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First Earth-sized planets beyond Solar System reported

Dec. 20, 2011
Courtesy of the Harvard-Smithsonian Center for Astrophysics
and World Science staff

As­tro­no­mers us­ing NASA’s Kep­ler mis­sion have iden­ti­fied the first Earth-sized plan­ets be­yond our so­lar sys­tem. 

While probably too hot to live on, they’re still a likely mile­stone in the search for al­ien life, the re­search­ers say. The find­ing is thought to br­ing sci­en­tists a step clos­er to find­ing a twin Earth in the hab­it­a­ble zone of a near­by star, the ar­ea where the warmth is nei­ther too hot nor too cold for liq­uid wa­ter.

The above di­a­gram shows the es­ti­mat­ed sizes of the two new­found plan­ets (their real col­ors are not known) as com­pared to Earth and Ve­nus. Left to right: Kep­ler-20e, Ve­nus, Earth and Kep­ler-20f. (Cred­it: NA­SA/JPL-Caltech/T. Pyle )


“Prov­ing the ex­ist­ence of Earth-sized exoplan­ets is a ma­jor step to­ward achiev­ing that goal,” said Fran­cois Fressin of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass., whose sci­en­tists worked on the stu­dy. An ex­o­plan­et is a plan­et out­side our so­lar sys­tem.

A pa­per de­scrib­ing the find­ings is to be pub­lished in the jour­nal Na­ture.

The two plan­ets, dubbed Kep­ler-20e and 20f, are the small­est plan­ets found to date. They are 6,900 miles and 8,200 miles wide — equiv­a­lent to 0.87 times Earth, or slightly smaller than Ve­nus, and 1.03 times Earth. They are ex­pected to have rocky make­ups. Both worlds cir­cle Kep­ler-20, a star slightly cool­er than the Sun and lo­cat­ed 950 light-years from Earth. (It would take the space shut­tle 36 mil­lion years to trav­el to Kep­ler-20.)

Kep­ler-20e or­bits eve­ry 6.1 days at a dis­tance of 4.7 mil­lion miles and Kep­ler-20f or­bits eve­ry 19.6 days at a dis­tance of 10.3 mil­lion miles, sci­en­tists say. Due to their close-in or­bits, they are heat­ed to an esti­mated 1,400 and 800 de­grees Fahr­en­heit, re­spec­tive­ly.

In ad­di­tion to the two Earth-sized worlds, the Kep­ler-20 sys­tem is be­lieved to con­tain three larg­er plan­ets. All five have or­bits clos­er than Mer­cu­ry in our so­lar sys­tem. They al­so show an un­ex­pected ar­range­ment. In our so­lar sys­tem small, rocky worlds or­bit close to the Sun and large, gas gi­ant worlds or­bit far­ther out. In con­trast, the plan­ets of Kep­ler-20 are or­gan­ized in al­ter­nat­ing size: big, lit­tle, big, lit­tle, big.

“We were sur­prised to find this sys­tem of flip-flopping plan­ets,” said co-author Da­vid Char­bon­neau of the Cen­ter. “It’s very dif­fer­ent than our so­lar sys­tem.”

The Kep­ler space­craft iden­ti­fies “ob­jects of in­ter­est” by look­ing for stars that dim slight­ly, which can oc­cur when a plan­et crosses the star’s face. To con­firm such an event, as­tro­no­mers look for the star to wob­ble as it is gravita­t­ionally tugged by its or­biting com­pan­ion, a meth­od known as ra­di­al ve­locity. But the ra­di­al ve­locity sig­nal for ap­prox­i­mately Earth-sized plan­ets is too small to de­tect with cur­rent tech­nol­o­gy, so oth­er con­firma­t­ion tech­niques must be used.

A va­ri­e­ty of situa­t­ions could mim­ic the dim­ming from a tran­sit­ing plan­et. For ex­am­ple, an eclips­ing binary-star sys­tem whose light blends with the star Kep­ler-20 would cre­ate a si­m­i­lar sig­nal. To rule out such im­posters, the team sim­u­lat­ed mil­lions of pos­si­ble sce­nar­i­os with spe­cially de­vel­oped soft­ware. They con­clud­ed that the odds are strongly in fa­vor of Kep­ler-20e and 20f be­ing plan­ets.

Fressin and Tor­res al­so used Blend­er to con­firm the ex­ist­ence of Kep­ler-22b, a plan­et in the hab­it­a­ble zone of its star that was an­nounced by NASA ear­li­er this month. How­ev­er, that world was much larg­er than Earth.


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Homepage image: Artist's conception of Kepler 20-e. (Credit: NASA/JPL-Caltech/T. Pyle)








 

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Astronomers using NASA’s Kepler mission have identified the first Earth-sized planets beyond our solar system. While probably too hot to live on, they’re still a likely milestone in the search for alien life, the researchers say. The finding is thought to bring scientists a step closer to finding a twin Earth in the habitable zone of a nearby star, the area where the warmth is neither too hot nor too cold for liquid water. “Proving the existence of Earth-sized exoplanets is a major step toward achieving that goal,” said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., whose scientists conducted the study. An exoplanet is a planet outside our solar system. The paper describing the finding will be published in the journal Nature. The two planets, dubbed Kepler-20e and 20f, are the smallest planets found to date. They are 6,900 miles and 8,200 miles wide — equivalent to 0.87 times Earth, or slightly smaller than Venus, and 1.03 times Earth. They are expected to have rocky makeups. Both worlds circle Kepler-20, a star slightly cooler than the Sun and located 950 light-years from Earth. (It would take the space shuttle 36 million years to travel to Kepler-20.) Kepler-20e orbits every 6.1 days at a distance of 4.7 million miles. Kepler-20f orbits every 19.6 days at a distance of 10.3 million miles. Due to their close-in orbits, they are heated to temperatures of 1,400 and 800 degrees Fahrenheit, respectively. In addition to the two Earth-sized worlds, the Kepler-20 system contains three larger planets. All five have orbits closer than Mercury in our solar system. They also show an unexpected arrangement. In our solar system small, rocky worlds orbit close to the Sun and large, gas giant worlds orbit farther out. In contrast, the planets of Kepler-20 are organized in alternating size: big, little, big, little, big. “We were surprised to find this system of flip-flopping planets,” said co-author David Charbonneau of the Center. “It’s very different than our solar system.” The Kepler spacecraft identifies “objects of interest” by looking for stars that dim slightly, which can occur when a planet crosses the star’s face. To confirm such an event, astronomers look for the star to wobble as it is gravitationally tugged by its orbiting companion, a method known as radial velocity. But the radial velocity signal for approximately Earth-sized planets is too small to detect with current technology, so other confirmation techniques must be used. A variety of situations could mimic the dimming from a transiting planet. For example, an eclipsing binary-star system whose light blends with the star Kepler-20 would create a similar signal. To rule out such imposters, the team simulated millions of possible scenarios with specially developed software. They concluded that the odds are strongly in favor of Kepler-20e and 20f being planets. Fressin and Torres also used Blender to confirm the existence of Kepler-22b, a planet in the habitable zone of its star that was announced by NASA earlier this month. However, that world was much larger than Earth.