Exoplanets orbiting pre-main sequence stars are laboratories for studying planet evolution processes , including atmospheric loss , orbital migration , and radiative cooling .
V1298 Tau , a young solar analog with an age of 23 \pm 4 Myr , is one such laboratory .
The star is already known to host a Jupiter-sized planet on a 24 day orbit .
Here , we report the discovery of three additional planets — all between the size of Neptune and Saturn — based on our analysis of K2 Campaign 4 photometry .
Planets c and d have sizes of 5.6 and 6.4 R _ { \oplus } , respectively and with orbital periods of 8.25 and 12.40 days reside 0.25 % outside of the nominal 3:2 mean-motion resonance .
Planet e is 8.7 R _ { \oplus } in size but only transited once in the K2 time series and thus has a period longer than 36 days , but likely shorter than 223 days .
The V1298 Tau system may be a precursor to the compact multiplanet systems found to be common by the Kepler mission .
However , the large planet sizes stand in sharp contrast to the vast majority of Kepler multis which have planets smaller than 3 R _ { \oplus } .
Simple dynamical arguments suggest total masses of < 28 M _ { \oplus } and < 120 M _ { \oplus } for the c-d and d-b planet pairs , respectively .
The implied low masses suggest that the planets may still be radiatively cooling and contracting , and perhaps losing atmosphere .
The V1298 Tau system offers rich prospects for further follow-up including atmospheric characterization by transmission or eclipse spectroscopy , dynamical characterization through transit-timing variations , and measurements of planet masses and obliquities by radial velocities .