How to Build the Machine That Will Find Another Living Earth

Conceptual illustration of the High Definition Space Telescope gives a sense of its enormous size, necessary to zero in on the exceedingly faint glow of another Earth. (Credit: STScI/AURA) The 20-year stretch since the discovery of the first exoplanet—a planet circling a star other than the sun—has seen a wholesale relocation of cool ideas from science fiction over to science fact. By 1999, scientists found a way to study the composition of exoplanets. By 2004, they identified a rocky planet, broadly similar to Earth in structure. By 2014, they tracked down an Earth-size world that is the right distance from its star for liquid water. The Star Trek vision of a galaxy full of habitable planets doesn’t look farfetched anymore. Despite that remarkable progress, we are still far from answering the big question: Is anybody (or anything) out there? What we all want to find is not just an Earth-size world, or an Earth-temperature world, or a vaguely Earth-like world. We want to find Earth 2.0: a wet, warm, living, breathing planet just like our own. To do that, we need to go far beyond the search techniques that astronomers have used so far. There needs to be a more ambitious plan. And there is. The huge limitation of the current research is that all of the Earthlike planets seen so far have not really been “seen” at all. We’ve detected their gravity or their shadow, but have not directly glimpsed the planets themselves. One of the standard search techniques, called the Doppler method, measures the back-and-forth motion of a star caused by the gravity of its planet. The main alternative approach, the transit method, measures the slight shadow created when a planet passes in front of its star as seen from Earth. Together, these two techniques have located some 5,400 likely exoplanets and delivered the first meaningful census on the varieties of solar systems out there. Unfortunately, neither technique can tell you much about those planets beyond the gross details of their orbital periods and their sizes or masses. For Earth-size planets, we know virtually nothing about their atmospheres or surface compositions. To understand the importance of such details, you need look no farther than the nearest planet, Venus. It is almost identical to Earth in size and density, and it orbits a reasonable distance from the sun--which is, of course, a very reasonable star. We would unquestioningly classify it as a “potentially habitable” planet based on the kinds of information we can now glean about worlds around other stars. In reality, Venus is a 900-degree F, sulfur-cloud-choked hellhole. (See my recent post about the mysteries of Earth’s wayward twin.)