TESS hunt for young and maturing exoplanets (THYME). III. A two-planet system in the 400 Myr Ursa major group

Andrew W. Mann*, Marshall C. Johnson, Andrew Vanderburg, Adam L. Kraus, Aaron C. Rizzuto, MacKenna L. Wood, Jonathan L. Bush, Keighley Rockcliffe, Elisabeth R. Newton, David W. Latham, Eric E. Mamajek, George Zhou, Samuel N. Quinn, Pa Chia Thao, Serena Benatti, Rosario Cosentino, Silvano Desidera, Avet Harutyunyan, Christophe Lovis, Annelies MortierFrancesco A. Pepe, Ennio Poretti, Thomas G. Wilson, Martti H. Kristiansen, Robert Gagliano, Thomas Jacobs, Daryll M. Lacourse, Mark Omohundro, Hans Martin Schwengeler, Ivan A. Terentev, Stephen R. Kane, Michelle L. Hill, Markus Rabus, Gilbert A. Esquerdo, Perry Berlind, Karen A. Collins, Gabriel Murawski, Nezar Hazam Sallam, Michael M. Aitken, Bob Massey, George R. Ricker, Roland Vanderspek, Sara Seager, Joshua N. Winn, Jon M. Jenkins, Thomas Barclay, Douglas A. Caldwell, Diana Dragomir, John P. Doty, Ana Glidden, Peter Tenenbaum, Guillermo Torres, Joseph D. Twicken, Steven Villanueva

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

68 Citations (Scopus)


Exoplanets can evolve significantly between birth and maturity, as their atmospheres, orbits, and structures are shaped by their environment. Young planets (<1 Gyr) offer an opportunity to probe the critical early stages of this evolution, where planets evolve the fastest. However, most of the known young planets orbit prohibitively faint stars. We present the discovery of two planets transiting HD 63433 (TOI 1726, TIC 130181866), a young Sun-like (M∗=0.99±0.03) star. Through kinematics, lithium abundance, and rotation, we confirm that HD 63433 is a member of the Ursa Major moving group (τ=414±23 Myr). Based on the TESS light curve and updated stellar parameters, we estimate the planet radii are 2.15±0.10R⊕ and 2.67±0.12R⊕, the orbital periods are 7.11 and 20.55 days, and the orbital eccentricities are lower than about 0.2. Using HARPS-N velocities, we measure the Rossiter-McLaughlin signal of the inner planet, demonstrating that the orbit is prograde. Since the host star is bright (V=6.9), both planets are amenable to transmission spectroscopy, radial velocity measurements of their masses, and more precise determination of the stellar obliquity. This system is therefore poised to play an important role in our understanding of planetary system evolution in the first billion years after formation.
Original languageEnglish
Article number179
Number of pages18
JournalAstronomical Journal
Issue number179
Publication statusPublished - 24 Sept 2020


  • Young star clusters
  • Exoplanet evolution
  • Transits
  • Exoplanet astronomy
  • Stellar activity
  • Stellar rotation
  • Exoplanet dynamics


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