The European planet-hunter Corot has spotted an object orbiting a star that is quite unlike anything seen before.
It is about the size of Jupiter, but packs more than 20 times the mass.
Scientists say they are not sure if the object is a planet or a type of failed star known as a brown dwarf - neither description really seems to fit.
The Corot orbiting observatory detected the object by the way it dimmed the light of its parent star as it moved in front of its disc, as seen from Earth.
The odd-ball has been designated Corot-exo-3b. It takes just four days and six hours to orbit its parent star, which is slightly larger than our Sun.
The object's size, close proximity and short orbit made it easier to detect.
"We were taken by surprise when we found this massive object orbiting so close to its parent star," commented Dr Magali Deleuil from the Laboratoire d'Astrophysique de Marseille (LAM), who led the discovery team.
She added: "Corot-exo-3b is really unique - we're still debating its nature."
Planet-hunters have found "big Jupiters" before; and they have found plenty of small, failed stars - balls of gas that do not have sufficient mass to initiate the nuclear reactions that drive sunlight.
But Corot-exo-3b is an in-between object. As a planet, it would be the most massive and the densest found to date - more than twice as dense as lead.
"Corot-exo-3b might turn out to be a rare object found by sheer luck," said team-member Dr Francois Bouchy, from Institut d'Astrophysique de Paris (IAP).
"But it might just be a member of a new-found family of very massive planets that encircle stars more massive than our Sun. We're now beginning to think that the more massive the star, the more massive the planet," he said.
Corot is a co-operative project between the French space agency (Cnes) and international partners including Esa (European Space Agency), Austria, Belgium, Brazil, Germany and Spain.
It was launched in 2006 and placed in a vantage point some 830km above the Earth.
Using its 27cm aperture telescope, it is observing many tens of thousands of stars, looking for the slight drop in light caused by the transit of a planet.
This is a rare event - it relies on the chance alignment of the star and the planet with Earth.
Corot's instrumentation is also designed to detect the subtle variation in a star's light caused by sound waves rippling across the surface. These waves are the equivalent of seismic waves on the Earth.
By studying these "starquakes", astronomers can gain a detailed insight into the internal conditions of the star.
The latest results are due to appear in the journal Astronomy and Astrophysics.