Another Earth, full of life. The idea inspires awe and wonder, as well as sober reflection on the savage thrashing we give our own blue oasis. These notions are fuel for science fiction. But recent missions have inched Earth-like planets closer to science fact.
The task of identifying another Earth is not trivial. Planets are not easy to identify. Though humans have been viewing the night sky for millennia, our own solar system’s full complement was not resolved until the nineteenth century. The first exoplanet (the term for planets orbiting other stars) was not discovered until 1988, though speculation regarding Earth-like exoplanets quickly followed.
Candidate Earth-like planets should be rocky orbs of size and mass roughly equivalent to our own world. Its orbit should also lie within a star’s so-called “habitable zone,” the region around each star where a rocky planet could theoretically accumulate and maintain both a complex atmosphere and water. Identifying the habitable zone around a star bears a striking resemblance to Goldilocks’s quest for the perfect porridge and perfect bed. Planets too close to their stars are too hot to accumulate thick atmospheres or harbor life-giving water and organic compounds. Planets too distant from their stars essentially cool into frigid, barren hulks. The habitable zone is a theoretical “just right” range, neither too close to cook nor too far to freeze. The location of a habitable zone varies by the star’s energy output, size, luminosity, and age, and rests on key (but possibly incorrect) assumptions about factors that made Earth such a welcoming refuge for life. The location and extent of our own sun’s habitable zone is still a matter of debate, with astronomers only able to agree that Earth definitely resides within it.
In 2009, NASA launched the Kepler observatory to find habitable-zone exoplanets. Kepler orbits our own sun and stares at nearly 150,000 distant stars within the constellations Cygnus, Draco, and Lyra. Kepler searches for regular fluctuations in each star’s luminosity that could be caused by an exoplanet passing in front of it. Astronomers use these luminosity fluctuations to approximate an exoplanet’s orbital period, orbital distance, and size. The Kepler mission has been a huge success, identifying nearly 3,000 suspected and confirmed exoplanets to date. Its instruments are sufficiently sensitive to detect smaller, more Earth-like worlds than the earlier exoplanet missions, which tended to identify swift-moving gas giants. Kepler has even found a handful Earth-sized exoplanets that could lie within a star’s habitable zone. Most recently, the observatory identified one such habitable-zone planet orbiting the star Kepler-62.
A rocky planet within a star’s habitable zone is not guaranteed to be Earth-like. Ideally, scientists would examine the atmospheres of candidate Earth-like worlds (such as the one orbiting Kepler-62) in the hopes of detecting water molecules and other compounds necessary for life. This task relies on observing how an exoplanet’s atmosphere interacts with light from its parent star; since different chemicals absorb, reflect, and scatter light in unique and tractable ways, astronomers can infer atmospheric composition and thickness indirectly by measuring how an exoplanet reflects and scatters light from its parent star.
Unfortunately, Kepler’s target stars are too distant and too dim for atmospheric divination. But, the same is not true for stars closer to home. Hawaii’s Keck Observatory has measured the atmospheric content of several gas giant planets orbiting HR 8799, a young and bright star near our own solar system. Two of these planets even displayed traces of water molecules and carbon compounds. Thus, these measurements are possible, but there are no known Earth-like candidates close by to study. An ideal future mission would use a Kepler-like approach to identify Earth-like exoplanets around close stars so astronomers could also measure the chemical content of their atmospheres.
In the coming decade, NASA will launch several missions capable of such surveys. The Transiting Exoplanet Survey Satellite (TESS) will search nearby solar systems for Earth-like worlds. TESS’s target stars should be close enough for Earth-based observatories or NASA’s James Webb Space Telescope (set for launch in 2018) to point their instruments to candidate exoplanets and divine the makeup of their atmospheres. These missions won’t resolve the potential Earth-like status of exoplanets in the Kepler-62 system or other Kepler stars; those worlds await the next generation of astronomers.
In the end, some scientists doubt there are many Earth-like exoplanets out there. In our own solar system, the record of success is just one out of eight. Mercury found itself too close to the sun (though it does contain a wisp of an atmosphere and some ice here and there). Venus might have showed early promise, but long ago its nascent atmosphere succumbed to a runaway greenhouse effect. Though we salivate at the idea that Mars may have once harbored life, Earth is the only true success story we know of. Given the “just right” Goldilocks requirements (not too close, not too far) for Earth-like planets to form, evolve, and endure as life-nourishing blue marbles, in the end these exoplanet surveys may simply reveal that Earth is a rare diamond in the rough.
Image credit: NASA/Ames/JPL-Caltech.