challenges and Opportunities in Constraining the Bulk Properties of Super-Earths with Transmission Spectroscopy Eliza Kempton (Formerly: Miller-Ricci) Assistant Professor of Physics Grinnell College, Grinnell, IA Image Credit: Zach Berta

2 Super-Earths have diverse bulk properties Hadden & Lithwick, ApJ, submitted

3 Super-Earths have diverse bulk properties H2-He atmospheres Hadden & Lithwick, ApJ, submitted

4 Super-Earths have diverse bulk properties H2-He atmospheres Hadden & Lithwick, ApJ, submitted

5 Super-Earths have diverse bulk properties H2-He atmospheres - rocky planet, H2 atmosphere - ice/rock planet, H2O atmosphere - + other volatiles? Hadden & Lithwick, ApJ, submitted

6 With JWST we anticipate the type of data that are now coming in for hot Jupiters from HST WASP-43b HD b Deming et al., ApJ 2013 Kreidberg et al., ApJ 2014

7 With JWST we anticipate the type of data that are now coming in for hot Jupiters from HST WASP-43b HD b Deming et al., ApJ 2013 Kreidberg et al., ApJ 2014 Water detections for transiting exoplanets

8 Measurements of molecular and atomic features in transmission spectra can lead to degenerate interpretations (ΔD) Mean molecular weight Kempton et al., PASP, (2017)

9 Measurements of molecular and atomic features in transmission spectra can lead to degenerate interpretations (ΔD) Mean molecular weight Kempton et al., PASP, (2017)

10 Atmospheric observations for low-mass planets reveal the challenges to interpretation HD 97658b GJ 1214b HD b Knutson et al., ApJ, 2014 HAT-P-11b Kreidberg et al., Nature 2013 Fraine et al., Nature, 2014

11 Many of the recently discovered super-earths most amenable to atmospheric follow-up do not have mass measurements

12 Many of the recently discovered super-earths most amenable to atmospheric follow-up do not have mass measurements ΔD ΔD ~ 2 H R pl / R * Scale height: H = kt μg mg Planetary Mass: Mpl = ktrp 2 μgh

13 Many of the recently discovered super-earths most amenable to atmospheric follow-up do not have mass measurements ΔD ΔD ~ 2 H R pl / R * Scale height: H = kt mg Planetary Mass: Mpl μg = from Teq ktrp 2 μgh from transit 2.3 for gas giants =?? for super-earths from ΔD

14 Many of the recently discovered super-earths most amenable to atmospheric follow-up do not have mass measurements ΔD ΔD ~ 2 H R pl / R * Scale height: H = kt mg Planetary Mass: Mpl μg = from Teq ktrp 2 μgh from transit 2.3 for gas giants =?? for super-earths from ΔD

15 Parameter space study of super-earths with H2-H2O atmospheres T atm = 400 K R pl = 1.5 R log[h2o/h2] = g = 5 25 m/s 2 (Mpl = 1 5 M ) Batalha, Kempton, & Mbarek, ApJL, 2017

16 Parameter space study of super-earths with H2-H2O atmospheres T atm = 400 K Degeneracy region R pl = 1.5 R log[h2o/h2] = g = 5 25 m/s 2 (Mpl = 1 5 M ) Batalha, Kempton, & Mbarek, ApJL, 2017

17 Degenerate spectra as observed with 200 hours of JWST time T obs = 200 hr M4.5 host star (T = 3,000 K, R = 0.2 R ) J = 8 JWST simulations w/ PandExo (Batalha et al. PASP, in press) Batalha, Kempton, & Mbarek, ApJL, 2017

18 Degenerate spectra as observed with 200 hours of JWST time Volatile-rich water world, water-rich atmosphere Rocky planet, outgassed H2-rich atmosphere T obs = 200 hr M4.5 host star (T = 3,000 K, R = 0.2 R ) J = 8 JWST simulations w/ PandExo (Batalha et al. PASP, in press) Batalha, Kempton, & Mbarek, ApJL, 2017

19 The degeneracies are further enhanced by the presence of aerosols Batalha, Kempton, & Mbarek, ApJL, 2017

20 The degeneracies are further enhanced by the presence of aerosols You too can model transmission spectra! Exo-Transmit available publicly on Github Kempton et al., PASP (2017) Batalha, Kempton, & Mbarek, ApJL, 2017

21 Planet formation N-body simulations predict the existence of water-rich super-earths Raymond et al., Icarus, 2006

22 Degeneracies in the mass-radius relationship for low-mass planets provide a challenge to diagnosing bulk water abundances GJ 1214b Lissauer et al., Nature, 2011 (modified c/o E. Lopez)

23 The low bulk density of GJ 1214b informs us of its possible atmospheric composition 1. Mini-Neptune Scenario: Rock / ice interior + hydrogen-dominated atmosphere (mostly H2+ trace H2O, CH4, etc.) 2. Water World Scenario: Mostly H2O - ice interior + steam atmosphere (Rogers & Seager, ApJ, Nettelmann et al. 2011)

24 The low bulk density of GJ 1214b informs us of its possible atmospheric composition 1. Mini-Neptune Scenario: Rock / ice interior + hydrogen-dominated atmosphere (mostly H2+ trace H2O, CH4, etc.) % H2 / He by mass 2. Water World Scenario: Mostly H2O - ice interior + steam atmosphere > 60% H2O by mass (Rogers & Seager, ApJ, Nettelmann et al. 2011)

25 Transit Depth (%) The power of complementary methods: Atmospheric spectroscopy + interior structure modeling Raymond et al., Icarus, Wavelength (microns)

26 Takeaway: 1) Atmospheric studies of small-radius planets should not be undertaken without knowledge of the planet s mass 2) Independent mass measurements (via RVs or TTVs) are necessary in many cases for interpreting the spectra of low-mass exoplanets 3) JWST will provide the first opportunities to unambiguously discover water worlds

27