European Astronomical Society Meeting, 2024

SS13: Young and Mischievous: close-in exoplanets around young stars

1. Adina Feinstein, Sagan Fellow University of Colorado Boulder Michigan State University Adina Feinstein, Sagan Fellow University of Colorado Boulder Probing the atmospheric composition of young planets Informing planet formation & evolution 1 July 5, 2024 EAS SS13
2. One of the most populous type of exoplanets are sub-Neptunes (2 - 4 REarth). 2 Sub-neptunes
3. Scenario 1: Sub-Neptunes are water worlds. 3 H2O snowline Rock & ice 
 core • Planets form at or beyond the water ice line, where accretion rates are accelerated. • Planets migrate in after formation to their current-day locations. • Planetary atmospheres would be volatile & water enriched. Morbidelli et al. (2015); Johansen et al. (2017); Batygin & Morbidelli (2023)
4. Scenario 2: Sub-Neptunes are dry, gas worlds. 4 H2O snowline Rock core • Planets form within the water ice line • Pebbles which migrated inwards are partially or fully devolatalized. • Planetary atmosphere would be primarily H & He and volatile depleted. Boley, Morris, & Ford (2014); Chatterjee & Tan (2014, 2015); Batygin & Morbidelli (2023)
5. While planetary densities can provide insights into what sub- Neptunes are composed of, atmospheric observations are more ideal. 5 Adapted from Luque & Palle (2022) Radius (R⨁ ) Mass (M⨁ ) 0.1 1 5 10 30 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 50% H2O Earth-like 300K 700K Earth-like + 0.1% H/He Earth-like + 1% H/He 300K 700K
6. Our insights into the compositions of sub- Neptunes has been limited. 6
7. Extended ionized metals Puffy hydrogen envelope The field of characterizing young planetary atmospheres is only now taking off. 7 Escaping helium Molecular composition
8. What have we learned so far about young planetary atmospheres from HST & JWST? 8 ADS Library for young planet atmospheric characterization publications Extended ionized metals Puffy hydrogen envelope Escaping helium Molecular composition
9. We have a very limited sample of young planets with near-infrared observations. 9 K2-33 HIP 67522 V1298 Tau 17 23 7.5 System Age [Myr] Orbital Separation [a/Rstar] 0 11.7 27.1
10. The young planets with atmospheric observations have radii between Jupiter and Neptune. 10 K2-33 b V1298 Tau c V1298 Tau b HIP 67522 b
11. HST + Spitzer gave us the first glance of young planetary atmospheres. 11 HST GO 14887 - PI B. Benneke HST GO 16083 - PI Kamen Todorov Spitzer ID 2498 - PI A. Mann
12. K2-33 b: a 7.5 Myr planet with a perplexing transmission spectrum. 12 K2-33 b
13. There is no obvious water feature in the HST spectrum of K2-33 b. 13 (Thao et al. 2022)
14. The optical and IR transit depths of K2-33 b raise another interesting question. 14 (Thao et al. 2022) K2 MEarth Spitzer HST
15. The spectrum of K2-33 b can be explained by: starspot contamination, hazes, and/or a dusty torus. 15 (Thao et al. 2022) CO Soot CO Tholin
16. The discovery of young multi-planet systems enables testing possible differences in formation and evolution mechanisms. 16 V1298 Tau c V1298 Tau b
17. The HST transmission spectrum of V1298 Tau b revealed the first confident detection of H2O in a young planet. 17 (Barat et al. 2024) Saugata Barat University of Amsterdam
18. The HST spectrum of V1298 Tau c does not provide the same glimpse of its atmosphere. 18 (Barat et al. submitted) Saugata Barat University of Amsterdam
19. The difference in cloud/haze properties could be due to the different levels of instellation. 19 (Barat et al. 2024; Barat et al. submitted) Saugata Barat University of Amsterdam SPlanet b = 35 S⨁ SPlanet c = 146 S⨁
20. JWST is revolutionizing our understanding of gas- dominated exoplanets. 20 JWST GO 2149 - PI J.-M. Désert JWST GO 2498 - PI A. Mann
21. By observing in the IR, we can place constraints on the C/O (and formation history?) of the planet. 21 HIP 67522 b
22. There is evidence of starspot crossing events in the JWST light curve of HIP 67522 b. 22 Pa Chia Thao University of North Carolina Chapel Hill (Thao, Mann, Feinstein et al. under review)
23. The first near-infrared transmission spectrum of a 17 Myr planet reveals a huge CO2 and no significant SO2 feature. 23 Pa Chia Thao University of North Carolina Chapel Hill (Thao, Mann, Feinstein et al. under review) H2O CO2 SO2
24. The simultaneous optical transit of HIP 67522 b constrained the starspot properties. 24 (Thao, Mann, Feinstein et al. under review) fs=0.3 TSpot = 3000K fs=0.1 TSpot = 4000K fs=0.3 TSpot = 4000K
25. The wavelength coverage of JWST allows us to begin drawing comparisons between short- and long-period exoplanets. 25 HIP 67522 b HIP 67522 b
26. The CO2 feature in HIP 67522 b shows a "double peak". 26 (Thao, Mann, Feinstein et al. under review)
27. What trends are we starting to see with 4 data points? 27
28. The transmission spectra of young planets favor lower masses than what is expected from the mass-radius relationship for mature planets. 28 V1298 Tau c V1298 Tau b HIP 67522 b
29. V1298 Tau c V1298 Tau b HIP 67522 b Where are the hot Jupiter progenitors? 29
30. We are seeing tentative evidence that atmospheric metallicity increases with age. 30 Uranus, Neptune, & GJ 3470 b V1298 Tau b & 
 HIP 67522 b Atmospheric Metallicity [x Solar] Age [Myr] 0 120 60 30 90 0 4600 100 1000 10 ? (Beatty et al. 2024)
31. While some young spectra favor cloudless models, others do not. 31 (Mature planet observations from Sing+2016 & Brande+2024)
32. We need to test, against many parameters, when young planets become cloud/haze dominated. 32
33. Extended ionized metals Puffy hydrogen envelope We still have a lot to explore and a lot to learn. 33 Escaping helium Molecular composition ADS Library for young planet atmospheric characterization publications
34. In the next year, we will triple our sample of NIR/ IR observations of young planetary atmospheres. 34 JWST GO 5311, 5959 - Co-PIs Feinstein & Welbanks JWST GO 5882 - Co-PIs Dai & Petigura V1298 Tau cde TOI 451 cd TOI 2076 bcd 120 205 23 System Age [Myr] Orbital Separation [AU] 0 0.08 0.3 0.1 22 AU Mic b 0.2
35. We have some answers. But we have more questions! 35 adina.feinstein@lasp.colorado.edu -How does atmospheric metallicity evolve? -When do young planets transition from cloudless to cloudy? -Where are all the primordial hot Jupiters? -What do all of these new pieces of information tell us about the diversity of planet formation, both in the same system and between systems? ADS Library for young planet atmospheric characterization publications