There has been quite a lot of news coverage suggesting that astronomer have, for the first time, discovered planets in another galaxy. It’s from a paper by Xinyu Dai and Eduardo Guerras called Probing Planets in Extragalactic Galaxies Using Quasar Microlensing.
It uses gravitational lensing, which is a consequence of Einstein’s Theory of General Relativity, which I discussed previously in this post. General Relativity proposes that objects with mass distort spacetime, which then manifests itself as gravity (i.e., objects with mass will fall towards each other in this curved spacetime). Light too is bent if it passes near massive bodies. Therefore, objects with mass can act as lenses, amplifying the light from more distant objects and, in some cases, producing multiple images of these objects.Gravitational lensing has actually been used to detect planets in our galaxy for quite some time now. Known as microlensing, an example is shown in the figure on the right. If two stars happen to come into alignment (when viewed from the Earth) the nearer star can act as a lens, focusing some of the light from the more distant star, causing it to appear to get brighter. As they move closer and closer into alignment, the amplification increases, and then decreases again as they move apart.
However, if the nearer star (lens) happens to have a planetary companion, it can cause an additional amplification. This is the small blip, about 10 days after 31 July 2005 in the figure on the right, that lasts about a day. This can then be analysed to infer the properties of the planet. In this case, the planet is about 5.5 Earth masses and orbits about 2.6 times further from its star than the Earth is from the Sun.
What they’ve done in this recent paper is related, but somewhat different. In this case, the lensing is of a quasar (the active nucleus of a very distant galaxy) by a nearer galaxy. This event actually produces multiple images of the distant quasar. There also appears, however, to be some variability which they explain as being due to planets in the lens galaxy.
Now, this is certainly plausible. We know there are planets in our own galaxy, so it’s not really a surprise that there are planets in other galaxies. A problem I have, though, is that even if this is a plausible explanation, it seems to be quite a long way from being an actual discovery (and, yes, they appear to use discover in their own press release). They’re also not really discovering specific planets, they’re suggesting that there has to be a population of planets, in addition to a population of brown dwarfs (stellar objects that don’t ignite fusion in their cores), and a population of stars. There are quite a large number of parameters.
Also, these need to either be free-floating planets, or planets on very wide orbits around their parent stars; if they were too close to the star, the influence of the star would dominate over that of the planet. Again, this is certainly possible; we’re aware of free-floating planets in our own galaxy. However, their analysis suggests something like 2000 planets, with masses between that of the Moon and Jupiter, per star. If we assume that planets typically form in discs around stars, then this would seem to be a rather large number of planets per star. Yes, some stars will have formed planets and then died, but I don’t think this is enough to explain these numbers.
Observations in our own galaxy do suggest that most stars host a planet of some kind, and that mult-planet systems are indeed reasonably common. However, we don’t really expect there to be thousands of planets per star. Also, only a fraction of those planets that form in a disc are likely to be ejected – maybe 20-40%. As a rough estimate, we might expect there to be a similar number of free-floating planets, as bound planets – something like a few per star.
Okay, so maybe my title is a little unfair, and maybe I’m missing something. I do think it’s great when people try to find innovative ways to explain observations. However, I also think it’s important to not make claims that are stronger than are warranted (may have found evidence for, rather than discovered). Maybe the only plausible explanation for these observations really is that there are thousands of free-floating planets per star in this galaxy, but I think I’m going to need a bit more convincing. To be clear, I have no real doubt that there are planets in this galaxy (would be very surprised if there weren’t) I’m just not convinced that this analysis has really demonstrated their existence.