|The original, and still the champ.|
It also gives me a perfect excuse to insert this QI clip...
The definition of a moon is "... a celestial body that orbits another celestial body of greater mass (e.g. a planet, star, or dwarf planet), called its primary". It doesn’t even have to be round. Earth definitely only has one that fully fits that definition. But, as well as the Moon, Earth has a loose family of tiny worlds that keep it company…
Tag along (co-orbital) asteroids:
Above: The weird path of a co-orbital asteroid, relative to Earth.
The best known of this group is an asteroid called 3753 Cruithne. Cruithne has an orbit which crosses ours and takes one year like ours, so it’s always in a predictable position relative to us and is influenced by Earth’s gravity… but doesn’t orbit us (it seems asteroids can have comet-ment problems … geddit?*). From our perspective it follows a funny little horseshoe path, and so do a handful of other asteroids…
Above: 2016HO3's quasi-orbit of Earth.
These have been known about for a while, and the new find fits into this category. The last one before this drifted away about ten years ago... Which is why they get called ‘quasi’ moons: Although they are connected to Earth gravitationally, and follow an orbit of sorts about the planet, they’re not permanently bound to Earth. Over time they wander away, and new ones replace them. 2015 HO3 is one of the more stable ones, having been looping around Earth for over a century and set to stay for the foreseeable ffuture.
|Above: A map of Jupiter's mysterious Trojan asteroids.|
Because the planets move about the Sun the Lagrange points do to, dragging their asteroid occupants with them. So 2010TK7 is under the influence of Earth’s gravity, and has a stable relationship with Earth, but also isn’t in an orbit around us.
|Above: Triton, Neptune's weirdest moon - one which was once a planet in its own right.|
There are other worlds that have captured a moon - Neptune and Mars have both done it - when a wandering world happened to stumble into that planet's gravity at the right angle and speed to enter a stable orbit. And, with plenty of objects out there it’s easy to imagine that one day Earth could pick up a second Moon in this way.
So, how many moons do we have?
One… but Earth also has an entourage of other hangers on, and ‘one’ is only the situation today….
* Seriously Science, I'm getting you a book of baby names or something.