Earth actually shares its orbit with a huge ring of dust particles. Alien astronomers would likely notice the dust ring long before seeing Earth itself, which could mean the best way to find alien planets is to look for dust.
We're barely aware of our dust ring's existence, but that's just because we're living right in the center of it. NASA scientist Mike Werner explains just what it is:
"As Earth orbits the sun, it creates a sort of shell or depression that dust particles fall into, creating a thickening of dust – the tail – that Earth pulls along via gravity. In fact, the tail trails our planet all the way around the sun, forming a large dusty ring."
You can see a computer simulation of the dust tail below. This is how the tail would look to astronomers outside our solar system. In general, the tail is very faint and not very dense, but its sheer size makes it more likely to be noticed than just the Earth itself. We've only recently been able to map what the dust ring looks like, thanks to observations made by the Spitzer Space Telescope as it moved through the dust a couple months back.
Now that we know what Earth's looks like, we can make assumptions about similar tails around other planets. As Werner explains, that's good news for exoplanet hunters:
"Planets in distant solar systems probably have similar dust tails. And in some circumstances these dust features may be easier to see than the planets themselves. So we need to know how to recognize them. A dust tail like Earth's could produce a bigger signal than a planet does. And it could alert researchers to a planet too small to see otherwise."
Of course, dust rings can form without the help of a planet's gravity to keep it in line. So how can we tell which rings are hiding planets? Fellow NASA scientist Mark Clampin has the answer:
"In some stars' dust disks there are bumps, warps, rings, and offsets telling us that planets are interacting with the dust. So we can 'follow the dust' to the planets. So far, we've seen about 20 dust disks in other solar systems. And in some of those cases, following the dust has already paid off."