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."