Update: On the trail of an exomoon
I feel I need to do a tl;dr here: New observations have been made of an exoplanet that might have an exomoon. There are no results on whether the moon exists or not just yet; this is an update in the work.
From article, (We've found thousands of planets orbiting other stars, and they come in all sorts of flavors: Big, small, hot, cold, orbiting single stars, binaries, one star in a binary, and so on. But never has there been any evidence of a moon orbiting one of those planets.
The reason for this is pretty simple: They're ridiculously hard to observe. We detect most of these exoplanets using the transit method. From our point of view on Earth, the orbits of planets around other stars can be tilted every which way. But if the geometry is right, and we coincidentally happen to see that orbit edge-on, then once every exoplanet orbit it will pass directly in front of the star. This blocks a teeny amount of the starlight, usually 1% or less, but that's detectable using current tech.
Mind you, that 1% number is what you expect for a big planet like Jupiter blocking a Sun-like star (Jupiter is 1/10th as wide as the Sun, and the area blocked goes as the square of the ratio of their diameters). A smaller planet blocks less light, and that makes the dip in light harder to detect.
You can see (or can't see, I suppose, ha ha) where this is going. Moons are smaller than planets*, so they block even less light. A planet like Earth is hard enough to see using the transit method; our Moon is about ¼ the size of Earth and would block only about 7% as much light as Earth would, making it almost impossible to detect.
But, if you have a really big planet and it happens to have a really big moon, then maaaaayyyyyybe you could detect it.
The planet in question here is Kepler-1625b, which happens to be about the same size as Jupiter. When the astronomers plotted the brightness of the star, the planet's transit is obvious … but weird. It's ever-so-slightly misshapen, and different every time it was observed. They found that a moon about the size of Neptune orbiting the planet would explain the shape of the graph.
That's very cool! We may know in just a few months whether the data support the idea of this being an exomoon! Or, it may not. We'll see.
We know moons form easily around planets, especially big ones —Jupiter, Saturn, Uranus, and Neptune have well over a hundred between them, and they come in astonishing varieties. There's no doubt that exomoons outnumber exoplanets in the galaxy, probably by a factor of 10 or more. And there are probably hundreds of billions of exoplanets. That means in our galaxy alone there may be trillions of moons. Trillions.
[Astronomers] not quite ready to say anything just yet. He's very clear about that (even to the point of lamenting the media coverage of their first paper).
And it brings me to a point. Two, actually. One is that astronomy is hard. You're not just pointing a telescope at some object, getting a cool picture, and the making some claim about it. You have to understand the camera you're using in extremely fine detail, because eensy weensy problems can completely swamp the effect you're looking for. And no matter how well these detectors are made, they all have issues like that.)
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