How many planets are there in the Milky Way Galaxy? At least double the estimated number of a few weeks ago. A team of astronomers from the United States, Japan, and New Zealand announced the discovery of planets apparently untethered to any star. See story at
About a month before the paper was published in the journal Nature, astronomer Philip Yock from the University of New Zealand in Auckland contacted me about creating an image for them to accompany the press release announcing their discovery (see image below).
The technique they used to find the new planets was at least as interesting as the planets themselves. Gravitational microlensing takes advantage of the bending of light by objects between the observer and the observed. If the alignment is perfect, the light from the background object will be focused into a ring, a so-called Einstein ring. If the alignment is just a bit less than perfect, the background image will split into two images bracketing object doing the lensing. Close study of this split image tells the mass of the lens.
We don’t know ahead of time where gravity lenses will appear—it’s a chance alignment and you might have to look at millions of stars to find even one. That’s precisely what the astronomers did. They looked towards to galactic center, where millions of stars were on the field of view. Careful searching of this alignment where the gravity field of view found a few, chance alignments where the gravity lens was visible.
My picture shows one of the planets along with the lensed image of a yellow background star. The mass of the lens is similar to the mass of Jupiter, so the lens must be a planet, not a star. Yet no star is visible near the planet. While it is hard to put a precise figure on the distance to the planet, it is large, possible as large as 15,000 light years, more than halfway to the center of the galaxy, making it the most distant exoplanet yet discovered.
Were the astronomers incredibly lucky to find a rare, starless planet? That seems very unlikely. If they found any at all, statistics suggests that they are not very rare. If we found one, there must be many billions roaming interstellar space.
But since planets are thought o form around stars, how did one cut loose from its parent and head for interstellar space? There are in fact many mechanisms that could fling a planet out of its solar system including gravitational encounters with other planets or stars and the sudden disappearance of a star’s mass in a supernova explosion. If the planet was not itself destroyed, the sudden radical decrease in the mass of the central star could fling a planet like a stone from a sling.
So, fellow galactic citizens, the galaxy just got even larger and more interesting. What a time to be alive!