Astronomers Scott Sheppard, David Tholen and Chadwick Trujillo were not hunting for new Jupiter moons. The trio were conducting a survey looking far beyond Jupiter and past the Kuiper Belt. They were looking for distant objects including new planets in the outer reaches of the solar system.
In March 2016 and 2017, Jupiter happened to be in the area they were looking in. Images captured by telescopes in Chile and Hawaii showed many of the moons we already know about. These include moons dubbed “lost moons.” A “lost moon” is a satellite that was observed previously, but not enough orbit data is known to accurately predict where they are now. Several of them were discovered in 2003.
The observations show five of these “lost” moons that link back to the 2003 observations. But two are entirely new. S/2016 J1 and S/2017 J1 were observed in March 2017 and March 2017. The astronomers double checked the data to make sure the moon pair were not part of the 2003 lost moon set. They were not. Jupiter officially has two extra moons.
Shepard, Tholen and Trujillo can’t rule out a few more in their observation data. But more observations will be needed in March 2018 to confirm if these are new moons or part of the “lost” set.
So what do we know about this new moon pair? Both are only about 1-2 kilometers in size. There’s a reason we haven’t heard about them until now. All the big ones were discovered long ago. The only ones left undiscovered are much smaller.
The small size of both moons means they’re both irregular. They’re not big enough to form into round moons like the four Galilean moons.
S/2016 J1 and S/2017 J1 orbit about 21 million km and 24 million km from Jupiter.
The pair of satellites don’t look like much, but it does remind us just how much gravitational power Jupiter has. That 69 will probably go even higher next year. And who knows, maybe it’s just one of many discoveries that will include new distant planets.