We’re spending quite a bit of time now looking for signals from aliens, or signs of life on a world outside the Solar System. But we haven’t really stopped to consider the opposite; what worlds are we visible to, and who might be trying to get in contact?
One team of astronomers has sought to address this problem. René Heller, an astronomer at the Max Planck Institute for Solar System Research in Göttingen, Germany, and Ralph Pudritz, an astronomer at McMaster University in Hamilton, Canada, report their findings in the April issue of Astrobiology Magazine.
Their argument is as follows. Our main method of finding planets at the moment is the transit method, where we look for the dip in light from distant stars as a planet passes in front. This relies on a planet’s orbital plane being visible from Earth; if we are seeing the system face on, the planet is undetectable via this method.
By the same token, any aliens residing on planets in the direction of our North and South poles would be unable to use this method to find us. Thus, in efforts of making first contact – through initiatives like the Breakthrough Listen project – the researchers say we should look for planets in the direction of our orbital path around the Sun, our “transit zone.”
There are, of course, other methods of detecting planets. These include the radial velocity method, looking for gravitational “wobbles” in a star from the presence of a planet, and direct imaging. But Heller says that the ease of a transit method might make it attractive to a race at a similar technological level to us.
“The transit method is a relatively cheap one to use, because the stellar transit of a planet yields a lot of information about the planetary atmosphere without the need to actually see the planet,” Heller told IFLScience. “Using transit spectroscopy, we have already identified traces of certain gases in the atmospheres of exoplanets.
“Hence, others might also find it more comfortable and less ‘expensive’ to find us via our solar transits.”
The transit method relies on spotting dips in brightness from an orbiting planet in our line of sight. NASA
The researchers found 82 Sun-like stars that fall in the transit zone within 3,260 light-years from Earth, but they predict hundreds of thousands more could be viable targets. Two-way communication at these distances would be pretty difficult, but if an alien race sprung up a long time ago, perhaps they sent us a message from one of these worlds when they noticed we were a habitable planet with life – albeit more primitive – on the surface.
For this reason, the researchers argue in their paper that the transit zone is an “ideal region” to be studied in detail by projects such as Breakthrough, alongside existing projects like the Search for Extraterrestrial Intelligence (SETI), which uses the Allen Telescope Array in California. However, there are no current plans by the Breakthrough project to look in this region.
“If any of these planets host intelligent observers, they could have identified Earth as a habitable, or even as a living, world long ago, and we could be receiving their broadcasts today,” the researchers write.
Heller noted to IFLScience that direct observations of a planet, using “large-large space telescopes,” is something that might be possible in 100 years, and another race who has developed this technology would not need to rely on transit zones. But until then, it’s reasonable to think others might have progressed in a similar way to us, and at some point relied on the transit method.
So as we continue to study other worlds, perhaps it is important to ask: Is anyone studying us?