Habitable Or Barren? Nearby Planet Is Lighter Than Astronomers Thought

It’s Now Far More Plausible That This Exoplanet Is Earth-Like

A planet once written off as too heavy to be Earth-like has just gotten a dramatic downgrade, and that’s good news for scientists hunting for life beyond the solar system. Astronomers have revised its estimated orbit and minimum mass, cutting the planet’s heft nearly in half. The lighter world makes a rocky composition more plausible, and its shorter orbit still falls within the zone where liquid water could theoretically survive on a surface, assuming it has one at all.

While all of that is legitimately promising, the planet also sits almost exactly on what astronomers call the cosmic shoreline, a boundary separating worlds that can hang onto an atmosphere from those stripped bare by their star’s radiation. Without an atmosphere, a well-placed planet is likely just a dead rock. Nobody yet knows which side of that line this one falls on, and that uncertainty is what has researchers so interested.

Known as GJ 3378 b, the planet orbits a small, dim red dwarf star just 25 light-years from Earth. Small stars like this are prime hunting grounds for habitable planets, since their habitable zones sit close enough in that Earth-sized worlds can be detected. Detecting a planet and understanding it are different things, though, and its true size, composition, and atmosphere remain unknown.

The Planet’s Weight Just Got Cut in Half

When scientists first announced the discovery in 2024, they pegged its minimum mass at 5.26 Earth masses, with a year of roughly 24.73 days. A new study in The Astrophysical Journal, led by astronomer Paul Robertson and colleagues, overturned those figures using a far larger pool of data from four telescopes.

Robertson’s team found a shorter period of 21.45 days and a minimum mass of just 2.3 Earth masses, roughly half the original estimate. At that lighter weight, the planet is more plausibly rocky, though its true mass could still be higher, since this method only pins down a minimum, and its radius and composition remain unknown.

Turns out, the instruments were disagreeing with each other. The 2024 announcement relied solely on SPIRou, in Hawaii. Adding three more instruments, including the Habitable-zone Planet Finder in Texas and NEID in Arizona, produced a different result: a 21.45 day period showing up in every combination of three or more instruments. That earlier signal likely traces to undiagnosed interference in the SPIRou data.

A Stellar Wobble Reveals a Faster, Lighter Orbit

Astronomers found the planet using the radial velocity method, which measures tiny wobbles in a star’s motion from an orbiting planet’s gravity. As a planet tugs on its star, the star moves toward and away from Earth in a repeating rhythm, similar to how a siren sounds higher pitched approaching and lower retreating. That shift in starlight reveals a planet’s period and minimum mass.

Behind the study sits 137 observations from the Habitable-zone Planet Finder, plus 18 from NEID, 78 from CARMENES in Spain, and 170 from SPIRou, an unusually large dataset for one star. To rule out the star itself as the source, since starspots can mimic a planetary wobble, the team checked timing against activity indicators and found no connection. That signal held steady for years across independent instruments.

Researchers also checked NASA’s TESS telescope, watching the star during four windows, for a brightness dip signaling a transit. None turned up, so the planet’s radius is still unknown.

Sitting Right on the Cosmic Shoreline

With a revised mass and orbit, the team turned to a deeper question: could this world support life? Its estimated surface temperature, assuming no atmosphere, comes in just below the freezing point of water, near the inner edge of the zone where liquid water could theoretically survive. Whether it actually does depends entirely on an atmosphere to trap heat, something nobody has measured.

Red dwarf stars like this one blast nearby planets with radiation that can strip away atmospheres over billions of years. The team calculated that the planet has absorbed roughly 48 times the radiation Earth took in from the sun over its history, placing it directly on the cosmic shoreline. As the researchers write, “the presence of an atmosphere on this world cannot be ruled out based on simple scaling relationships.” They call the metric a rough guide, meaning its status remains genuinely open.

Bigger Telescopes Will Have to Settle the Question

Whether a closer look is possible soon remains open. Even future giant ground-based telescopes, the 30-meter-class observatories now being built, would find directly studying a planet this close to its star extremely challenging. It is the kind of nearby, possibly rocky habitable-zone world astronomers say is worth characterizing carefully regardless, since it could also affect what scientists learn about any other planets that might be sharing the same system.

This planet’s story shows that revising old data can matter as much as gathering new data. A world that seemed settled in 2024 looks entirely different two years later, not because telescopes saw farther, but because researchers combined more instruments. What emerged is a lighter, closer-orbiting planet that might be rocky, sitting on a line that could decide whether it holds an atmosphere.

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