Suppose there really are aliens out there who are creeping around on the surface of some faraway planet and have managed to survive everything space has thrown at them so far. How could we find out they exist?
The answer might lie in how they would (hypothetically) see us. We may never know whether there really are intelligent beings who have spotted our planet as it passed by the sun, but observing it from their perspective could help us see through extraterrestrial eyes. This is the objective of the Earth Transit Observer (ETO) mission concept. Led by a research team from the Johns Hopkins Applied Physics Laboratory (APL), ETO will watch Earth in transit as if it was a spacecraft sent out here by other intelligent beings.
Researchers Noam Izenberg and Kevin Stevenson, who will be the project leads if this mission becomes reality, and co-led a study recently presented at the 52nd Lunar and Planetary Science Conference, and Laura Mayorga, who also co-led the study, believe that observing Earth from the perspective of a being who never knew it existed could give us new insight on how to look for habitable—and possibly inhabited—planets.
"While an earth transit observer will not explicitly help us detect exoplanets, it will help us understand them and tease out their possible habitability signatures better. There is a fundamental problem in exoplanet science in that we only know planets as well as we know their stars," they said.
Astronomers have been using the transit method of finding exoplanets since 1999. This method determines when a planet is orbiting in front of its star, which causes starlight to dim while it is being obscured and brighten again when the planet has moved on. Particles in a planet’s atmosphere can also absorb starlight at some frequencies, and the light that makes through can tell the observer how much was absorbed and whether that is a sign of a planet in transit. What the ETO team wants to know is whether there is more that can be applied to this method in the future.
The problem with stars is that they have varying degrees of brightness throughout, with starspots (like the sunspots in our own star) can warp what scientists think they are seeing in a transiting planet. However, the dark spots on our Sun are documented and followed, so scientists know how they change over time. Starspots on distant bodies remain largely unknown. When there is no clear idea of their size or distribution, they can mess with obsevations. Most exoplanets are also too close to their stars to make out the continents and oceans they may have.
"Even if you can separate the planet from the star, the Earth is reflecting and absorbing sunlight at different wavelengths," the researchers said. "You’re always using the Sun as a reference to say the planet is dark because it is absorbing light here or it is dark because the star is dark here and it is just reflecting that. For a star to be a good reference it either needs to be unchanging, or you need a good understanding of how it changes."
When trying to characterize exoplanets, space telescopes like TESS and Hubble (and the James Webb Space Telescope when it gets off the ground) search for what may be signs of a planet’s habitability potential. The same atmospheric particles that absorb starlight can also give away whether said planet has anything close to the air we breathe—or not. Such data has told us about exoplanets where it rains everything from metal to lava, even one so hot that it not only vaporizes metal, but tears the vapor molecules until the dismembered atoms are blown away to its cooler night side and get a chance to regroup.
"Future NASA astrophysics missions, such as JWST, will require stacking dozens of transmission spectra to build up sufficient signal for the relatively small atmospheres on terrestrial planets," the ETO scientists said. "Variations in the planet's atmosphere and on the star are like background noise when trying to discern signals. An earth transit observer will test how well stacking can be done or if other strategies are needed."
What ETO is likely to see is Carl Sagan’s pale blue dot, watery and covered in clouds, with an atmosphere of nitrogen and oxygen in which water vapor precipitates. Reflected light can also give away whether a planet has oceans and may be even planet life, which has very low reflectivity. Any celestial body with water is seen by Earthlings as having a higher chance of being habitable. There are also traces of methane in our atmosphere, and methane is an organic substance that could tell an alien observer that the unknown blue planet they are staring at could possibly be swarming with life.
"Looking at Earth from afar, we’d be looking for evidence (or signs) of life as we know it, which is exactly what we do when looking for life on other planets. For example, we look for gases that are either produced only by life as we know it or are required by life as we know it. On Earth those include oxygen, methane, ozone, and water. These gasses each leave their own signature in the observed spectrum of the planet's atmosphere."
However, Earth may not be so easy to demystify from an alien’s perspective. Stars and planets are dynamic. Flares and storms are always happening on stars, which also vomit plasma in coronal mass ejections that could mess with how transiting planets are seen. Then there are the constantly changing seasons on planets that can also throw off observations.An alien astronomer trying to detect Mars would come up as barren as the planet itself.
"We can make an exoplanet-style observation and know exactly what the Earth, the Sun, the spacecraft are doing and how that affects our final product," said the researchers. "These lessons are then directly transferable to exoplanet observations."
Of course, the Gaian model is only a reflection of what is necessary for life to flourish on Earth. Nobody knows if there are things out there which stay alive by breathing in poison. When even some Earth bacteria can eat rocks and metabolize methane, you have to expect anything.