With so much human hope and endeavour focused on the task of identifying extraterrestrial life on worlds outside our Solar System, it can be pretty disheartening when scientists point out how slim the prospects could really be.
Which is why a recent finding that Earth itself would only rate an 82 percent chance of being habitable – per a new calculation designed to rate exoplanet habitability – could actually be considered rather positive news. Wait… what?
Yes, strange as it sounds, researchers say that their 'habitability index', developed to help scientists prioritise the search for alien life on exoplanets, doesn't actually award Earth a 100 percent score in terms of the likelihood of being able to sustain life.
Which is more than a little ironic, when you think about it, given it's the only planet we 100 percent know for sure does sustain life. So why the less-than-perfect rating?
"Basically, where we lose some of the probability, or chance for life, is that we could be too close to the [Sun]," said astronomer Rory Barnes from the University of Washington. "We actually are kind of close to the inner edge of the habitable zone. If we spotted Earth with our current techniques, we would reasonably conclude that it could be too hot for life."
Barnes and his fellow researchers developed their index last year, which ranks exoplanets based on habitability-influencing factors such as whether their atmospheric pressure would be sufficient for liquid water to exist on the surface, how rocky they are, and how much energy they absorb or reflect from their host star.
When these details are considered, the researchers say Earth would look good but not great to alien astronomers observing our home from afar. Chiefly, this is because our proximity to the Sun might lead them to conclude that we're simply too close to the superheated inner edge of our star's habitable zone.
"Remember, we have to think about Earth as if we don't know anything about it," said Barnes. "We don't know that it's got oceans, and whales and things like that – imagine it's just this thing that dims some of the light around a nearby star when it passes."
According to the researchers, this could mean astronomers on other planets – if they were to arrive at the same conclusions – might focus their efforts on other planets offering better habitability scores than Earth and overlook us entirely, having taken our home as being a little on the crispy side.
"The point of the paper is that the [higher-ranking planet] is the best to spend our time on. Because it's less in danger," said Barnes. "But, it's obviously based on this very limited information."