Thanks to the Intergovernmental Panel on Climate Change Fifth Assessment Report, we’ve recently heard a great deal about how the Earth’s climate is changing. The IPCC’s cautious assessment of the situation is that we now know, with 95 percent certainty, that human greenhouse gas emissions are causing global warming—but did you know that the actions of other bodies in our solar system also have huge effects on our climate?
To be fair, this type of warming and cooling takes place over tens or hundreds of thousands of years, far longer than a human lifetime. But, although effects on this scale can feel irrelevant to us right now, we may well be able to use this information to help us in our future search for life on other planets.
And while we don’t currently know of any planets that could be considered truly Earth-like beyond our own solar system, it is only a matter of time before such planets are discovered, and the search for life beyond our solar system begins in earnest.
Throughout the history of astronomy, it is almost always the case that, though the discovery of the first of a given class of object is incredibly challenging, many more of that same class will follow quickly on the heels of the first.
In the few years that followed, three more asteroids were found—2 Pallas, 3 Juno, and 4 Vesta. The fifth asteroid, 5 Astraea, was not discovered until 1845—but since that discovery, the population of known asteroids has grown rapidly, such that more than 600,000 have been found to date.
More relevant to our current story, the first planet discovered orbiting a sunlike star, 51 Pegasi b, was found just 18 years ago—yet we already know of more than 700 (or more than 900, depending on which catalog you use) such planets.
The search for life on these distant worlds will be an incredibly challenging process. In the past decade, several measurements have been made of the atmospheres of planets orbiting other stars.
Those observations have generally targeted “hot Jupiters”—planets far larger and more massive than Earth, orbiting much closer to their host stars. Both these factors make the kind of observations carried out easier than they would be for an Earth-like planet.
But how will we decide which planets represent the best prospects for the detection of life beyond our solar system? While it is possible to imagine an enormous variety of life occupying ecological niches vastly different to those found on Earth (something well-serviced by science fiction), the one place that we know life exists and thrives is our own planet.
It therefore makes sense to direct our search to the most Earth-like planets (exo-Earths) we can find, since those will offer the best odds of us making a positive detection.
We need to build a “checklist of habitability” by which we can assess the many exo-Earths we will detect, and rank them such that the most promising can be identified and targeted by the exhaustive observations needed in the search for life.