Some exoplanets (planets beyond our solar system) would have a direct line of sight to observe our planet, according to a team of astronomers.
Several missions – such as Kepler or TESS to name a few – have led to the discovery of thousands of exoplanets. NASA's James Webb Telescope and Roman Telescope will also soon have a chance to find more. Naturally, astronomers and other curious people ask themselves the following question:could some of them harbor life? Also, much research has been undertaken in recent years to try to answer it. A team of researchers asked themselves a completely different question:could we be visible from the outside?
To unearth an exoplanet, telescopes rely on several methods. One of them is the so-called "transit". Concretely, the idea consists in measuring the curve of light emitted by the stars. Periodic dips in brightness , sometimes observed from Earth, can then testify to the passage of one or more planets in front of said stars.
In this study, astronomers Lisa Kaltenegger of Cornell's Carl Sagan Institute and Joshua Pepper of Lehigh University therefore sought to find out whether the transit of the Earth in front of the Sun could be visible from other planets . That is, if other worlds could properly align with Earth's ecliptic, the geometric plane containing Earth's orbit around the Sun.
As part of this work, astronomers have selected more than a thousand stars that could in theory host Earth-like planets in their habitable zone. All planets position themselves at approximately 100 parsecs (326 light years) from the Solar System.
Why did they choose such "close" worlds? Because the Earth is a small world compared to the Sun. To give a simple example, seen from a distant star, Jupiter, which is the largest planet in our System, would only block about 1% of sunlight during its transit. Our poor little Earth would block one hundred times less .
Thus, at our level of technology, only relatively close civilizations could be able to notice the presence of our planet.
Researchers then discovered that many of these exoplanets have a direct line of sight to Earth. They also add that, if these worlds are indeed home to sufficiently advanced extraterrestrial civilizations, they could theoretically be able to detect signs of biosphere in the Earth's atmosphere from their point of view.
Note that most of the stars selected here are red dwarfs (29 are G-type stars like the Sun), and that we have not yet discovered any exoplanets orbiting them . Still, it's fun to imagine that an alien civilization could in theory be able to find us before we find them.
Details of this work are published in the Monthly Notices of the Royal Astronomical Society.
