Gliese 3378b: A Super-Earth 25 Light-Years Away That Astronomers Want a Closer Look At

On 1 July 2026 a team of astronomers announced they had found a planet roughly 2.3 times the mass of Earth orbiting a red dwarf 25 light-years away. Its name is Gliese 3378b, also catalogued as GJ 3378b, and it sits inside its host star habitable zone, receiving about 90 percent of the radiation Earth receives from the Sun. On paper that is remarkably close to our own situation. Whether it means anything at all depends on a question nobody has answered yet.

The question is whether the planet has an atmosphere. Everything else about it, the promising mass, the tidy orbit, the flattering energy budget, stays provisional until that one is settled.

How Gliese 3378b was found

The discovery team was led by Dr Michael Endl of the University of Texas at Austin and Dr Paul Robertson of the University of California, Irvine, working with colleagues. They used two instruments: the Habitable-zone Planet Finder on the Hobby-Eberly Telescope at McDonald Observatory in Texas, and the NEID Spectrometer on the WIYN Telescope at Kitt Peak National Observatory in Arizona.

Both are precision spectrographs, and both were doing the same job, which is measuring radial velocity. The method rests on a simple physical fact. A planet does not really orbit its star so much as the two of them orbit a shared centre of mass, which means the star traces a small circle of its own. Seen from Earth, the star creeps very slightly towards us and then very slightly away, over and over, and that motion shows up as a rhythmic shift in the wavelengths of its light.

Pick that rhythm out of the noise and you get the orbital period and a figure for mass. Here the period came out at 21.45 days and the mass at about 2.3 times Earth. Using two instruments at two observatories was not redundancy for its own sake. Red dwarfs are restless stars, and starspots and flares can counterfeit the signal of a planet. Independent confirmation is how you tell one from the other.

What a super-Earth is, and why the label is slippery

A super-Earth is a planet more massive than Earth but lighter than the ice giants. That is a definition by mass alone, and it says nothing about what the thing is made of. A world in that range might be rocky with a thin skin of air. It might be rocky under a crushing envelope of hydrogen. It might be a water world. The category is a bracket, not a description.

Super-Earths also have no representative in our own solar system. There is nothing between Earth and Neptune here, so every expectation about how such a world behaves comes from theory and from observing distant examples. At 2.3 Earth masses this one sits in the lower part of the range, on the side that models generally expect to be rocky. Generally expect is not the same as know.

The star is the complication

Gliese 3378 is a red dwarf. Red dwarfs are the most common type of star in the galaxy, which by itself makes them worth studying, and they are extremely long lived, burning slowly enough to outlast stars like the Sun many times over. If life needs deep time, red dwarf systems have more of it on offer than anywhere else.

The catch is that they can be violent. Red dwarfs are often magnetically active and prone to flares, and because their habitable zones sit close in, any planet inside one takes that activity at short range. Over hundreds of millions of years, that steady battering can strip gas from a planet upper atmosphere and throw it out into space.

The habitable zone itself is a narrower idea than the name suggests. It is the orbital range where liquid water could exist on a surface, given a suitable atmosphere. That last clause carries the entire argument. Take away the atmosphere and the habitable zone is just a distance.

Why the atmosphere question decides everything about Gliese 3378b

The planet sits at what astronomers call the cosmic shoreline, and it is a useful image. On one side are worlds that have held onto their air. On the other are worlds that lost it. The line between them is drawn roughly by the balance between a planet gravity, which holds gas down, and the radiation it receives, which knocks gas loose.

Planets near that boundary could plausibly fall either way, and this one is near it. The comparison astronomers reach for is Mars, which is thought to have had a thicker atmosphere long ago and to have lost most of it, leaving a cold, thin, dry world where the chemistry of life would have a very hard time. A planet of 2.3 Earth masses has considerably more gravity than Mars, which helps. Sitting close to an active red dwarf does not.

So the 90 percent figure has to be read carefully. A world receiving about 90 percent of the radiation Earth gets from the Sun is in the right energy neighbourhood, but energy received is not the same as energy retained. With no atmosphere to trap heat and hold pressure, a planet in that position would be bare surface swinging between extremes, with nothing to keep liquid water stable. The radiation figure describes the delivery, not what happens afterwards.

What comes next for Gliese 3378b

Radial velocity finds planets and weighs them. It does not photograph them and it cannot read their air. Working out whether this world has an atmosphere is a separate observational problem needing different techniques and, in all likelihood, a great deal of telescope time competing against everything else astronomers want to point at.

Its main advantage is distance. Twenty five light-years is close by the standards of exoplanet astronomy, which is exactly why systems like this become priority targets. The nearer the system, the better the odds that a future instrument can gather enough light from it to say something definitive.

Common questions

How far away is Gliese 3378b?

Twenty five light-years, in the constellation Camelopardalis. That counts as nearby in astronomical terms, though it remains far beyond the reach of any spacecraft that exists or is seriously planned.

Is Gliese 3378b habitable?

Nobody knows. It sits inside the habitable zone of its star and receives about 90 percent of the radiation Earth receives from the Sun, but the habitable zone only describes an orbital range where liquid water could exist given a suitable atmosphere. Whether this world has one is unresolved.

What does a 21.45 day orbital period tell us?

That the planet goes around its star in a little over three weeks, which puts it far closer to its star than Earth is to the Sun. Red dwarfs are cool and dim, so their habitable zones sit much nearer in. A very short year is what a temperate orbit looks like around that kind of star.

How was it detected?

By radial velocity, using the Habitable-zone Planet Finder on the Hobby-Eberly Telescope and the NEID Spectrometer on the WIYN Telescope. Both measure the small wobble a planet induces in its star by tracking shifts in the star light.

A test case rather than a destination

It is tempting to read announcements like this one as a countdown to finding somewhere else to live, and that framing misses the real value. Gliese 3378b is useful to astronomers as a test, not a destination. It sits at a boundary where theory makes a specific prediction and observation can go and check it.

If a planet this size, receiving this much radiation, from a star this active, turns out to have kept its atmosphere, that says something important about how durable air is around the most common stars in the galaxy. If it turns out to be bare rock, that is nearly as informative and considerably more sobering, because it narrows down where anyone should bother looking next.

Either answer moves the field forward. That is what makes a nearby world sitting at the edge of the shoreline worth the telescope time, whatever is or is not wrapped around it.