We have discovered more than 4,000 planets orbiting distant stars. They are a diverse group, from hot Jupiters orbiting red dwarf stars in a few days to rocky, Earth-like worlds orbiting sun-like stars. With spacecraft like Gaia and TESS that number will rise rapidly, which may one day lead to the discovery of a world where intelligent life could flourish. But if we can discover strange worlds, life could find us on other planets. Not every star nearby would have a good view of our world, but some of them would. New work in the Royal Astronomical Society's monthly bulletins tries to find out which ones.
The orientation of an orbit determines whether it traverses. Photo credit: Las Cumbres Observatory Global Telescope Network
Most of the exoplanets discovered were found using the transit method. Here the planet moves between its star and our view from Earth, blocking some of the starlight. As the planet goes by, the star darkens slightly. It is through this periodic dimming that we know that the planet exists. There are other ways to find exoplanets, such as the Doppler method, but the transit method is the simplest.
The transits of Jupiter and Earth compared. Photo credit: Kepler Mission
Unfortunately, the method of transit only works if the planet's orbit is precisely aligned. If it is slightly tilted relative to us, so that the planet passes above or below the star from our point of view, we will never know that it is there. Another challenge is that the earth is a small world compared to the sun. Seen from a distant star, Jupiter would only block about 1% of the sunlight if it passed through. But that would be 100 times more than the amount that Earth is blocking. At our technological level, only relatively close stars could see the small decrease in brightness caused by the earth.
In this study, the team examined known stars within 100 parsecs (326 light years) of the Sun. In this area there are about a thousand main sequence stars that are positioned so that they can observe an earth transit. Earth transit would take at least 10 hours for 508 of them. If you measured the sun's brightness over time, you would see a small drop in brightness every 365 days. Thus, they would know that the sun has a rocky planet orbiting in the potentially habitable zone.
Most of the stars in this group are red dwarfs, but 29 are G stars like the sun. We don't know of any exoplanets orbiting these stars, but it's fun to imagine an alien civilization looking for us as we look for them.
Reference: L. Kaltenegger and J. Pepper. "Which stars can see Earth as transit exoplanets?" Royal Astronomical Society monthly bulletin 499.1 (2020): L111 – L115