For The First Time, A Starless Wandering Planet Has Been Precisely Measured

This is the culmination of a long-term hunt. In a study published on January 1, 2026 in the journal Sciencean international coalition of astronomers (Peking University, University of Warsaw and Ohio State University) has drawn up a sketch of a celestial object that haunted the theories of astrophysics for more than 30 years.

For the very first time, they calculated the exact mass and distance of a wandering planeta solitary star located 10,000 light years from us. With no sun to warm it, it measures about the size of the great Saturn and wanders the void of space. Since this star is invisible to our instruments, researchers were forced to trick with the laws of physics to achieve their ends.

Orphan planets: billions of ghosts

If this announcement has the effect of a small bombshell in the scientific community today, it is because we have suspected the existence of these exiled worlds since the 1990s. At the time, the first numerical models of planetary formation already suggested that young star systems were real arenas. In these chaotic areas of the cosmos, many protoplanets (planetary embryos) are condemned to be ejected by violent gravitational interactions.

A large planet can brush against another, smaller one, modifying its trajectory to the point of propelling it definitively outside the gravitational influence of its star. This is why they are called wanderers : because they are deprived of the anchorage of a star system. They drift, without any source of external thermal radiation, which made them, until now, virtually undetectable for our instruments.

We also knew that they were very numerous within our Milky Way, probably much more than the stars that populate our galaxy. According to Andrzej Udalski, an astronomer at the University of Warsaw Observatory and co-author of this study, estimates suggest that there is at least one wandering planet for every star in our galaxy. Some projections even put forward the figure of 10 to 20 vagrants per sun.

The trick of the loupe » d’Einstein

If they swarm by the billions, why have we never managed to focus our instruments on them and fully detect them for three decades? As explained in the introduction, the reason is purely physical: they do not emit any radiation.

In astronomy, we generally detect objects in one of two ways: either because they shine (like stars), or because they block the light of what shines behind them (like exoplanets). As a wandering planet is a thermally dead body on the surface, whose temperature is around absolute zero and without a nearby star to illuminate it, it therefore does not reflect any light. It doesn’t cause mini-eclipses either, since it doesn’t orbit around anything: looking for a shard of coal measuring a few millimeters in a perfectly black cellar would almost be simpler!

To get around this insurmountable obstacle, researchers had to rely on a property of the universe theorized by Albert Einstein: gravitational microlensing. As we cannot capture the light emitted by the planet, we are therefore forced to observe how its mass deforms its surrounding space.

According to the laws of general relativity, the more massive a body is, the more it bends space-time. When a wandering planet passes exactly between Earth and a distant star, it acts as a natural converging lens. Its gravity deflects and focuses light rays from the background star toward our instruments, causing a sudden burst of brightness.

But even if we capture this small flash of light, it can just as easily come from a small object close to us as from a giant star located at the edge of the galaxy. This is the whole problem with microlensing: the lack of perspective. A light, slow object can produce exactly the same light signature as a heavy, fast object. Until this publication, we were still in the same impasse, but the team behind it has achieved a real masterstroke.

OGLE, KMTNet and Gaia: the alliance of giants

They synchronized three of the world’s most powerful detection networks: the ground-based programs OGLE (based in Chile) and KMTNet (spread across three continents), with the eye of the European Space Agency’s Gaia satellite. This, to create a form of binocular vision of the solar system where this famous wandering planet was located. The OGLE and KMTNet ground-based telescopes recorded the flash from Earth; while Gaia captured the same signal from its observation post located at the Lagrange point L2, 1.5 million kilometers from us.

This allowed them to measure the parallax of the microlens, that is to say the difference in the apparent position of the object seen from two distinct observation points. By observing it from two radically different perspectives, astronomers were able to triangulate the object’s trajectory and, above all, precisely determine its mass and distance.

Using this method, astronomers were finally able to “ see »: a real planet, located 10,000 light years awaywith a mass equivalent to a fifth of that of Jupiter. Luck was still on their side, as Andrzej Udalski explains: “ Gaia only scans the same region of the sky every 30 days or so. In our case, it was pointing exactly at the right place precisely during the 16 hours that the peak of brightness lasted “. An extraordinary temporal synchronization which made it possible to draw his portrait, albeit summarily.

« For the first time, we have directly measured the mass of a body likely to be a wandering planet, thus bypassing simple statistical models. “, explains Subo Dong, researcher at Peking University and lead author of the study. “ We now know with certainty that this object is indeed a planet », he continues. Probably ejected from its native system, it has since started an eternity a lonely drift in the darkness of the Milky Way.

It’s already a historic event for astrophysicsbut when the future Nancy Grace Roman space telescope comes into operation in 2027, the hunt for vagrants will move up a gear. Equipped with a field of view 100 times greater than that of Hubble, it is specially designed to detect, at an industrial rate, the real population of these billions of planets in the Milky Way. This discovery was only a small taste, and it is now certain that the sky still hides from us a multitude of these castawayswhich we will be able to identify in areas which appeared to us, until recently, completely deserted.