What a wonderful and amazing Scheme have we here of the magnificent Vastness of the Universe! So many Suns, so many Earths, and every one of them stock’d with so many Herbs, Trees and Animals, and adorn’d with so many Seas and Mountains! And how must our wonder and admiration be encreased when we consider the prodigious distance and multitude of the Stars?
— Christiaan Huygens, “Cosmotheoros”, 1698
Although the existence of planets outside our Solar System had been pondered since the time of the Ancient Greek civilisation, extrasolar planets remained a staple of science fiction and fantasy for most of human history. All this changed over 20 years ago, with the discovery of the first extrasolar planet around the pulsar PSR B1257+12. Ever since, the field of extrasolar planets has been one of the most flourishing and productive in astronomy, with new discoveries now coming at a regular basis. And this field of scientific study has been revolutionised in recent years by NASA’s Kepler space telescope.
Kepler was launched in March 2009. Its mission was a straight-forward one: to characterise how common Earth-sized planets were around other stars and to identify Earth-like planets inside the “habitable zone” of these stars. Although it can be argued that the conditions for habitability might be found in a broader range of environments, scientists nonetheless wanted to look for planets inside the so-called “Goldilocks” orbits, where conditions are just right for liquid water to exist on their surfaces—a key prerequisite for life as we know it. Kepler hunted for planets using the “transit method,” where the spacecraft constantly observed a certain small patch of the sky containing 150,000 stars, looking for a dimming in a star’s brightness that would indicate that a nearby planet was crossing in front of it.
And the results of the mission could be described as nothing short of spectacular. The space telescope has already identified 167 confirmed exoplanets as of date and thousands more of planetary candidates. Fullfiling the objectives of its mission, all the data releases made by the Kepler science team since the telescope’s launch, show the rate of discovery of Earth-like planets to be steadily increasing as time passes. That is to be expected, since Earth-type planets in long-period orbits are more difficult to detect and require many years of observations. Notable discoveries include the detection of the first-ever terrestrial exoplanet to be found, Kepler-10b, the first potentially habitable Earth-type planet around a Sun-like star, Kepler-22b, the first single-planet (Kepler-16b), and multi-planet (Kepler-47) systems around binary stars and two planetary systems both containing super-Earth-sized planets inside the systems’s “habitable zones,” (Kepler-62 and Kepler-69).
Although Kepler was sadly crippled by a malfunction last May in its second of four gyroscopes that were responsible for keeping it accurately pointed at the same patch of the sky, the amount of science data that yet await to be processed and analysed will keep astronomers occupied for many years to come.
This couldn’t be more evident than during the second Kepler Science Conference, held earlier this month at NASA’s Ames Research Center in Moffett Field, Calif., where scientists from all over the world gathered to discuss the telescope’s latest findings. These findings, presented on the first day of the Conference, showed that since the previous Kepler update back in January, there has been an increase of 29 percent in the detection of candidate planets, with 833 more added to the list, bringing the total to 3,538 planets that await confirmation by ground-based telescopes. More importantly, 78 percent of the newly discovered candidates are Earth-sized. It becomes evident as more data from Kepler is being processed, that Earth-like worlds may be even more common than previously thought. Past statistical studies have supported this notion, showing that every single star in the Milky Way must contain at least one planet.
At the same day that the newest Kepler findings were being presented, another study based on previous Kepler observations, was published by scientists working at the Keck Observatory in Hawaii. In the study, scientists showed that potentially habitable Earth-like worlds in the Galaxy may number in the billions. What the scientists did was to use Keck’s twin telescopes to obtain the spectra of only the Sun-like stars out of the total 150,000 Kepler was observing. That left them with approximately 42,000 stars. The scientists used sophisticated algorithms and software to weed out false positives and added artificial planetary signals in the Kepler data to check the software’s detection accuracy. In the end, the scientists came up with 603 candidate planets, out of 42,000 stars. From the candidates being discovered, ultimately 10 were found to be Earth-sized and lying inside their star’s habitable zone. This number may sound rather small, but we should keep in mind that the stars examined are such a tiny fraction of the Milky Way’s total, and that by using the transit method, only those planets can be detected whose orbits are edge-on to our line-of-sight. We can only speculate as to how many more are out there, awaiting discovery. Indeed, taking these factors into account, the scientists extrapolated that since the area of the sky Kepler was observing is a fairly typical one in the galaxy, at least 22 percent of all Sun-like stars in the Milky Way should harbor Earth-like planets. What this study implies cannot be overstated: there are hundreds of Sun-like stars within an 100-light-year radius of the Solar System and the nearest Earth-like planet could be located only a dozen light-years away.
Exoplanet discoveries may today seem routine, since new ones are being announced almost at a monthly basis. Yet this wasn’t always the case. We should always remember that, as mundane as they may seem to many, these discoveries represent one of the biggest triumphs of the human spirit.