A young progenitor for the most common planetary systems in the Galaxy

Abstract

The Galaxy's most common known planetary systems have several Earth-to-Neptune-size planets in compact orbits. At small orbital separations, larger planets are less common than their smaller counterparts by an order of magnitude. The young star V1298 Tau hosts one such compact planetary system, albeit with four planets that are uncommonly large (5 to 10 Earth radii). The planets form a chain of near-resonances that result in transit-timing variations of several hours. Here we present a multi-year campaign to characterize this system with transit-timing variations, a method insensitive to the intense magnetic activity of the star. Through targeted observations, we first resolved the previously unknown orbital period of the outermost planet. The full 9-year baseline from these and archival data then enabled robust determination of the masses and orbital parameters for all four planets. We find the planets have low, sub-Neptune masses and nearly circular orbits, implying a dynamically tranquil history. Their low masses and large radii indicate that the inner planets underwent a period of rapid cooling immediately after dispersal of the protoplanetary disk. Still, they are much less dense than mature planets of comparable size. We predict the planets will contract to 1.5-4.0 Earth radii and join the population of super-Earths and sub-Neptunes that nature produces in abundance.

Figures (3)

• Figure 1: Transit timing variations in the V1298 Tau system. Top left, points show the transit times of planet c measured against a reference linear ephemeris; error bars represent 1$\sigma$ uncertainties. Grey curves show credible transit times drawn from the $N$-body models described in the text. Bottom left, same as above but for planet d. The interactions between c and d are nearly sinusoidal and anticorrelated. Top and bottom right, the same but for planets b and e. The TTVs of b and e are also sinusoidal and anticorrelated.
• Figure 2: Planetary radius versus orbital period and planetary mass.a,b, Planetary radius versus orbital period (a) and planetary radius versus planetary mass (b) for the V1298 Tau system (red filled circles); error bars represent 1$\sigma$ uncertainties. The low-density planets of the Kepler-51 system are shown for comparison (purple squares), along with kernel density estimates of the distributions of well-characterized exoplanets (shaded contours), drawn from the NASA Exoplanet Archive (n=624 planets with mass and radius uncertainties less than 20%, P $<$ 150 days, and host $T_{\mathrm{eff}}$ = 4500–6500 K to exclude M dwarfs). The parameters of the Kepler-51 planets were sourced from the 'outside 2:1' solution in Table 6 of Masuda2024. Theoretical radius evolution tracks from Poppenhaeger2021 are shown as vertical dashed lines. The terminal radii at 5 Gyr from that work are shown as open triangles. The colour indicates the assumed core mass (red for 5 $M_{\oplus}$ and black for 10 $M_{\oplus}$). The orientation represents the stellar extreme-ultraviolet activity level (upwards for high activity, downwards for low activity). The black dashed line in a depicts the observed location of the radius valley vanEylen2018. Theoretical mass–radius relations for different planet compositions from Zeng2019 are shown in b as dashed lines. Grey dotted lines indicate theoretical mass-radius relations for Earth-like cores with H/He envelopes with various mass fractions from LopezFortney2014, calculated for an age of 100 Myr and an insolation of 10 F$_\oplus$.
• Figure 3: Posterior distributions for the initial properties of the V1298 Tau planets. The posteriors are derived by applying the planetary evolution and mass loss framework of Owen2020 to our measured masses and radii for planets c (red), d (orange), b (green), and e (blue). Left, initial envelope mass fraction versus core mass. Right, initial Kelvin-Helmholtz cooling timescale versus core mass. Contours show the 1$\sigma$ and 2$\sigma$ credible regions. (Note that the jagged appearance of some contours is a numerical artefact of the discrete core mass grid used in our analysis; see Methods for more details.) The vertical dotted line in the right panel at 10 Myr marks the approximate upper limit for standard high-entropy formation models. These models are strongly disfavoured for the inner planets c and d, whereas for the less-irradiated outer planets b and e, the method lacks the statistical power to distinguish between high- and low-entropy scenarios.