Planetary Hunt Yields Four New 'Earth Twins'

1-6-15 Earth-like planet
This artist’s conception depicts an Earth-like planet orbiting an evolved star that has formed a stunning “planetary nebula.” Earlier in its life, this planet may have been like one of the eight newly discovered worlds orbiting in the habitable zones of their stars. David A. Aguilar/CfA

Astronomers have gotten one step closer to answering the question that occupies scientists and sci-fi enthusiasts alike: "Are we alone in the universe?" A team from the Harvard-Smithsonian Center for Astrophysics (CfA) has identified eight new planets that are within the habitable zones of the stars they orbit, and could hold the potential for life.

Researchers announced the discovery Tuesday at a meeting of the American Astronomical Society in Seattle. Four of those planets discovered are likely to be rocky and similar in size to Earth, scientists believe.

"It's another major milestone along a journey that astronomers have undertaken for 20 years," says David Kipping, an astronomer at the CfA and co-author of the research. "We've proved that there are numerous objects [that] could potentially support life."

Kipping and his colleagues, including lead author Guillermo Torres and Francois Fressin, set out a year-and-a-half ago to study a list of a dozen objects observed by NASA's Kepler spacecraft to determine whether they were genuine planets and whether they might be inhabitable.

Two of the objects, several hundred light years away, were identified by researchers as being the most Earth-like planets of any discovered to date.

Kepler monitors stars beyond our own solar system, looking for dips in the brightness level of those stars. That would signal the transit of an object between the instrument and the star. To determine whether the objects on their list were planets rather than other stars, the researchers looked at the shape of the light curve caused by the transits; U-shaped curves generally indicate a planet, while V-shaped curves would usually indicate another star interacting with the one being observed, Kipping explains.

Of the original 12 candidates, the team was able to establish that 11 are planets with almost 100 percent confidence, using a computer program developed by Torres and Fressin.

Three of those candidates were validated by other teams during the year-and-a-half Kipping's worked on its study. That leaves eight new exoplanets, or planets that orbit around a star other than the sun (also called extrasolar planets), that were announced for the first time in the CfA study.

Once they confirmed that the objects in question are planets, Kipping explains, the team checked whether they fulfilled two basic criteria for life as we know it: Are they within the habitable zone of the star they orbit? And are they the right size?

The habitable zone—sometimes referred to the "Goldilocks" zone by the media and the surface water liquid zone by scientists—is the range of distance from the star in which a planet orbiting it could have liquid water on its surface. In other words, the range in which water, if it existed on the surface of the planet, would neither boil off nor freeze—but stay just right.

The researchers estimated the distance of the exoplanets from the stars they orbit by calculating the number of days it takes them to complete one orbit (we call that a "year" here on Earth), as well as estimating the mass of the star itself. Using Kepler's Laws and Newton's laws, researchers could then estimate the distance of each planet from its star. All eight were found to be within the habitable zone.

The researchers also had to determine whether the planets were the right size: If they were giant, they'd probably be gaseous, like Jupiter or Saturn, and not rocky like Earth. Based on previous research, planets up to 50 percent larger than Earth are still likely to be rocky, says Kipping. Any bigger, and you get gas.

Though scientists cannot directly measure the size or the mass of planets so far away, they can estimate the size based on how much light they block out during their transit across the star they orbit. The team at the CfA found that of the eight planets within habitable zones, four were very likely to be rocky. All four could be called "Earth twins," says Kipping, or "an Earth-size planet in the habitable zone of a sunlike star," Kepler scientist Tom Barclay explained to Space.com in April last year.

The two new exoplanets most similar to Earth are Kepler-438b and Kepler-442b, the CfA scientists said Tuesday. The former has a diameter only 12 percent larger than Earth's, a 70 percent chance of being rocky, and receives 40 percent more light than Earth. The latter is one-third larger than Earth with a 60 percent chance of being rocky, and it receives two-thirds as much light as the Earth. Both of them orbit red dwarf stars smaller than our yellow dwarf sun.

Before the discovery of these eight exoplanets, the planets Kepler had identified that were most similar to Earth were 186f and 62f, but 438b and 442b—470 light years and 1,100 light years away respectively—surpass these to become the most Earth-like planets of any discovered to date.

Of course, the parameters for "habitable" are broad, and the recent findings do not mean that 438b and 442b do in fact have water or life on their surfaces—only that they could theoretically support the kind of life with which we are familiar.

"For our calculations we chose to adopt the broadest possible limits that can plausibly lead to suitable conditions for life," lead author Torres is quoted as saying in a press release.

The next step might be to study what makes up the planets' atmospheres to see if their makeups are consistent with an ability to host carbon-based life forms. However, new tools will need to be designed, funded, and built before scientists can pursue next steps. "This is as close as we're ever going to get [using] the Kepler data," says Kipping.

To detect an unambiguous signal of something like oxygen, for example, is beyond the scope of any existing telescope, says Kipping. But the discovery of what kinds of potentially habitable planets are out there and how far away they are will inform the designs of the next generation of telescopes.

"We're the mapmakers," says Kipping, and the findings announced today are "one milestone along the journey."