A team using the European Southern Observatory’s Very Large Telescope in Chile has identified the strongest candidate yet for a moon outside the Solar System. The object, at least 0.9 times the mass of Jupiter, circles the brown dwarf CD-35 2722 B every 170 days at a distance of roughly one-fifth the Earth–Sun separation. Detected via radial-velocity wobbles with the CRIRES+ spectrograph, the finding was published in Nature on 22 July and marks the first time the exoplanet-hunting technique has yielded evidence of a satellite around a substellar body.

The discovery immediately unsettles existing nomenclature. The host is a brown dwarf—too massive to be a planet, too light to sustain fusion—while the companion itself is planet-sized but orbits a non-stellar object. “Being the third wheel in this system makes us want to call it a moon, even if it is nothing like the small, rocky moons we have in our system,” said lead author Kevin Hoy of Universidad Diego Portales and the Millennium Nucleus of Young Exoplanets and their Moons. Researchers in Santiago and Padua, where co-author Alice Zurlo originates, are therefore using the provisional term “exosatellite” until a formal definition is agreed. The system, just 71–73 light-years away, challenges the tidy hierarchy that separates stars, planets and moons.

Elsewhere in the galaxy, a different kind of anomaly is emerging. British astronomers at the University of Warwick have confirmed that V445 Puppis, the Milky Way’s only known helium nova, is a white dwarf accreting material from a rare helium companion star. After a dust shell cleared two decades after the 2000 outburst, the team observed clumps of oxygen-rich gas shooting outward at up to 20 million miles per hour. “The origin of these ‘bullets’ is a mystery,” said PhD student John Mills. “Bullets of this kind have not been observed in any other nova.” The system is a potential progenitor of Type Ia supernovae, the standard candles used to measure cosmic distances.

Closer to home, Earth itself delivered a surprise on 22 July, when the planet completed one rotation in slightly less than 86,400 seconds, making it one of the shortest days since atomic-clock records began in 1960. The International Earth Rotation and Reference Systems Service attributes the acceleration to a combination of Chandler wobble variations, polar ice melt redistributing mass toward the equator, and fluid motions in the outer core. Meanwhile, observations of the exoplanet LHS 1140 b, a rocky world 48 light-years away, confirm it retains an atmosphere after roughly 3 billion years, detected via helium escape signatures. Harvard’s Robin Wordsworth noted the finding answers a key question about whether terrestrial planets in habitable zones can hold onto their gaseous envelopes.

All four lines of inquiry will sharpen as next-generation instruments come online. The Extremely Large Telescope, with its 39-metre mirror, is expected to resolve smaller exomoons and probe exoplanet atmospheres in finer detail, while continued monitoring of Earth’s rotation will determine whether a negative leap second is required for the first time.