A missão Kepler da NASA descobriu os primeiros planetas com tamanho equivalente ao da Terra orbitando uma estrela semelhante ao Sol, fora do nosso sistema solar. Os planetas, chamados Kepler-20e e Kepler-20f, estão suficientemente próximos de sua estrela-mãe para se situarem na chamada zona habitável, onde é possível a existência de água em estado líquido na superfície planetária, mas eles são os menores exoplanetas de existência já confirmada ao redor de uma estrela como o nosso Sol.
A descoberta é um importante marco na história recente da busca por planetas como a Terra. Acredita-se que os novos planetas sejam rochosos. Kepler-20e é ligeiramente menor do que Vênus, com um raio de 0,87 vez o da Terra. Kepler-20f é um pouco maior do que a Terra, com um raio de 1,03 vez o do nosso planeta. Ambos os planetas situam-se em um sistema de cinco planetas chamado Kepler-20, aproximadamente 1.000 anos-luz distante da Terra, na constelação de Lira.
Kepler-20e orbita sua estrela-mãe a cada 6,1 dias, e Kepler-20f o faz a cada 19,6 dias. Esses períodos orbitais curtos indicam tratar-se de mundos muito quentes e inóspitos. Um dia em Kepler-20f, com temperatura de 427 graus Celsius, é parecido com um dia comum no planeta Mercúrio. A temperatura na superfície de Kepler-20e, superior a 760 graus Celsius, é suficiente para fundir o vidro.
"O objetivo principal da missão Kepler é encontrar planetas de tamanho semelhante ao da Terra dentro da zona habitável," disse Francois Fressin, do Centro Harvard-Smithsonian de Astrofísica em Cambridge, Massachussets, EUA, autor principal de um novo estudo publicado na revista Nature. "Essa descoberta demonstra pela primeira vez que planetas de tamanho equivalente ao da Terra existem ao redor de outras estrelas, e que é possível detectá-los."
O sistema Kepler-20 inclui outros três planetas, maiores que a Terra, mas menores que Netuno. Kepler-20b, o mais próximo, Kepler-20c, o terceiro, e Kepler-20d, o quinto planeta, orbitam sua estrela-mãe a cada 3,7 dias, 10,9 dias e 77,6 dias, respectivamente. Todos os cinco planetas têm órbitas situadas aproximadamente na mesma região ocupada por Mercúrio em nosso sistema solar. A estrela-mãe pertence à mesma classe do Tipo G que o nosso Sol, embora ela seja ligeiramente menor e mais fria.
O sistema tem um arranjo surpreendente. Em nosso sistema solar, os astros pequenos e rochosos orbitam as proximidades do Sol e os maiores, gasosos, percorrem órbitas mais distantes. Já os planetas de Kepler-20 estão dispostos alternadamente: grande, pequeno, grande, pequeno, e grande.
"Os dados da Kepler mostram que alguns sistemas planetários têm arranjos muito diferentes dos encontrados no nosso sistema solar," disse Jack Lissauer, cientista planetário assistente e integrante da equipe científica da member no Centro Ames de Pesquisas da NASA, em Moffett Field, Califórnia. "A análise dos dados da Kepler prossegue, devendo revelar novidades a respeito da diversidade de planetas e sistemas planetários em nossa galáxia."
Os Cientistas não têm certeza de como o sistema evoluiu, mas eles não creem que os planetas se formaram nos locais onde estão atualmente. Eles propõem que os planetas se tenham formado em locais mais distantes de sua estrela e então se moveram para o interior, provavelmente por meio de interações com o disco de matéria do qual se originaram. Isso teria permitido a esses astros manter um espaçamento regular, apesar dos tamanhos alternados.
A missão Kepler detecta planetas e candidatos a planeta medindo variações no brilho de mais de 150.000 estrelas, que ocorrem quando um planeta passa diante do disco solar (estelar), o que é chamado "trânsito". A equipe científica da Kepler necessita de um mínimo de três trânsitos para determinar se um sinal é mesmo de um planeta.
A equipe científica da Kepler utiliza telescópios baseados no solo e o Telescópio Espacial Spitzer para revisar observações de candidatos a planeta que a nave espacial Kepler descobre. O campo estelar obrsarvado pela Kepler nas constelações do Cisne e da Lira só são visíveis com telescópios baseados no solo durante o período que vai da primavera até o começo do outono. Os dados obtidos com essas outras observações ajudam a determinar quais candidatos podem ser confirmados como planetas.
Para validar Kepler-20e e Kepler-20f, os astrônomos utilizaram um programa de computador chamado Blender, que faz simulações que ajudam a descartar outros fenômenos astrofísicos que se assemelham a planetas.
Em 5 de dezembro, a equipe anunciou a descoberta de Kepler-22b na zona habitável de sua estrela-mãe. Mas ele parece ser grande demais para ter uma superfície rochosa. E embora Kepler-20e e Kepler-20f tenham tamanhos bem parecidos com o da Terra, eles estão próximos demais de sua estrela-mãe para poderem ter água em estado líquido na superfície.
"No jogo cósmico de esconde-esconde, encontrar planetas com o tamanho e temperatura certos parece ser apenas uma questão de tempo," disse Natalie Batalha, da equipe científica da Kepler.
NASA's Kepler mission has discovered the first Earth-size planets orbiting a
sun-like star outside our solar system. The planets, called Kepler-20e and
Kepler-20f, are too close to their star to be in the so-called habitable zone
where liquid water could exist on a planet's surface, but they are the smallest
exoplanets ever confirmed around a star like our sun.
The discovery marks the next important milestone in the ultimate search for planets like Earth. The new planets are thought to be rocky. Kepler-20e is slightly smaller than Venus, measuring 0.87 times the radius of Earth. Kepler-20f is slightly larger than Earth, measuring 1.03 times its radius. Both planets reside in a five-planet system called Kepler-20, approximately 1,000 light-years away in the constellation Lyra.
Kepler-20e orbits its parent star every 6.1 days and Kepler-20f every 19.6 days. These short orbital periods mean very hot, inhospitable worlds. Kepler-20f, at 800 degrees Fahrenheit (427 degrees Celsius), is similar to an average day on the planet Mercury. The surface temperature of Kepler-20e, at more than 1,400 degrees Fahrenheit (760 degrees Celsius), would melt glass.
"The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of a new study published in the journal Nature. "This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them."
The Kepler-20 system includes three other planets that are larger than Earth but smaller than Neptune. Kepler-20b, the closest planet, Kepler-20c, the third planet, and Kepler-20d, the fifth planet, orbit their star every 3.7, 10.9 and 77.6 days, respectively. All five planets have orbits lying roughly within Mercury's orbit in our solar system. The host star belongs to the same G-type class as our sun, although it is slightly smaller and cooler.
The system has an unexpected arrangement. In our solar system, small, rocky worlds orbit close to the sun and large, gaseous worlds orbit farther out. In comparison, the planets of Kepler-20 are organized in alternating size: large, small, large, small and large.
"The Kepler data are showing us some planetary systems have arrangements of planets very different from that seen in our solar system," said Jack Lissauer, planetary scientist and Kepler science team member at NASA's Ames Research Center in Moffett Field, Calif. "The analysis of Kepler data continues to reveal new insights about the diversity of planets and planetary systems within our galaxy."
Scientists are not certain how the system evolved, but they do not think the planets formed in their existing locations. They theorize the planets formed farther from their star and then migrated inward, likely through interactions with the disk of material from which they originated. This allowed the worlds to maintain their regular spacing despite alternating sizes.
The Kepler space telescope detects planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets crossing in front of, or transiting, their stars. The Kepler science team requires at least three transits to verify a signal as a planet.
The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planet candidates the Kepler spacecraft finds. The star field Kepler observes in the constellations Cygnus and Lyra can be seen only from ground-based observatories in spring through early fall. The data from these other observations help determine which candidates can be validated as planets.
To validate Kepler-20e and Kepler-20f, astronomers used a computer program called Blender, which runs simulations to help rule out other astrophysical phenomena masquerading as a planet.
On Dec. 5, the team announced the discovery of Kepler-22b in the habitable zone of its parent star. It is likely to be too large to have a rocky surface. While Kepler-20e and Kepler-20f are Earth-size, they are too close to their parent star to have liquid water on the surface.
"In the cosmic game of hide and seek, finding planets with just the right size and just the right temperature seems only a matter of time," said Natalie Batalha, Kepler deputy science team lead and professor of astronomy and physics at San Jose State University.
The discovery marks the next important milestone in the ultimate search for planets like Earth. The new planets are thought to be rocky. Kepler-20e is slightly smaller than Venus, measuring 0.87 times the radius of Earth. Kepler-20f is slightly larger than Earth, measuring 1.03 times its radius. Both planets reside in a five-planet system called Kepler-20, approximately 1,000 light-years away in the constellation Lyra.
Kepler-20e orbits its parent star every 6.1 days and Kepler-20f every 19.6 days. These short orbital periods mean very hot, inhospitable worlds. Kepler-20f, at 800 degrees Fahrenheit (427 degrees Celsius), is similar to an average day on the planet Mercury. The surface temperature of Kepler-20e, at more than 1,400 degrees Fahrenheit (760 degrees Celsius), would melt glass.
"The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of a new study published in the journal Nature. "This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them."
The Kepler-20 system includes three other planets that are larger than Earth but smaller than Neptune. Kepler-20b, the closest planet, Kepler-20c, the third planet, and Kepler-20d, the fifth planet, orbit their star every 3.7, 10.9 and 77.6 days, respectively. All five planets have orbits lying roughly within Mercury's orbit in our solar system. The host star belongs to the same G-type class as our sun, although it is slightly smaller and cooler.
The system has an unexpected arrangement. In our solar system, small, rocky worlds orbit close to the sun and large, gaseous worlds orbit farther out. In comparison, the planets of Kepler-20 are organized in alternating size: large, small, large, small and large.
"The Kepler data are showing us some planetary systems have arrangements of planets very different from that seen in our solar system," said Jack Lissauer, planetary scientist and Kepler science team member at NASA's Ames Research Center in Moffett Field, Calif. "The analysis of Kepler data continues to reveal new insights about the diversity of planets and planetary systems within our galaxy."
Scientists are not certain how the system evolved, but they do not think the planets formed in their existing locations. They theorize the planets formed farther from their star and then migrated inward, likely through interactions with the disk of material from which they originated. This allowed the worlds to maintain their regular spacing despite alternating sizes.
The Kepler space telescope detects planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets crossing in front of, or transiting, their stars. The Kepler science team requires at least three transits to verify a signal as a planet.
The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planet candidates the Kepler spacecraft finds. The star field Kepler observes in the constellations Cygnus and Lyra can be seen only from ground-based observatories in spring through early fall. The data from these other observations help determine which candidates can be validated as planets.
To validate Kepler-20e and Kepler-20f, astronomers used a computer program called Blender, which runs simulations to help rule out other astrophysical phenomena masquerading as a planet.
On Dec. 5, the team announced the discovery of Kepler-22b in the habitable zone of its parent star. It is likely to be too large to have a rocky surface. While Kepler-20e and Kepler-20f are Earth-size, they are too close to their parent star to have liquid water on the surface.
"In the cosmic game of hide and seek, finding planets with just the right size and just the right temperature seems only a matter of time," said Natalie Batalha, Kepler deputy science team lead and professor of astronomy and physics at San Jose State University.
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