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Earth-sized, ‘Goldilocks-zone’ planet found in distant solar system

The hunt for Earth’s alien twin reached a new milestone with the discovery of a faraway exoplanet that’s not much bigger than our own globe and is theoretically capable of retaining liquid water.

The planet is the first Earth-sized sphere found outside our solar system that also resides in the “Goldilocks zone” – the habitable range where the temperature is neither too hot nor too cold. In other words, the right conditions for life to potentially thrive.

The planet, called Kepler-186f, meets what researchers believe are two basic requirements for life. One is that its size is similar to Earth’s, which increases the chance it has a rocky, rather than gaseous, surface. The other is that it gets the right amount of stellar radiation to support liquid water, as opposed to ice or vapor.

“We don’t fully understand what makes a planet habitable, so we look for what we know,” said theoretical astrophysicist Brad Hansen of UCLA, who was not involved in the finding. “The basic assumption is that you need to have a rocky surface to stand on and liquid water for life to use.”

Using data gathered by NASA’s Kepler space-based telescope, the team of astronomers discovered a group of five planets orbiting a star 500 light-years from Earth. The star, called Kepler-186, is a relatively cool red dwarf about half the size of our sun. Four of the planets venture extremely close to the star, making them too hot for liquid water – and therefore, life as we know it. But the outermost planet soaks in just enough energy for surface water to stay liquid.

Last year, the Kepler spacecraft discovered three exoplanets, all larger than Earth, within the habitable zone of two different stars. One of these three, Kepler-62f, is 40 percent larger than Earth, and previously held the record for the habitable exoplanet that is closest to the size of our planet. The newly found Kepler-186f set a new record by being only 10 percent larger than Earth.

“We thought it was special when we first saw the little blip in the data,” said study author and astronomer Elisa Quintana of the SETI Institute. The findings were published online yesterday in the journal Science.

To find Kepler-186f, Quintana and her colleagues sifted through the mounds of data gathered by the telescope as it scrutinized one patch of the sky continuously for four years, looking for signs of planets outside our solar system.

Because the telescope can’t see exoplanets directly, astronomers use a technique called the transit method to infer their presence. The light intensity from a star will normally read as continuous and flat – but if a planet happens to pass between the telescope’s field of view and that star, it will block some of the light and show up as a dip in the data.

So Kepler-186f may be close to the Earth in size, but is it truly Earth’s twin? Most likely, no.

For one thing, the planet is colder than Earth. The amount of stellar energy it receives is only a third of the energy that the Earth gets from the Sun.

“This planet actually receives less warmth than Mars does,” said astronomer David Kipping at the Harvard-Smithsonian Center for Astrophysics, who was not involved in the study.

Also, the transit method provides information about a planet’s size – but not about its mass.

“Because you don’t get the mass, you don’t know if it’s a big rock, or a small rock with a big, gaseous atmosphere,” Hansen said.

While a smaller radius does mean Kepler-186f has a higher probability of having a rocky rather than gaseous surface, scientists at this point can only speculate about its physical composition.

Tidal locking – or a planet orbiting with the same side always facing its star – also has not been ruled out for Kepler-186f. The absence of a day-night cycle wouldn’t rule out life entirely, but it would make for a world very different from ours.

“It would have one sunny side and the other would be permanently dark, meaning it wouldn’t have an Earth-like climate,” said atmospheric scientist James Kasting of Penn State University, also not a part of the Kepler team.

Quintana says that the planet is “more of an Earth cousin” than a twin – but experts seem to agree that further composition analysis of bodies like Kepler-186f is a necessary next step to find out what these Earth-like exoplanets are really like.

“Earth-like planets are very common – they’re actually all over the place,” said Kipping, whose specialty is exomoons. “Now we just need to find the closest one.”

In 2017, NASA will launch the Transiting Exoplanet Survey Satellite, which will use an array of wide-field cameras to identify nearby exoplanets for further mass, composition and atmospheric analysis. Then, when the James Webb Space Telescope is launched in 2018, it will serve as the fine-toothed comb that measures the physical and chemical properties of those planets to assess the potential for life.

But some scientists argue that the transit method, which TESS will also use, has drawbacks. In some ways, it requires a happy accident to work – the planet has to be aligned edge-wise along the observer’s line of sight, passing between the observer and the star. A tighter orbit around the star or a larger planet-to-star size ratio can increase the probability of seeing the characteristic dip in light values.

But this leaves many exoplanets and star systems largely undetected. For instance, even Earth itself – given its distance from the Sun and comparatively small size – would likely not be easy to spot using the transit method from afar.

Legacy Comments6

The easiest way to lie to folks is on the subjects of science and health. Both of those subjects require folks to get informed. You want to know what foods to eat, you have to read labels. You want to find out about climate issues, good luck, the info is based on political agendas. Folks do not want to take the time to be informed. That is why we are settling for lousy politicians. And by that I mean all politicians. No honest discussion about anything. Just political agendas. No wonder we fix nothing.

Jim's suggestion is "regressive" not "progressive". Yes, Jim, let's perfect that windmill and those trains before we take one more look at space!

I guess the dams around this country are regressive. Flood control is regressive. Bringing fresh water to drought prone farm lands is regressive. Growing food is regressive. Creating jobs is regressive. In manufacturing to "perfect" something is called efficiency, a better product at lower cost..... If wanting to grow/produce more food, stop floods, make things better, and create jobs where we actually live makes me a liberal then I guess I'll take the name.......You talk of smaller government and a more efficient government and yet your fine spending trillions looking for a speck 500 light years away. I say explore here and fix the problems we have here - becasue there are plenty.

It is a real shame that all this money is not spent on fixing the real problems on this planet. People are going hungry every day, farming areas are going through droughts, we have the technology to fix these problems but we but we would rather spend the money watching a planet 500 light years away and pretending to find answers. There are floods every year in the same areas, rather than build reservoirs and aqueducts to control the water we let it run off and then rebuild on the same land, just to wait for the next flood. Tens of thousands of jobs could be created and actually help people survive on this one problem alone. If Mans great quest is to seek knowledge of the unknown then why not seek the knowledge to fix the problems where we actually live, not dream about places 500 light years away.

I disagree. Science--whether space, medical, earth, or other, represents the best of what we are as humans. Curiosity, the quest to understand, is part of what makes us human. The first pictures of Earth from space helped spark the environmental movement. Our "pale blue dot" in space is all we have. If humanity were more cognizant of this one fact, it would change human behavior for the better. BTW; the spending on space science is a drop in the bucket compared to what we humans squander on the mindless quarrels we call war, and their antecedents, which we call 'defense'. Our own trillion dollar defense budget is Exhibit A for wasteful spending in the grand scheme of things. But alas...we humans have always behaved this way. To quote Oscar Wilde: "We're all in the gutter, but some of us are looking at the stars."

Gee, for once I agree with Currie. We need to explore more and understand. Of course we are talking real science not politically motivated science like global warming, which is, politics, ideology, science and speculation concocted into a bitter brew. I expected that liberals would come on here, however and say that we need to keep spending and spending and spending on social issues, etc.

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