It may have a radius about 2.4 times that of our home planet, but NASA scientists have confirmed that Kepler-22b — depicted in the artist's conception up top — is the first planet we've ever confirmed orbits within the so-called "habitable zone" of a Sun-like star, making it the most Earth-like planet we've yet discovered.
In astronomy, the habitable zone (also known as the "Goldilocks zone") is the region surrounding a star in which an orbiting planet could maintain liquid water (and, by extension, life) on its surface. And as the "Goldilocks" moniker implies, whether or not a planet resides inside a habitable zone has everything to do with whether the planet is a little too cold, a little too hot, or just right, temperature-wise.
Take Kepler-21b, for example, whose discovery was announced last week by the National Optical Astronomy Observatory. Kepler-21b is even closer to the size of Earth than Kepler-22b, but it orbits far too close to its sun to sustain any form of life we're familiar with; as this conception of K-21b by artist Ron Miller clearly illustrates, surface temperatures on the planet are estimated to reach as much as 3000-degrees Fahrenheit — that's hot enough to melt iron, not to mention any hope of us ever calling K-21b "Earth 2.0."
But Kepler-22b is a different story. Sure, the planet orbits about 15% closer to its star than Earth does to the Sun, but its star is also significantly cooler, dimmer, and smaller than ours. And while scientists have yet to determine K-22b's composition — be it rocky, gaseous or liquid — they estimate that surface temperatures on K-22b average a very Earth-like 72-degrees Fahrenheit.
"This is a major milestone on the road to finding Earth's twin," said Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington. "Kepler's results continue to demonstrate the importance of NASA's science missions, which aim to answer some of the biggest questions about our place in the universe."
NASA's Kepler mission (which is charged with identifying Earth-like planets throughout the Milky Way galaxy) has certainly turned up habitable zone planet candidates in the past, but Kepler 22-b is the first of these candidates to be officially confirmed.
And it certainly won't be the last. In fact, the confirmation was made on top of another announcement: that the Kepler mission has now discovered 1,094 additional potential planets (many of which could very well be Earth-like), bringing the total number of planet candidates discovered to date to 2,326.
Let's pause and consider that number for a moment. The Kepler space telescope has been in operation for less than three years, and already its findings stand to quadruple the number of worlds known to exist beyond our solar system. If the rate of discovery continues on its present course, the identification of more and more Earth-like planets stands to ramp up in a big way.
"The tremendous growth in the number of Earth-size candidates tells us that we're honing in on the planets Kepler was designed to detect: those that are not only Earth-size, but also are potentially habitable," explains Natalie Batalha, Kepler deputy science team lead at San Jose State University. "The more data we collect, the keener our eye for finding the smallest planets out at longer orbital periods. We are really zeroing in on the true Earth-sized habitable planets."
In other words, we're getting closer and closer to finding Earth's twin — and that's assuming we haven't found something incredibly close already. All that's left now is coming up with a way to make the 600-light-year trip to Kepler-22b and we can set up camp.