Next Door Earth Like Planets

This artist’s conception shows a hypothetical habitable planet with two moons orbiting a red dwarf star. Astronomers have found that 6 percent of all red dwarf stars have an Earth-sized planet in the habitable zone, which is warm enough for liquid water on the planet’s surface. Since red dwarf stars are so common, then statistically the closest Earth-like planet should be only 13 light-years away. // David A. Aguilar (CfA)

By Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts

Published: February 6, 2013
 
Using publicly available data from NASA’s Kepler space telescope, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, have found that 6 percent of red dwarf stars have habitable Earth-sized planets. Since red dwarfs are the most common stars in our galaxy, the closest Earth-like planet could be just 13 light-years away.

“We thought we would have to search vast distances to find an Earth-like planet. Now, we realize another Earth is probably in our own backyard waiting to be spotted,” said Courtney Dressing from CfA.

Red dwarf stars are smaller, cooler, and fainter than our Sun. An average red dwarf is only one-third as large and one-thousandth as bright as the Sun. From Earth, no red dwarf is visible to the naked eye.

Despite their dimness, these stars are good places to look for Earth-like planets. Red dwarfs make up three out of every four stars in our galaxy for a total of at least 75 billion. The signal of a transiting planet is larger since the star itself is smaller, so an Earth-sized world blocks more of the star’s disk. And since a planet has to orbit a cool star closer in order to be in the habitable zone, it’s more likely to transit from our point of view.

Dressing culled the Kepler catalog of 158,000 target stars to identify all the red dwarfs. She then reanalyzed those stars to calculate more accurate sizes and temperatures. She found that almost all of those stars were smaller and cooler than previously thought.

Since the size of a transiting planet is determined relative to the star size, based on how much of the star’s disk the planet covers, shrinking the star shrinks the planet. And a cooler star will have a tighter habitable zone.

Dressing identified 95 planetary candidates orbiting red dwarf stars. This implied that at least 60 percent of such stars have planets smaller than Neptune. However, most weren’t quite the right size or temperature to be considered truly Earth-like. Three planetary candidates were both warm and approximately Earth-sized. Statistically, this means that 6 percent of all red dwarf stars should have an Earth-like planet.

“We now know the rate of occurrence of habitable planets around the most common stars in our galaxy,” said David Charbonneau from CfA. “That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought.”

Locating nearby Earth-like worlds may require a dedicated small space telescope or a large network of ground-based telescopes. Follow-up studies with instruments like the Giant Magellan Telescope and James Webb Space Telescope could tell scientists whether any warm, transiting planets have an atmosphere and further probe its chemistry.

Such a world would be different from our own. Orbiting so close to its star, the planet would probably be tidally locked. However, that doesn’t prohibit life since a reasonably thick atmosphere or deep ocean could transport heat around the planet. And while young red dwarf stars emit strong flares of ultraviolet light, an atmosphere could protect life on the planet’s surface. In fact, such stresses could help life evolve. “You don’t need an Earth clone to have life,” said Dressing.

Since red dwarf stars live much longer than Sun-like stars, this discovery raises the interesting possibility that life on such a planet would be much older and more evolved than life on Earth. “We might find an Earth that’s 10 billion years old,” said Charbonneau.

The three habitable-zone planetary candidates identified in this study are Kepler Object of Interest (KOI) 1422.02, which is 90 percent the size of Earth in a 20-day orbit; KOI 2626.01, 1.4 times the size of Earth in a 38-day orbit; and KOI 854.01, 1.7 times the size of Earth in a 56-day orbit. All three are located about 300 to 600 light-years away and orbit stars with temperatures between 5700° and 5900° Fahrenheit (3100° and 3300° Celsius). For comparison, our Sun’s surface is 10000° F (5500° C).

New Secrets of Super-Earths

Fig : A diagram comparing Earth, at left, to a cross-section of a super-Earth on the right. The super-Earth has a relatively small rocky core, an atmosphere of methane, water, and hydrogen, and an extended hydrogen envelope. // Credit: H. Lammer

By Royal Astronomical Society, United Kingdom


Published: February 4, 2013

In the past two decades, astronomers have found hundreds of planets in orbit around other stars. One type of these so-called “exoplanets” is the super-Earths that are thought to have a high proportion of rock but at the same time are significantly bigger than our world. Now, a new study led by Helmut Lammer of the Space Research Institute (IWF) of the Austrian Academy of Sciences suggests that these planets are actually surrounded by extended hydrogen-rich envelopes and that they are unlikely to ever become Earth-like. Rather than being super-Earths, these worlds are more like mini-Neptunes.

Super-Earths follow a different evolutionary track from the planets found in our solar system, but the question is whether they can evolve to become rocky bodies like the terrestrial planets Mercury, Venus, Earth, and Mars. To try to answer this, Lammer and his team looked at the impact of radiation on the upper atmospheres of super-Earths orbiting the stars Kepler-11, Gliese 1214, and 55 Cancri.

These planets are each a few times more massive and slightly larger than Earth and orbit close to their respective stars. The way in which the mass of planets scales with their sizes suggests that they have solid cores surrounded by hydrogen or hydrogen-rich atmospheres, probably captured from the clouds of gas and dust — nebulae — from which the planets formed.

The new model suggests that the short wavelength of extreme ultraviolet light — much bluer than the blue light we see with our eyes — of the host stars heats up the gaseous envelopes of these worlds so that they expand to several times the radius of each planet, and gas escapes from them fairly quickly. Nonetheless, most of the atmosphere remains in place over the whole lifetime of the stars that they orbit.

“Our results indicate that although material in the atmosphere of these planets escapes at a high rate, unlike lower-mass Earth-like planets, many of these super-Earths may not get rid of their nebula-captured hydrogen-rich atmospheres,” said Lammer.

Rather than becoming more like Earth, the super-Earths may more closely resemble Neptune, which together with Uranus is a smaller “gas giant” in our solar system. If the scientists’ results are right, then super-Earths farther out from their stars in the “habitable zone,” where the temperature would allow liquid water to exist, would hold on to their atmospheres even more effectively. If that happens, they would be much less likely to be habitable.