Cruithne isn't actually another Moon of Earth it is rather weirder.
Cruithne is approximately 5 kilometres (3.1 mi) in diameter, and its closest approach to Earth is approximately thirty times the separation between Earth and the Moon(12 Gm or twelve million kilometres)., i orbit around the Sun in approximate 1:1 orbital resonance with the Earth. Thus, in relation to Earth it doesn't orbit around us but in a strange horseshoe shape, near us. It was discovered in 1986.
There has been a history of announced discoveries of new Moons. Not particularly surprising when you realise how unusual the real one is, being both enormous compared to its primary and alone.
The best known disappearing planet is Vulcan (no not that one|) which was assumed to be within the orbit of Mercury. The assumption was reasonable since it was based on calculations by the man who discovered Neptune by the same method.
In the early 1840s the brilliant young mathematician Urbain Le Verrier (1811-71) spent months carrying out painstaking calculations (by hand with ink and paper), analysing the position of Uranus to pinpoint where the hypothetical planet’s effect on Uranus began. (At the same time as he was investigating the anomalous motion of Uranus in the distant outer darkness, he was also studying the innermost Solar System which had a mystery of its own.)
By 1846, Le Verrier completed his calculations, which showed that the planet ought to lie between Aquarius and Capricorn, and posted them to the Berlin Observatory (no French observers were interested, such was the novelty of Le Verrier’s reasoning). Within a day of receiving Le Verrier’s missive German astronomers led by Johann Galle (1812-1910) used his calculations to discover the no longer theoretical planet on 23 September 1846. It was with 1° of the calculated position. The planet was duly named Neptune and Galle and Le Verrier were lauded as the planet’s co-discoverers...
Buoyed by this triumph, he further investigated the inner Solar System anomaly alluded to earlier. Sometimes Mercury wasn’t where it should be! The orbits of the planets could be predicted perfectly- apart from Mercury’s.
Planetary orbits are not circles, but ellipses and Mercury’s orbit was the least circular (or most “eccentric”) known. Mercury varies from as close as 46 million km from the Sun (“perihelion”) before climbing as far as 70 million km from the Sun when it is at the opposite side of its orbit (“aphelion”). This was all in accordance with the laws of planetary motion discovered by Kepler and explained by Newton. The problem was that Mercury kept reaching the low point of its orbit early and the high point late. The discrepancy was tiny (so tiny as to invoke awe for the observational skills of the astronomers of the period who measured it) but disconcerting. Could Newton have been wrong?
Le Verrier said not, but after thoroughly studying observations of Mercury going back to the 1600s, he also proved that Mercury’s odd shifting orbit was not due to the gravitational pulls of Venus, Earth and Mars. Instead a previously unseen planet between Mercury and the Sun must be to blame.
Vulcan was discovered transiting the Sun in March 1859 by a physician and amateur astronomer named Edmond Modeste Lescarbault (1814 -94)....Le Verrier formally announced Vulcan’s discovery in March 1860. In France Le Verrier and Lescarbault were hailed as heroes
In fact it didn't exist. The even more incredible truth was that Newton could be wrong. These same calculations were, much later, part of Einstein's proof of Relativity.
Finally it turns out that Earth has a Trojan satellite.
Lagrangian points, lie along a planet's orbit — one ahead of the planet and one behind it. Drawing straight lines between the Earth, the sun and a Lagrangian point produces a triangle whose sides are equal in length. An asteroid there would hover in the sky at a 60-degree angle from the sun.
Any object that close to the sun would be difficult to see from Earth because it would be overhead mostly during broad daylight, as invisible as the stars.
But Martin Connors, a space scientist at Athabasca University in Alberta, Canada, had an idea. Maybe NASA's Wide-Field Infrared Survey Explorer, which aims its lens 90 degrees away from the sun, would be able to pick up an oddball Trojan with an eccentric orbit.
Indeed it did. Connors found one candidate whose strange path over six days in late 2010 seemed to match the unevenly elongated orbit typical of Trojans. His team confirmed the Trojan's identity by spotting it a few months later with another telescope in Hawaii.
"This is pretty cool," said Amy Mainzer, a scientist at the Jet Propulsion Laboratory who wasn't involved in the study, which was published online Wednesday (ie 27th July 2011) by the journal Nature. "It's a new class of near-Earth object that's been hypothesized to exist."
And if more Trojan asteroids can be found, researchers said, they could be ideal for astronaut visits and the mining of precious resources. (This particular asteroid is too tilted with respect to the solar system to make a good candidate, Mainzer said.)
Cool indeed. If there is one there are going to be more. For a long time authors, from Gerry Anderson to John Norman have speculated about a planet on the opposite side of Earth's orbit and thus permanently behind the Sun. Actually that would not be a mathematically stable point and the perturbations of Jupiter would eventually move it out of that position. Depending on the planet's mass (Trojan points are only stable for masses much smaller than the parent body) the effect might be spectacular.
However with the technology we have now we are perfectly capable of building L5 settlements out of asteroids. We may not have found Earthlike planets but we can build Earthlike settlements.