V1298 Tauri
V1298 Tauri is a young weakly-lined T Tauri star that is part of the Taurus-Auriga association in the Taurus Molecular Cloud. Alternatively it is part of a proposed moving group, called Group 29 that is slightly older. The system has four transiting exoplanets, discovered with the Kepler space telescope in the K2 mission. One of the planets was discovered in August 2019 and the other three were discovered in November 2019 by the same team.
Stellar characteristics
V1298 Tauri has a spectral type of K0 - K1.5 and it has a mass of about 1.1. The star appears in x-rays from ROSAT data and it does show strong lithium absorption lines, both signatures of youth and therefore it was a proposed member of Taurus-Auriga. On the other hand it does not show signs of accretion and it lacks infrared excess. Instead it shows H-alpha in absorption.In 2007, Konstantin Nikolaevich Grankin et al. announced their discovery that V1298 Tauri is a variable star. It was given its variable star designation in 2011. The brightness of V1298 Tauri varies in an unpredictable way between a maximum visual magnitude of 10.31 and a minimum of 10.54. The light curve of the star shows quasi-periodic variability that was interpreted as stellar rotation and starspots. The light curve also showed several flares.
Based on Gaia DR2 data this star is part of a co-moving pair, together with HD 284154. The star is included in an analysis of the 93 Tau group, which finds an age of.
Planetary system
The planets around V1298 Tauri are super-puffs with some of the lowest densities ever recorded, which is attributed to their young age. Over time the planets will shrink and become super-Earths and sub-Neptunes. Additionally planet c is suspected to have a ‘boil-off’ phase when the disk around the star disperses.V1298 Tauri has four confirmed planets of which planets c, d and b are near a 1:2:3 resonance. The system is very young and might be a precursor of a compact multiplanet system. The 2:3 resonance suggests that some close-in planets may either form in resonances or evolve into them on timescales of less than 10 Myr. The planets c, d in the system have a size between Neptune and Saturn. Planets b, e have sizes similar to Jupiter. Planet e was recovered and detected with K2, LCO, TESS and possibly CHEOPS. It has an orbital period of 48.7 days and it has a 2:1 commensurability with planet b, which shows large TTVs due to the interaction. Orbits of the planets b and c are nearly coplanar and planet c is not inclined to the equatorial plane of the star, misalignment equals to 2 degrees.
Models predict that the planets have a minimum core mass of 5 and are surrounded by a thick envelope that make up 20% of their mass. The total mass of planet c and d was predicted to be 2 - 28 and the total mass of planet d and b was predicted to be 9 - 120. In a follow-up paper the mass of V1298 Tauri b was constrained to <2.2. The masses were measured with transit timing variations and are in the range of super-Earths to sub-Neptunes.
The planet c was suspected to be shedding mass due to intense irradiation by the host star, but hydrogen tail existence was refuted by 2021. Additionally planet c has a non-detection of helium, which provided limits for mass-loss rates. A Hubble WFC3 transmission spectrum was obtained for planet c and hazes could not be ruled out for planet c. Planet e could be a planet with a water-rich core and an substantial hydrogen envelope.