A ‘Goldilocks’ world?

Astronomers have spotted an exoplanet that looks a lot like home. Slightly larger than Earth, this distant, alien world could have liquid water, scientists say. That’s important because water is a key ingredient for life.

It’s too soon to know if E.T. can call this planet home, and people will not be visiting any time soon. The planet orbits a star so far away that light, the fastest traveler in the universe, takes nearly 500 years to zip between that star and Earth. So the fastest spacecraft would need about 15 million years to make even a one-way trip.

The new world, Kepler-186f, was named for the Kepler space telescope that first snapped its picture. The number in its name refers to the star it orbits. Kepler-186f is the smallest exoplanet found so far that orbits within the “habitable zone” of its star. That means it’s at the right distance for water to remain liquid.

If a planet is too close, the extreme heat of its sun would evaporate liquids. If it’s too far, the extreme cold would freeze any water. But if it’s neither too far nor too close — in what some scientists refer to as the Goldilocks zone — water could pool as a liquid. Planet-hunting astronomers have been seeking habitable-zone planets because they’re the best places to look for life. All forms of known life need water for basic processes, such as transporting materials in and out of cells. So to look for life, scientists follow the water.

Astronomers reported finding the planet on April 18 in Science. They found evidence for it among data collected by the Kepler space telescope. This exoplanet-hunting observatory in space carefully watched for changes in the light coming from distant stars. As a planet passed between a star and the telescope, the star’s light would dim. Kepler picked up these dips in light. Astronomers then analyzed those changes to estimate the size and orbit of the planet. 

Kepler-186f orbits a type of star called an M dwarf. A red dwarf, this star gives off less heat and light than our sun, which is a yellow dwarf star. Because an M dwarf is cooler, any potentially habitable planets would orbit the star at a closer distance than Earth orbits our warm sun. Kepler-186f orbits its star at a distance closer than Mercury is from our sun. And four other planets orbit Kepler-186 closer still.

Close-in planets were the easiest for Kepler to spot. That’s because distant planets are less likely than closer ones to transit — or pass between the telescope and its star.

“M dwarfs are now becoming everybody’s darlings,” astronomer Jill Tarter told Science News. Although she was not involved in the discovery, she has spent her entire career studying alien worlds. She’s the former director of the Center for SETI Research in Mountain View, Calif.

When SETI scientists first heard about Kepler-186f, they directed telescopes to search for any radio transmissions. They listened for two weeks but heard no greetings from distant aliens.

M dwarf stars are the most common in our Milky Way. “If we find that Earth-sized planets around habitable zones of M stars are common, that means they’re common throughout the galaxy,” Thomas Barclay told Science News. An astronomer at NASA Ames Research Center in Moffett Field, Calif., he helped analyze the newfound planet. 

Scientists already know life can exist on a small, rocky planet like Earth. That makes Earth-sized planets good candidates for the search for alien life.

But Barclay offers a warning: “Just because it’s in the habitable zone doesn’t mean it’s habitable.” Scientists can’t be sure the planet actually has water. And the planet orbits so closely that bursts of radiation from the star might have wiped out any chance of life.

The Kepler space telescope stopped working in 2013. Still, Barclay says its pioneering observations will guide the future of planet hunting. A mission scheduled to launch in 2017, for example, will search for worlds around the M dwarfs closest to Earth. And maybe they’ll find a home-like planet much closer to home.

Power Words

alien       A non-native organism. (in astronomy) Life on or from a distant world.

exoplanet    A planet that orbits a star outside the solar system.

habitable     A place suitable for humans or other living things to comfortably dwell.

light-year    The distance light travels in a year, about 9.48 trillion kilometers (almost 6  trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

M dwarf    The most common type of star in the Milky Way. M dwarfs are smaller, cooler and fainter than yellow dwarfs, of which our sun is an example.

Milky Way  The galaxy in which Earth’s solar system resides.

planet   A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish      ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

red dwarf     A type of smallish star that is relatively cool (and hence emits reddish light). Dwarfs are the most common size stars in the Milky Way.

SETI      An abbreviation for search for extraterrestrial life, meaning life on other worlds.

transit     (in astronomy) The passing of a planet across the face of a star, or of a moon or its shadow across the face of a planet.

yellow dwarf    A medium-sized star. The Sun is an example of a yellow dwarf.