Scientists were looking for objects on the fringes of the solar system previous year when they pointed their telescopes close to Jupiter's backyard, according to Scott Sheppard of the Carnegie Institute for Science in Washington.
The tiny moon's orbit takes it both inside and outside of where the other new moons orbit, putting it at a high risk of colliding with them.
The moonlets mostly follow Jupiter's known patterns: Located far beyond the planet's large primary moons (purple), two of the new moonlets belong to a grouping (blue) that spins in the same direction as the planet, all of which are believed to be the fragments of one large shattered moon.
The most interesting of the new moons is Valetudo (pronounced val-eh-TOO-doh), named after the ancient Roman god Jupiter's great-granddaughter, the goddess of health and hygiene.
The team is calling one of the new moons an "oddball" because of its unusual orbit.
Jupiter's newly-discovered moons are all less than two miles wide, which may explain why they've only just been discovered now.
Twelve new moons orbiting Jupiter - one may collide with the others
We've got a nice little moon orbiting Earth, haven't we?
A moon is defined as any object, regardless of size, that orbits a planet, not the Sun. If small moons like these were around when the solar system was still thick with gas and dust, drag forces would have slowed them down and caused them to fall into Jupiter, never to be seen again.
The chance find brings Jupiter's tally of moons to 79, 17 more than Saturn, the planet with the second most.
For example, the discovery that the smallest moons in Jupiter's various orbital groups are still abundant suggests the collisions that created them occurred after the era of planet formation.
"Our other discovery is a real oddball and has an orbit like no other known Jovian moon", said Scott Sheppard of the Carnegie Institution for Science, who led the discovery team.
These astronomers said they came upon the new moons while searching the outer solar system for evidence of Planet Nine, a large, as-yet unseen planet thought by some scientists to exist in the far outer reaches of the solar system, far beyond Pluto. The 7 new retrograde moons join 45 other satellites that take 2-3 years to orbit.
Due to their sizes-one to three kilometers-these moons are more influenced by surrounding gas and dust. While the retrograde moons orbit in a clockwise direction, Valetudo travels in a counter-clockwise direction. Because the planet is so big and bright, researchers surmised that unrecorded moons could be faint, or even obscured, or quite far from the gas giant. Given the moons' stable orbits and kilometer-scale sizes, the collisions were likely chance events later in the solar system's history.
"If we do find this planet in the next few years, it would be a pretty incredible discovery for astronomy". Since they are still around, now, that means they formed after that gas and dust had been swept away by the solar wind.