A Neptune-sized transiting planet closely orbiting a 5–10-million-year-old star

Abstract

Theories of the formation and early evolution of planetary systems postulate that planets are born in circumstellar disks, and undergo radial migration during and after dissipation of the dust and gas disk from which they formed 1,2 . The precise ages of meteorites indicate that planetesimals -the building blocks of planets -are produced within the first million years of a star's life 3 . A prominent question is: how early can one find fully formed planets like those frequently detected on short orbital periods around mature stars? Some theories suggest the in situ formation of planets close to their host stars is unlikely and the existence of such planets is evidence for large scale migration 4,5 . Other theories posit that planet assembly at small orbital separations may be common [6][7][8] . Here we report on a newly-born, transiting planet orbiting its star every 5.4 days. The planet is 50% larger than Neptune, and its mass is less than 3.6 times Jupiter (at 99.7% confidence), with a true mass likely to be within a factor of several of Neptune's. The 5-10 million year old star has a tenuous dust disk extending in to about 2 times the Earth-Sun separation, in addition to the large planet located at less than 1/20 the Earth-Sun separation.USco 161014. 75-191909.3, hereafter K2-33, is a several million year old M-type star that was observed by NASA's Kepler Space Telescope during Campaign 2 of the K2 mission. The star was identified as one of more than 200 candidate planet hosts in a systematic search for transits in K2 data 9 . As part of our ongoing study of the pre-main sequence population of Upper Scorpius observed by K2, we independently verified and analyzed the planetary transit signal. We acquired radial velocity (RV) and high spatial resolution observations at the W. M. Keck Observatory to confirm the planet, hereafter K2-33b, and to measure its size and mass.2 Within the 77.5 day photometric time series of K2-33 (Kp = 14.3 mag), there are periodic dimmings of 0.23% lasting 4.2 hours and occurring every 5.4 d (Fig. 1). The ensemble of transits are detected at a combined signal-to-noise ratio of ≈ 32. During the K2 observations, cool, dark regions on the stellar surface (starspots) rotated in and out of view, producing semi-sinusoidal brightness variations of ∼3% peak-to-trough amplitude with a 6.3 ± 0.2 d periodicity (Extended Data Fig. 1). We removed the starspot variability prior to modeling the transit events. We fit the transit profiles using established methods 10, measuring the planet's size relative to its host star and its orbital geometry (Table 1).K2-33 is an established member of the Upper Scorpius OB association 11,12 , the nearest site , which we confirm from Keck spectra (Table 1). Furthermore, the stellar rotation rate we measure via broadening of absorption lines in the spectra and via the starspot period (Table 1), is rapid relative to field-age stars of similar mass 14 . We determined the star's systemic RV (Table 1) The inferred planet size and mass depend directly upon the host sta...

A Transient Transit Signature Associated with the Young Star RIK-210

“…Indeed, in Upper Sco itself, dozens of members exhibit such dimming behavior . At the age of Upper Sco, it is also possible to find fully formed planets at small orbital separations, as evidenced by K2-33b (David et al 2016b;Mann et al 2016), a transiting, Neptune-sized, short-period exoplanet around a low-mass member of Upper Sco.…”

“…David et al (2016b) pointed out that K2-33 b orbiting slightly interior to the corotation radius, with for the candidate ∼11-12M Jup planet around the T Tauri star CI Tau. Lastly, the planet V830 Tau b has a period ratio of » P P 1.8 orb rot (Donati et al 2016).…”

Rocky Planet Formation: Quick and Neat

“…Several recent studies identify massive planets orbiting several pre-main sequence stars (e.g., Mann et al 2016;Gaidos et al 2016;David et al 2016;Donati et al 2016;Johns-Krull et al 2016b,a). Identifying Earth-mass planets orbiting young stars would constrain the timescale of terrestrial planet formation relative to the production of debris.…”

Orbiting Clouds of Material at the Keplerian Co-rotation Radius of Rapidly Rotating Low-mass WTTs in Upper Sco

“…The data from that 78-day campaign form the largest set of precision YSO optical light curves that have ever been obtained. Those light curves provide a hunting ground for the search for newly formed planets via their transits (David et al 2016b;Mann et al 2016), for the identification of YSO eclipsing binaries Kraus et al 2015;Lodieu et al 2015;David et al 2016a), and for the determination of the rotational periods for a large sample of low-mass stars of 5-10 Myr age (L. Rebull et al 2017, in preparation). A small fraction of the Upper Sco YSOs have IR excesses and are likely classical TTauri stars (CTTs); a subgroup of "burster" light curves associated with some of these accreting systems is discussed in Cody et al (2017).…”