Two exoplanets have unexpectedly small radii

According to the core-accretion model of planetary formation, Jupiter, Saturn, and other gas giants formed by first building a rocky core and then accumulating an atmosphere. The planets then contracted slowly over hundreds of millions of years until they reached their final sizes. An evolving planet’s mass–radius combination is therefore correlated with its age. Exoplanet mass–radius data have been consistent with predictions so far but haven’t included any very young planets, which are difficult to characterize because the young stars they orbit are extremely active.

Now Alejandro Suárez Mascareño at the Institute of Astrophysics of the Canary Islands and his collaborators have measured the masses and radii of four young exoplanets. The star they orbit, V1298 Tau, is around 20 million years old, which is young given that planets take millions of years to form. But the researchers found that two of the exoplanets have mass–radius combinations that would seem to put their ages at hundreds of millions of years old. The inconsistency may indicate a need to revise existing assumptions about planetary contraction.

V1298 Tau was first observed by NASA’s Kepler space telescope in 2015. Then, in 2019, analysis of the data collected by Kepler uncovered the star’s four transiting planets, and by comparing the time series with models, the researchers established the planets’ radii and orbital periods. Measuring their masses required additional spectroscopic data. From April 2019 to April 2020, Suárez Mascareño and colleagues collected more than 260 measurements of V1298 using high-resolution spectrographs. Because the star is so variable, the researchers simultaneously tracked the star’s visible brightness with the Las Cumbres Observatory global telescope and used that data to adjust the spectroscopic measurements.

The resulting radial-velocity signals—Doppler shifts in the light emitted by the star in response to the orbiting planets’ gravitational pulls—yielded radii and periods in agreement with the Kepler data. The derived mass–radii combinations of the two smaller planets, c and d, were consistent with the low densities expected for young planets, as shown in the figure. But for the two larger planets, b and e, the parameters were closer to those of the old gas giants in our own solar system.

One explanation for the discrepancy is that giant planets may contract much more quickly than has been assumed. Another is that the planets are extremely enriched in heavy elements, which include anything heavier than helium. If the exoplanets had 5–20 times more of those elements than planets in our solar system, their seemingly high contraction rate would be consistent with predictions. It’s also possible that they’re just strange; mass and radius predictions are based on statistical models, and the planets could be outliers. New observations of V1298 from the Transiting Exoplanet Survey Satellite and data from more young exoplanet systems will help resolve the apparent conflict. (A. Suárez Mascareño et al., Nat. Astron., 2021, doi:10.1038/s41550-021-01533-7 .)