最も一般的な惑星タイプの形成過程をついに解明（We finally know how the most common types of planets are created）

2026-01-07 カリフォルニア大学ロサンゼルス校（UCLA）

Astrobiology Center, NINS
Astronomers have now witnessed four baby planets in the V1298 Tau system in the process of becoming super-Earths and sub-Neptunes.

＜関連情報＞

• https://newsroom.ucla.edu/releases/we-finally-know-how-the-most-common-types-of-planets-are-created
• https://www.nature.com/articles/s41586-025-09840-z

銀河系で最も一般的な惑星系の若い祖先 A young progenitor for the most common planetary systems in the Galaxy

John H. Livingston,Erik A. Petigura,Trevor J. David,Kento Masuda,James Owen,David Nesvorný,Konstantin Batygin,Jerome de Leon,Mayuko Mori,Kai Ikuta,Akihiko Fukui,Noriharu Watanabe,Jaume Orell Miquel,Felipe Murgas,Hannu Parviainen,Judith Korth,Florence Libotte,Néstor Abreu García,Pedro Pablo Meni Gallardo,Norio Narita,Enric Pallé,Motohide Tamura,Atsunori Yonehara,Andrew Ridden-Harper,… Lorenzo Pino
Nature

Abstract

The Galaxy’s most common known planetary systems have several Earth-to-Neptune-size planets in compact orbits¹. 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)^2,3. 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.