Illustration by NASA Ames/JPL-Caltech

It’s been just over two decades since astronomers announced the first discoveries of exoplanets—planets orbiting stars other than the sun—and their progress in the intervening years has been so routinely remarkable its recitation now seems mundane: There are now thousands of cataloged exoplanets, and hundreds of billions more probably await discovery in the Milky Way alone. The exoplanet searches, steadily growing in number and sophistication, are increasingly uncovering worlds that, in size and orbit, resemble our own. Such worlds appear large enough to hold on to substantial atmospheres, yet still small enough to be mostly rocky, like the Earth. They seem to circle neither scorchingly close nor frigidly far from their stars, and instead reside somewhere within a “habitable zone” between those extremes, in orbits where liquid water—and thus life as we know it—could exist.

Considering all this, you could rightly think that any day now astronomers will find a true twin of our planet, an “Earth-like” world of blue oceans, white ice caps, brown continents, and green plants, all flourishing beneath a yellow-looking Sun. Considering instead typical news reports and press releases, you might wrongly think they already had. Every few months now, a new, best candidate for an Earth-like planet is announced, replacing the last, previous best candidate for a short time until yet another, supposedly even more Earth-like successor comes along. As exciting as this progress is, it is only partially driven by empirical measurements. Each announcement usually includes an artist’s rendition of a blue-green planet so familiar that, squinting, you can almost glimpse your house hiding beneath a swirl of cloud, yet in actuality you could scribble the sum total of our salient knowledge about each world in fat-tipped marker on the palm of one hand. At present, in the very best of cases all we really know of any potentially “Earth-like” exoplanet is an age, an orbit, a radius, and a mass, all derived with some degree of uncertainty from the flickers and oscillations of its star.

The latest iteration of this cycle began on April 18th, when scientists affiliated with NASA’s planet-hunting Kepler space telescope announced they had found not one, but two seemingly rocky planets in the habitable zone of a single orange star, Kepler-62, a slightly smaller and cooler star than our own Sun located some 1,200 light-years away. One planet, Kepler-62e, is 60 percent larger than Earth and orbits the star every 122 days, near the habitable zone’s inner edge. The other, Kepler-62f, illustrated above, is 40 percent larger than Earth and orbits the star every 267 days, close to the habitable zone’s middle. Each world “transits” its star (crosses directly between Earth and the star), casting a shadow across the light-years toward our solar system that reveals the planet’s size. Both worlds are wishfully called “super-Earths,” but as of yet astronomers have not been able to measure either planet’s mass or composition—not to mention take their pictures—over such a great interstellar distance. On balance, this is a great discovery, and it bodes well for finding more promising planets in the near future. There are compelling reasons to believe both planets are in fact rocky, and could be rather like our own. That said, those beliefs must confront the fact that the greatest constant throughout all our years of searching for someplace like home has been the shattering of earthbound expectations.

Each announcement includes a rendition of a blue-green planet so familiar that, squinting, you can almost glimpse your house hiding beneath a swirl of cloud. Yet in actuality you could scribble the total of our knowledge about it in fat-tipped marker on the palm of one hand.

For instance, the first confirmed exoplanets orbited a pulsar, a whirling sphere of degenerate neutronium left behind after a massive star exploded in a supernova. No one had predicted planets could exist around such exotic objects, and yet there they were. The next planets to be discovered also upended expectations. They were around stars like our sun, but in very close orbits, with years only as long as a few Earth days, and they were also as large as our own Jupiter. The dominant planet-formation theories of the time had been built to explain the architecture of our own solar system. They could not easily explain how such “hot Jupiters” came to be, and so all our theories had to be revised. They are now being revised again, after Kepler and other planet-hunting surveys recently revealed another unexpected class of objects: Most planetary systems now being found are very different from our own, harboring several intermediate-mass, sub-Neptune-sized planets all in compact, close-in orbits where our own arrangement only has diminutive Mercury and empty space. Our own familiar architecture may be the exception rather than the rule. Whether the majority of those strange new worlds are gassy “mini-Neptunes” or rocky super-Earths remains unknown, though through a bit of luck within a decade we might begin uncovering the truth. The unifying theme of all these discoveries is that none could have been predicted by extrapolation from our local circumstances here in the solar system, and they should collectively serve as a humbling reminder of how much we still have to learn.

The next time a potentially Earth-like world is announced in the media, I would encourage you to substitute a different picture for whatever lurid illustration is included in the press release. Below is an image of Venus, taken by the Japanese spacecraft Hinode as the planet transited across the face of the Sun in June of last year. There are no green continents or blue oceans to be seen here, only the planetary silhouette and a faint ring of sunlight refracting through the atmosphere, roughly analogous to what we actually see of distant transiting exoplanets. As recently as the 1950s, it was scientifically plausible to believe that Venus was a mirror Earth. It was almost the exact same size as our own planet, albeit slightly closer to the Sun. Venus would thus be a warmer world than Earth, but the blanket of clouds that hid the Venusian surface from view gave astronomers hope, for the clouds could also serve as a shield to reflect much of the sunlight back into space before it could heat the planet. Perhaps Venus was a jungle world, filled with lush forests and steamy swamps. After the dawn of the Space Age and the robotic reconnaissance of Venus, those dreams were shattered; Venus was revealed to be a hellish place, red-hot and parched beneath a dense atmosphere of almost pure carbon dioxide sprinkled with droplets of sulfuric acid. In the transition from telescopic to more direct observations, Mars, too, experienced a similar Space-Age fall from Earth-like grace. What some had suspected to be a planet of vegetation-covered plains and ancient canal-building civilizations instead became a frozen and desiccated world of rust where life, if it existed at all, eked out survival on the slimmest of margins.

Photograph by JAXA/NASA

The hard reality we must face today is that it will probably be a very long time before planet-hunters can observe any potentially habitable exoplanets nearly as well as our 19th-century forebears observed our nearest neighboring worlds. In the meantime, it’s wise to keep in mind that, viewed through some alien equivalent of Kepler from light-years away, Venus, Earth, and Mars could easily be interpreted as a trio of “habitable” planets orbiting a distant yellow star. The truth, of course, would be more complicated, and hopefully would be discovered before any eager extraterrestrials packed their starships for a Venusian vacation.

In my next posts, I’ll look at more closely at what we’re talking about when we talk about “Earth-like” planets, and also at our near-future options for investigating potentially habitable bodies.





Lee Billings is a freelance writer living in New York City. Five Billion Years of Solitude, his book on the search for Earth-like exoplanets, will be published this October by Current/Penguin.





