Sometimes an astronomical discovery is made not at the telescope but on second inspection. The exoplanet GJ 3378b, which orbits a red dwarf star in the constellation Camelopardalis (the Giraffe), was tracked down in 2024 by French astronomers using the Canada-France-Hawaii Telescope on Mauna Kea and classified as a mini-Neptune — a largely gaseous world, in other words, on which life would be scarcely conceivable. A team at the University of California, Irvine has now remeasured it. The result moves the planet into an entirely different category: it is a rocky world in its star's habitable zone, some 25 light-years from Earth.
Two corrected numbers made the difference. The original measurement had credited GJ 3378b with 5.26 Earth masses; the new observations with the Habitable-zone Planet Finder on the Hobby-Eberly Telescope at the McDonald Observatory in Texas and the NEID spectrometer on the WIYN Telescope at Kitt Peak in Arizona yielded only 2.3 Earth masses at roughly 1.3 Earth radii — a super-Earth. At the same time the orbital period shrank from 25 to a little over 21 days. That places the planet closer to its star than assumed, receiving about 90 percent of the radiation Earth gets from the Sun.
"This one's exciting. It's one of our closest cosmic neighbors," says Paul Robertson of UC Irvine. "Twenty-five light years sounds like a long way, but the Milky Way is about 100,000 light years across."
The open question is the atmosphere
Why this matters beyond a single planet: red dwarfs make up roughly 70 percent of all stars in the galaxy, as Michael Endl of the University of Texas at Austin points out — understand their planets and you understand the cosmic norm. Yet these same stars hurl out fierce gusts of radiation capable of tearing away a planet's gaseous envelope. GJ 3378b sits precisely on the "cosmic shoreline," the boundary beyond which an atmosphere is lost. Mars is the example on our own doorstep: it probably once had an Earth-like envelope, later destroyed by the Sun.
Whether GJ 3378b has one cannot yet be settled. Seen from Earth, the planet does not pass in front of its star; it was detected solely through the gravitational wobble it imposes on the star, betrayed as a Doppler shift in the starlight. Only the Habitable Worlds Observatory, which NASA plans for the 2040s, could examine the envelope directly.
Robertson illustrates what is at stake: shrink the Earth to the size of an apple and its atmosphere would be about as thick as the apple's skin — just enough to maintain the kind of surface pressure at which liquid water can exist. Only once such an envelope is confirmed, the team says, is the search for biosignatures worth the effort. The study appeared in The Astrophysical Journal.