The TESS mission recently uncovered a remarkable multi-planet system consisting of a super-Earth and two temperate mini-Neptunes around the bright M3 dwarf star TOI-270 (Günther et al., 2019). We will acquire near-infrared transmission spectra for the two mini-Neptunes (TOI-270 c&d) using HST WFC3 as part of GO-15814 (PI: T. Mikal-Evans).

Atmospheric characterization of two TESS mini-Neptunes formed in the same protoplanetary nebula using Hubble

Thomas Mikal-Evans,1,★ Ian Crossfield,1 Laura Kreidberg,2 Maximilian Günther,1 Nikku Madhusudhan,3 Vincent Van Eylen,4 Jason Dittmann,1 Tansu Daylan,1

Laetitia Delrez,3 Luis Welbanks,3 Benjamin Drummond,5 Amaury Triaud6

1MIT, 2CfA, 3Cambridge, 4Princeton, 5Exeter, 6Birmingham, ★tmevans@mit.edu

Radius distribution for planets with orbital periods <100 days, adapted from Günther et al. (2019). Colored vertical bars show radii of the three TOI-270 planets. TOI-270b is a super-Earth that likely has a predominantly rocky composition with a thin atmosphere, while TOI-270 c&d are exemplary mini-Neptunes, likely having thick H2-dominated atmospheres suitable for transmission spectroscopy with HST.

GJ1214b

GJ3470b

K2-266b

TOI-270c d

HD97658b

GJ9827d

HD3167c

LHS1140b

Figure 2. Known transiting planets with radii <4R⨁ as a function of host star brightness and transit depth change due to 1 atmospheric scale height. Circle sizes give relative SNR between targets, with unfilled circles indicating those targets already observed with HST. Slanting gray lines indicate constant SNR contours to guide the eye. TOI-270 c&d are the best unobserved mini-Neptunes with temperatures below 1000K.

Known transiting planets with radii <4Rꚛ as a function of host star brightness and 1 pressure scale height transmission signal. Circle size gives relative SN between targets, with unfilled circles indicating planets already observed with HST. Slanting gray lines show constant SN contours to guide the eye. TOI-270 c&d are the best unobserved mini-Neptunes with temperatures below 1000K.

Transmission spectrum models assuming chemical equilibrium and different metallicities. Synthetic WFC3 data points with anticipated measurement uncertainties of 30ppm are also plotted. Prominent spectral features due to CH4 and H2O may be detectable at high SN for metallicities <1000x solar. Photochemical haze could be present in one or both atmospheres. If so, our observations will provide a unique test of how photochemical haze production correlates with incident UV flux and planet temperature, given that both planets orbit the same star.

Figure 3. Transmission spectrum models assuming chemical equilibrium and different metallicities. Synthetic data points were generated from the 100x solar metallicity models using measurement uncertainties of 30ppm (see Description of Observations). Prominent spectral features due to H2O and CH4 will be detectable at high SN for metallicities <1000x solar. Comparison of two such spectra for planets in the same system will be unprecedented.

TOI-270cT=420Kg=10ms-2

TOI-270d T=350Kg=6ms-2

CH4

CH4

CH4 + H2O

CH4

CH4

CH4 + H2O

Figure 3. Transmission spectrum models assuming chemical equilibrium and different metallicities. Synthetic data points were generated from the 100x solar metallicity models using measurement uncertainties of 30ppm (see Description of Observations). Prominent spectral features due to H2O and CH4 will be detectable at high SN for metallicities <1000x solar. Comparison of two such spectra for planets in the same system will be unprecedented.

TOI-270cT=420Kg=10ms-2

TOI-270d T=350Kg=6ms-2

CH4

CH4

CH4 + H2O

CH4

CH4

CH4 + H2O

Figure 3. Transmission spectrum models assuming chemical equilibrium and different metallicities. Synthetic data points were generated from the 100x solar metallicity models using measurement uncertainties of 30ppm (see Description of Observations). Prominent spectral features due to H2O and CH4 will be detectable at high SN for metallicities <1000x solar. Comparison of two such spectra for planets in the same system will be unprecedented.

TOI-270cT=420Kg=10ms-2

TOI-270d T=350Kg=6ms-2

CH4

CH4

CH4 + H2O

CH4

CH4

CH4 + H2O

Figure 3. Transmission spectrum models assuming chemical equilibrium and different metallicities. Synthetic data points were generated from the 100x solar metallicity models using measurement uncertainties of 30ppm (see Description of Observations). Prominent spectral features due to H2O and CH4 will be detectable at high SN for metallicities <1000x solar. Comparison of two such spectra for planets in the same system will be unprecedented.

TOI-270cT=420Kg=10ms-2

TOI-270d T=350Kg=6ms-2

CH4

CH4

CH4 + H2O

CH4

CH4

CH4 + H2O

Figure 3. Transmission spectrum models assuming chemical equilibrium and different metallicities. Synthetic data points were generated from the 100x solar metallicity models using measurement uncertainties of 30ppm (see Description of Observations). Prominent spectral features due to H2O and CH4 will be detectable at high SN for metallicities <1000x solar. Comparison of two such spectra for planets in the same system will be unprecedented.

TOI-270cT=420Kg=10ms-2

TOI-270d T=350Kg=6ms-2

CH4

CH4

CH4 + H2O

CH4

CH4

CH4 + H2O

Figure 3. Transmission spectrum models assuming chemical equilibrium and different metallicities. Synthetic data points were generated from the 100x solar metallicity models using measurement uncertainties of 30ppm (see Description of Observations). Prominent spectral features due to H2O and CH4 will be detectable at high SN for metallicities <1000x solar. Comparison of two such spectra for planets in the same system will be unprecedented.

TOI-270cT=420Kg=10ms-2

TOI-270d T=350Kg=6ms-2

CH4

CH4

CH4 + H2O

CH4

CH4

CH4 + H2O

Figure 3. Transmission spectrum models assuming chemical equilibrium and different metallicities. Synthetic data points were generated from the 100x solar metallicity models using measurement uncertainties of 30ppm (see Description of Observations). Prominent spectral features due to H2O and CH4 will be detectable at high SN for metallicities <1000x solar. Comparison of two such spectra for planets in the same system will be unprecedented.

TOI-270cT=420Kg=10ms-2

TOI-270d T=350Kg=6ms-2

CH4

CH4

CH4 + H2O

CH4

CH4

CH4 + H2O

To learn more about the TOI-270 system: Günther et al., 2019, Nature Astronomy,

DOI: 10.1038/s41550-019-0845-5

Figure 3. Transmission spectrum models assuming chemical equilibrium and different metallicities. Synthetic data points were generated from the 100x solar metallicity models using measurement uncertainties of 30ppm (see Description of Observations). Prominent spectral features due to H2O and CH4 will be detectable at high SN for metallicities <1000x solar. Comparison of two such spectra for planets in the same system will be unprecedented.

TOI-270cT=420Kg=10ms-2

TOI-270d T=350Kg=6ms-2

CH4

CH4

CH4 + H2O

CH4

CH4

CH4 + H2O