The Kepler space telescope NASA launched in 2009 was designed to look for exoplanets - that is, planets that orbit stars other than our Sun. Specifically, it was designed to survey a portion of the Milky Way galaxy in search of Earth-size planets in or near their star’s habitable zone. The unspoken goal is to find another Earth, a planet equivalent to our own where life might exist.

The quest to find life on other planets has driven planetary science for decades. Missions are designed to “follow the water” since it’s one of the necessary ingredients for life as we know it. But the question of whether there might be another Earth somewhere in the Universe extends beyond the few decades we’ve been robotically visiting other bodies in our Solar System. It’s a centuries-old question, one that Kepler’s has shown not to be groundless.

The ancient and mediaeval universe

The idea that there might be another Earth somewhere in the Universe was first suggested by the Greek philosopher Epicurus in the third century B.C. At the time, philosophically speaking, the Universe was a bounded one in which the heavenly bodies - the Sun, the Moon, and the five planets visible to the naked eye - orbited in their own respective spheres. The outermost sphere was that of the fixed stars, which were also unmoving and unchanging. It was a cosmos that mimicked the perfection and simplicity of the divine mind, the view that had been propagated by Plato and picked up by Aristotle a century earlier.

Epicurus broke with this then-dominant idea by suggesting the opposite: the Universe wasn’t bounded and unchanging, it was infinite and filled with infinite other worlds. Some, he said, would be like our own while others would be entirely different. But all planets would share certain features, like a sky over the land and living creatures on its surface. Epicurus’ was an unorthodox view that never gained much traction; the Aristotelean cosmos dominated throughout the middle ages.

...there are terabytes of data from the telescope astronomers have yet to study, and that data might contain evidence of another Earth. Kepler may never gather new data again, but the mission is far from over. ,

But not everyone was content with a closed Universe. Nicolaus de Cusa was one of the first to revisit the Epicurean cosmos in the thirteenth century, a move that inspired Giordano Bruno nearly four centuries later. Bruno suggested built on the infinite universe by suggesting that the Sun isn’t unique. What Aristotle saw as fixed stars, he said, are actually distant suns. And each one has the potential to host an inhabited Earth-like planet.

Unfortunately for Bruno, his ideas were hardly more popular than Epicurus’. His proposal of the existence of distant Earth’s was among the heresies for which he was burned at the stake in 1600.

Finding other worlds

While the idea of distant Earths persisted, there was no evidence of any such planet. But the landscape of cosmology was changing. By the mid-nineteenth century the Aristotelean cosmos was falling out of favour as astronomers discovered the gas giants Uranus and Neptune and built telescopes that could resolve the moon of Jupiter and the rings of Saturn. The Universe was no longer the stoic and perfectly organised place it once was.

It was in this period of cosmological change that an astronomer first reported evidence of an exoplanet. In 1855, Captain W.S. Jacob of the East India Observatory in Madras was observing the binary star 70 Ophiuchi. In studying the stars’ brightness, he noticed periodic dips in the light. Subsequent calculations suggested the only thing that could cause the dip in light he saw was a planet orbiting the stars.

Jacob’s exoplanet turned out not to be an exoplanet at all. It was a false alarm, the first of many that appeared over the next century and a half. But amid the false sighting of exoplanets came the first concrete evidence that there might be worlds outside our own system. In 1925, Edwin Hubble found evidence that the Universe was expanding and that galaxies are groups of stars similar to our own Milky Way. This discovery changed the way scientists looked at the Universe, and offered the first evidence that exoplanets could really exist.

Evidence we might not be alone

But true evidence for exoplanets did not come with the opening of the space age. In the 1960s an 1970s, we landed probes on Mars and Venus and sent men to walk on the Moon, but we still didn’t have any evidence of planets orbiting distant stars.

Then, in 1992, everything changed.

Astronomers Aleksander Wolszczan and Dale Frail were in Puerto Rico measuring the pulsar (the pulsating remains of an exploded star) PSR1257 +12 with the Arecibo radiotelescope. The pattern of pulses they saw wasn’t consistent with a pulsar. Something else looked to be affecting the pulsar’s light; two somethings, actually. Calculations showed that two rocky planets were in orbit around the pulsar, a theory the astronomers soon confirmed.

These two exoplanets weren’t the other Earths Epicurus and his intellectual successors had hoped for. Pulsars shoot out such high levels of radiation that no life could possibly live on these planets. But they were still planets, and they were orbiting a star other than our Sun. It was another discovery that changed how we look at the Universe around us, and the first strong evidence that maybe, if we looked the right way, we could find that other Earth.

It wasn’t long before astronomers found more exoplanets. In 2009 there were already 374 confirmed exoplanets. Kepler, which launched in March of that year, has added 132 confirmed exoplanets to the tally and nearly 3,000 additional candidate bodies.

The current hunt for exoplanets

Kepler experienced a major setback a few weeks ago. The second of four reaction wheels that make up the spacecraft’s stabilizing system stopped working, leaving the telescope unable to focus on a fixed point in the sky. It’s effectively blind. At the moment, the telescope is “resting” while mission scientists work out what to do next. It’s unclear whether it will ever come back online as an exoplanet hunter.

The news was met with disappointment and frustration; reactions, particularly, reflected despair over the now unrealisable dream of finding another Earth. Which is patently untrue: there are terabytes of data from the telescope astronomers have yet to study, and that data might contain evidence of another Earth. Kepler may never gather new data again, but the mission is far from over.

But that’s hardly the point. Kepler was designed to find evidence of Earth-like exoplanets and it has. In fact, it’s found enough to suggest that rocky planets in a star’s habitable zone aren’t uncommon making it increasingly likely that we’ll find more. Whether Kepler, Hubble, or Hubble’s successor the James Webb Space Telescope finds another Earth will be icing on the exoplanet cake. We’ve found Earth-like planets outside our solar system and gathered ample evidence to support Epicurus’ 2,500 year old idea.

Amy Shira Teitel has an academic background in the history of science and now works as a freelance science writer specialising in spaceflight history. She maintains her own blog, Vintage Space, and contributes regularly to Discovery News, Scientific American, Motherboard, DVICE.