On Tuesday, astronomers using NASA's Kepler space telescope announced the discovery of two Earth-size worlds orbiting a star 1,000 light-years away.
Does this mean Kepler's mission has been accomplished? Is this proof that Earth 2.0 exists? It is after all Kepler's prime objective to seek out Earth-like worlds — or "Earth analogs." Isn't this exoplanetary duo "Earth-like"?
Right Size, Wrong Place
Though both Kepler-20e and Kepler-20f will go down in the history books as the first confirmed Earth-sized exoplanets, that is where the similarity ends.
Both worlds orbit very closely to Kepler-20 (their parent star), so they have very short orbital periods. In fact, the entire star system of five exoplanets orbit within the distance Mercury orbits the sun!
Kepler-20e and Kepler-20f orbit their star every 6.1 and 19.5 days respectively. Compared with our planet's 365 days to complete an orbit around the sun, there is nothing familiar about a 6.1- or 19.5-day "year."
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As a consequence, both worlds are extremely hot as they are much closer to their star than the innermost limit of their habitable zone — the zone at which water can exist in a liquid state on the planet's surface.
In the quest for Earth 2.0, the potential to support liquid water on a planetary surface is of paramount importance. Liquid water, after all, is crucial for terrestrial biology.*
Kepler-20e and Kepler-20f may be the right size, but both are in the wrong place to be called "Earth analogs."
Now, turn the clock back to Dec. 5, when another Kepler announcement confirmed the discovery of a world orbiting its star within the habitable zone.
Naturally, the mere fact that Kepler-22b orbits smack bang in the middle of the habitable zone surrounding a sun-like star was enough to get the world excited about the potential for life. Kepler-22b also has an orbital period approaching the length of one Earth year — 290 days.
It's sounding pretty Earth-like, right? Sadly, there's a catch.
Wrong Size, Right Place
Astronomers consider Kepler-22b to be a "super-Earth," as it's 2.4 times wider than our planet, but they're not sure whether it's a small gas giant (like a dinky Neptune) or a large rocky world … or something in between. It may be the smallest world discovered orbiting within a star's habitable zone, but physically it's not very Earth-like.
Kepler-22b is in the right place … but it's the wrong size.
Kepler's mission is to find a world (or worlds) of Earth-like dimensions orbiting in an Earth-like period around a sun-like star. Only then can we describe the discovery as being "Earth-like" (a term that is widely misused by the popular press).
But when will a bona fide Earth-like exoplanet be discovered? Well, such a world may be announced in 2012.
2012: An Earth 2.0 Discovery?
Kepler detects exoplanets by constantly staring at the same patch of sky, along the Cygnus arm of our galaxy. Approximately 100,000 stars are in its field of view.
Exoplanets can only be detected if they orbit in front of their parent stars from Kepler's perspective — an event known as a "transit." Therefore, star systems with their ecliptic planes tilted in any other direction cannot be spotted by Kepler. Of the many stars Kepler is monitoring, a few are tilted just right to allow their worlds to be spotted.
As an exoplanet passes in front of its star, Kepler will see a slight dimming of starlight. But the detection of just one transit (one dimming event) reveals very little about the nature of the exoplanet's orbit. A single dimming event may also be an error or some random event on the surface of the distant star.
These transits are known as "candidates," and follow-up transit detections are needed. Only when multiple (periodic) transits are recorded can a "candidate" exoplanet be confirmed as a discovery.
Now, for the confirmed discovery of a bona fide Earth-sized world, with an Earth-like orbit, around a sun-like star, Kepler requires the detection of four transits. This can only happen after 3.5 years of continuous observations of the same patch of sky.
Kepler was launched in March 2009 and began taking observations the following month. Assuming April 2009 was the earliest possible time for scientific observations, 3.5 years from that date is October 2012. (The primary mission duration for Kepler is 3.5 years — the minimum possible time for the confirmed detection of a real Earth analog.)
So by the end of 2012, we could see the first hints of a confirmed world orbiting a sun-like star, with Earth-like dimensions, inside the habitable zone. Only then can we start celebrating the profound discovery of an Earth-like world orbiting another star: a real Earth 2.0.**
*Just because the search for extraterrestrial life (or potential habitats for extraterrestrial life) focuses on the hunt for liquid water, it doesn't mean scientists are ignoring the possibility that other forms of life wildly different from Earth Brand™ Life may exist. It's just that searching for "life as we know it" happens to be the best place to start.
**However, the discovery will not be definitive. Follow-up observations by other instruments will be required to see if the Earth analog even has an atmosphere. As for the potential for alien life, well, we'll probably have to travel there to find out.
Image: An Earth analog is out there … somewhere. (Credit: NASA).