Voyager 1 at the speed it moved outward over the year to 2026-10-25 (JPL Horizons). Driving: nonstop, never refuelling.
How it was found
First planet found in 2017, by transit (NASA Exoplanet Archive).
Its planets
K2-138 c — 2.3 times Earth's width, a 'sub-Neptune' — bigger than Earth, smaller than Neptune; its year lasts 4 days; about 740 °C, hotter than boiling water; found in 2017 by transit.
K2-138 d — 2.4 times Earth's width, a 'sub-Neptune' — bigger than Earth, smaller than Neptune; its year lasts 5 days; about 620 °C, hotter than boiling water; found in 2017 by transit.
K2-138 b — 1.5 times Earth's width, about Earth-sized — probably rocky; its year lasts 2 days; about 880 °C, hotter than boiling water; found in 2017 by transit.
K2-138 e — 3.4 times Earth's width, a 'sub-Neptune' — bigger than Earth, smaller than Neptune; its year lasts 8 days; about 480 °C, hotter than boiling water; found in 2017 by transit.
K2-138 g — 3 times Earth's width, a 'sub-Neptune' — bigger than Earth, smaller than Neptune; its year lasts 42 days; about 170 °C, hotter than boiling water; found in 2021 by transit.
K2-138 f — 2.9 times Earth's width, a 'sub-Neptune' — bigger than Earth, smaller than Neptune; its year lasts 13 days; about 380 °C, hotter than boiling water; found in 2017 by transit.
NASA Exoplanet Archive, Planetary Systems Composite Parameters (fetched 2026-11-02). Composite values: where a planet's size or mass was not measured, the archive estimates it from the other.
This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with NASA under the Exoplanet Exploration Program.