Earth-Likenesses: Have We Discovered Earth 2.0?

Astronomers have announced the groundbreaking discovery of an Earth-sized exoplanet, called Kepler-186f, orbiting a star within its habitable zone. Although this is an exciting finding - and a historic one at that - calling this world "Earth-like" is a little premature.

In fact, Kepler-186f could be completely alien.

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In 2011, Discovery News ran a series of articles predicting what scientific breakthroughs were most likely to occur in 2012. In my article "Big Question for 2012: Will We Find Earth 2.0?," I speculated that, some time in 2012, NASA's Kepler space telescope would have had enough time to have detected its first bona fide Earth-sized exoplanet orbiting a sun-like star within the habitable zone - the region surrounding a star where water, on a rocky planetary surface, could exist in a liquid state. On Earth, where there's liquid water, there's life, so the quest to find liquid water on another world is key to our quest to find life elsewhere in the Universe.

Alas, although Kepler did indeed have enough time to gather orbital data for many small worlds with Earth-like dimensions around their host stars, that announcement didn't come in 2012 (or in 2013) - although there were many near-misses.

Today, a little over two years later, exoplanetary science has caught up with the world's expectations and finally produced a world that, from 500 light-years distant, appears to be a ripe "Earth 2.0″ candidate.

"Previously, the exoplanet most like Earth was Kepler-62f, but Kepler-186f is significantly smaller," David Charbonneau, with the Harvard-Smithsonian Center for Astrophysics, told Discovery News' Irene Klotz. "Now we can point to a star and say, ‘There lies an Earth-like planet.'"

Why is Kepler-186f so Special?

During its primary mission, Kepler had a fixed stare on one tiny portion of the sky in the direction of the constellation Cygnus, carefully watching the brightness of 150,000 sun-like stars. Should an exoplanet drift in front of one of those stars, Kepler's sensitive optics registered it as a very slight dip in brightness, an event known as a "transit." As these exoplanetary candidates continued to orbit their host stars, Kepler registered more and more transits, leaving astronomers in little doubt that the signal is indeed an orbiting exoplanet and not some other transient dark feature like a "starspot."

ANALYSIS: ‘Earth-like' Planets May Be Nothing Like Earth

With followup observations by ground-based telescopes, these exoplanetary candidates could then be confirmed and added to the growing tally of confirmed small worlds orbiting other stars. There is little doubt that we are in a "Golden Age" of exoplanetary studies.

To find another planet with all the orbital and physical qualities of Earth, however, is a tall order. The sheer technological precision needed to make these detections is mindboggling, but as Kepler is proving, it is absolutely possible to detect worlds smaller than Earth in orbit around stars hundreds of light-years away.

And Kepler-186f has all the attributes that makes us believe that it is a world not so dissimilar to Earth: it orbits a sun-like star (well, the star Kepler-186 is a little smaller and redder than the sun), is approximately the same physical size as our planet (just 10 percent bigger) and has an orbit of 130-days, putting it right on the outside edge of its star's habitable zone. But it takes more than a planet's orbit and size to make it truly "Earth-like."

Exo-Artistic License As you may have noticed by the vivid artistic renderings of Kepler-186f accompanying today's announcement, the perceived life-giving potential of Kepler-186f is obvious. The view is from the planet's surface, looking up at its host star with other planets in the multi-planetary system in tow. On the planet's surface is an ocean lined with tree-like vegetation. There's another rendering (pictured top) of a blue world with a thick atmosphere and white fluffy clouds.

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The message is clear: ‘This is just like Earth; it's a planet in another part of the galaxy capable of supporting life as we know it.' But just as our solar system is a great example of possessing a life-giving world orbiting inside its habitable zone (Earth), there's two other examples of biologically ‘dead' planets that orbit inside the sun's habitable zone: Venus and Mars.

Although we are currently on a quest to work out whether Mars has ever had the potential to support basic forms of life, it's pretty clear from studies of the Red Planet's thin atmosphere and radiation-drenched surface that it is not particularly cozy for life. As for Venus (which, coincidentally, is almost the same physical size as Earth), it has an acidic atmosphere that is undergoing a crazy greenhouse effect that literally destroys water molecules. Neither of these "habitable zone" examples are, well, particularly habitable for life as we know it.

So how do we know that Kepler-186f has white fluffy clouds and pine tree-lined coastlines? Hint: We don't.

We currently have no means to study this fascinating world's atmosphere, let alone understand whether it has life-giving potential.

"Being in the habitable zone does not mean we know this planet is habitable," cautions Thomas Barclay, research scientist at the Bay Area Environmental Research Institute at NASA Ames, in a NASA news release. "The temperature on the planet is strongly dependent on what kind of atmosphere the planet has.

"Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth."

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In the future, space telescopes are planned to directly image worlds orbiting other stars, potentially measuring high-resolution spectra of atmospheric compositions. Until then, we won't know whether Kepler-186f is a barren Mars-like world or a choked Venus-like world. Perhaps it's a completely alien place that cannot even be compered with any other solar system example.

These concerns over the "Earth-like" moniker to one side, the fact that we are even able to have a clue about an alien planet's life giving potential is profound. Kepler-186f has just become the poster child for interstellar studies and a prime astronomical target for future observing campaigns - it will also likely be mainstream press headline fodder for some time to come. For now, it's best to take the term "Earth-like" in the spirit and excitement that it inspires - even though it may not be scientifically accurate.

So, for me, for now, Kepler-186f is "Earth-like" enough - it just carries some huge caveats.

Artist's impression of Kepler-186f in orbit around its M-class dwarf star.

Exquisite Exoplanetary Art

Sept. 19, 2011 --

They're alien worlds orbiting distant stars far out of reach of detailed imaging by even our most advanced telescopes. And yet, day after day, we see vivid imaginings of these extrasolar planets with the help of the most talented space artists. The definition of an extrasolar planet -- or "exoplanet" -- is simply a planetary body orbiting a star beyond our solar system, and nearly 700 of these extrasolar worlds have been discovered so far (plus hundreds more "candidate" worlds). With the help of NASA's Kepler space telescope, the ESO's High Accuracy Radial velocity Planet Searcher (HARPS), French COROT space telescope and various other advanced exoplanet-hunting observatories, we are getting very good at detecting these worlds, but to glean some of the detail, we depend on artist's interpretations of fuzzy astronomical images and spectral analyses. That's the way it will be until we build a vast telescope that can directly image an exoplanet's atmosphere or physically travel to an alien star system. So, with the flurry of recent exoplanet discoveries, Discovery News has collected a few of the dazzling pieces of art born from one of the most profound searches mankind has ever carried out: the search for alien worlds orbiting other stars; a journey that may ultimately turn up a true "Earth-like" world.

The Transit

As an exoplanet passes in front of its star as viewed from Earth, a very slight dip in starlight brightness is detected. Observatories such as NASA's Kepler space telescope use this "transit method" to great effect, constantly detecting new worlds.

Hot Jupiters

Some exoplanets orbit close to their parent stars. Due to their close proximity and generally large size, worlds known as "hot Jupiters" are easier to spot than their smaller, more distant-orbiting cousins.

Habitable Worlds

The primary thrust of exoplanet hunting is to find small, rocky worlds that orbit within their stars' "habitable zones." The habitable zone, also known as the "Goldilocks zone," is the region surrounding a star that is neither too hot nor too cold. At this sweet spot, liquid water may exist on the exoplanet's surface. Where there's water, there's the potential for life.

A Phantom

Usually, exoplanet hunters look for the slight dimming of a star or a star's "wobble" to detect the presence of an exoplanet. However, in the case of Kepler-19c, its presence has been detected by analyzing its gravitational pull on another exoplanet, Kepler-19b. Kepler-19c is therefore the Phantom Menace of the exoplanet world.

Keeping Warm

The habitable zone seems to be the pinnacle of extraterrestrial living. If you're an alien with similar needs to life on Earth, then you'll need liquid water. If your planet exists outside your star's habitable zone, well, you're in trouble. Either your world will be frozen like a block of ice, or boiling like a kettle. But say if your world had the ability to extend your star's habitable zone? There may be some atmospheric factors that might keep water in a comfy liquid state. Even better, if you like deserts, a dry world could even be oddly beneficial.

Crazy Aurorae

Planets with a global magnetic field, like Earth, have some dazzling interactions with the winds emanating from their stars. The high-energy particles bombard the planet's atmosphere after being channeled by the magnetism. A wonderful auroral lightshow ensues. But say if there's an exoplanet, with a magnetosphere, orbiting really close to its star? Well, stand back! The entire world would become engulfed in a dancing show, 100-1000 times brighter than anything we see on Earth.

The "Candidates"

"Candidate" exoplanets are often mentioned, especially when talking about detections by the Kepler space telescope. But what does this mean? As a world passes in front of its star, slightly dimming the starlight, this isn't considered a "confirmed" exoplanet detection. To make sure that signal is real, more orbital passes of the exoplanet need to be logged before a bona fide discovery can be announced. Until then, these preliminary detections are called exoplanet candidates.

Angry Suns, Naked Planets

Exoplanets come in all sizes and all states of chaos. Some might have wonky orbits, others might be getting naked. Other times, they're simply being ripped apart by X-rays blasted from their parent star. Bummer.

Super-Earths

Super-Earths get a lot of press. Mainly because "Earth" is mentioned. Sadly, most of these worlds are likely completely different to anything we'd call "Earth." And you can forget calling the vast majority of them "Earth-like." It's simply a size thing -- they're bigger than Earth, yet a lot smaller than Jupiter, hence their name, "super-Earth." Easy.

Let's Go!

For now, we have to make do with artist's renditions of exoplanets for us to visualize how they may look in their alien star systems. However, plans are afoot to send an unmanned probe to an interstellar destination. Although these plans may be several decades off, seeing close-up photographs of these truly alien worlds will be well worth the wait.