Andrew Mann

THYME: the TESS Hunt for Young and Maturing Exoplanets

The growing census of young transiting planets (points, colored by age) on top of the older planets found by Kepler (gray). By 2026 there are 51 young planets on this plot. Young planets sit noticeably higher (larger) than the Kepler population.

THYME is a collaboration that uses NASA's TESS mission to find transiting planets around young stars, then confirms and characterizes them with ground-based follow-up. I have been a co-PI of THYME since 2018. The goal is simple to state: if we can find planets at a range of known ages, we can watch how planets change over time, rather than only seeing the end product around stars billions of years old.

The trick is to look in young associations: clusters, moving groups, and streams of stars that formed together. Stars in an association share an age, so a planet in one inherits a well-measured age (see how we date them). Several THYME papers turned up new associations along the way, now named the MELANGE groups, by searching for stars that move together with a planet host.

Young stars make this hard. They are covered in spots and spin quickly, so their brightness swings by a few percent over a few days, often much more than the dip from a transiting planet. Finding and fitting transits under that noise is a big part of what we do.

A transit around a quiet star (left) versus a young, spotted star (right). The light curve at the end of the right-hand animation is real data for HIP 67522.

THYME discoveries include DS Tuc Ab, a planet in the 45 Myr Tucana-Horologium association; HIP 67522 b, a 17 Myr Jupiter-sized planet in Sco-Cen; and the three-planet system TOI-451 in the 120 Myr Pisces-Eridanus stream (see the Media page for coverage of each). Before TESS, we did the same thing with K2, which found K2-33 b around an 11 Myr old star in Upper Scorpius and K2-25 b in the Hyades.

Young planets are bigger

One clear result across our surveys (ZEIT with K2, THYME, and TIDYE with TESS) is that young planets are larger than mature planets on similar orbits. You can see this in the animation at the top of this page: the young planets sit above the bulk of the Kepler population. Where we can measure masses, the young planets are also remarkably low density. The best example is HIP 67522 b, which is about the size of Jupiter but only about 14 times the mass of Earth (The Featherweight Giant). This is what we expect if planets start out hot and puffy, with extended atmospheres that cool, contract, and in some cases escape over the first few hundred million years.

Papers

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