At Last, Earth-Sized Alien Worlds

Learn about the latest Kepler space telescope discovery of alien Earths, Kepler-20e and Kepler-20f, in this infographic.

NASA Discovers First Earth-size Planets Beyond Our Solar System

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 a bit 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, is similar to an average day on the planet Mercury. The surface temperature of Kepler-20e, at more than 1,400 degrees Fahrenheit, would melt glass.

Transit light curves

“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. 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.

Mass versus radius relation for small planets. Kepler-20 e and Kepler-20 f theoretical mass and observed radius ranges (1σ) are plotted as orange- and greenshaded areas, while the other transiting planets with dynamically determined masses are plotted in black, with 1σ error bars. The curves are theoretical constant-temperature mass–radius relations.

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 continue 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, 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 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. “We are on the edge of our seats knowing that Kepler’s most anticipated discoveries are still to come.”

Searching for Dark Matter in Exoplanet Data

An image taken by Kepler of star cluster NGC 6791, which is located 13,000 light years from Earth. The image has been color-coded so that brighter stars appear white, and fainter stars, red. Image credit: NASA/Ames/JPL-Caltech

Our galaxy could be filled with asteroid-size black holes that presumably formed shortly after the big bang. If they exist in large numbers, these so-called primordial black holes would serve as the dark matter that keeps stars gravitationally glued inside galaxies. None of these primordial black holes have been detected so far, but a new theoretical analysis described in Physical Review Letters demonstrates that a current planet-hunting mission is well placed to search for them.

As dark matter candidates go, primordial black holes are widely considered to be the dark horse. Previous astronomical searches for these objects came up empty, so many cosmologists put their money on the alternative candidate: a weakly interacting particle that physicists hope to find in accelerators or other experiments.

Still, there is a mass range of relatively small primordial black holes that has yet to be ruled out. Kim Griest, of the University of California in San Diego, and colleagues believe that part of this “observational gap” could be explored by piggybacking on a separate astronomy survey. NASA’s Kepler satellite was designed to search for planets around 150,000 stars (in a single field of view) that are relatively close to Earth. A planet passing in front of one of these stars dims the starlight by a small amount. Conversely, a black hole passing between us and a Kepler star would have the opposite effect: it would act as a lens and brighten the starlight. The authors calculate that Kepler is the first instrument sensitive enough to detect this so-called microlensing for black holes with masses of around 0.1% of an Earth mass. – Michael Schirbe

Read also: NASA satellite could reveal if primordial black holes are dark matter

One-Third of Sun-Like Stars Have Earth-Like Planets In Habitable Zone

Astronomers have calculated the likelihood of finding Earth-like planets around other stars using the latest data from the Kepler mission.

The period and radius of Kepler planets in the sample, around bright stars, are plotted. The lower right corner is relatively empty, probably owing to low SNR there, not because small planets are absent from long periods. The upper left corner is relatively sparse, in spite of an expected high SNR there, implying a deficit of large planets on shortperiod orbits. The left side of the diagram is relatively empty owing to an apparent paucity of planets of all sizes at periods less than 3 days. The right side of the diagram is not completely sampled in the current database, so should be ignored here.

The Kepler orbiting observatory is specifically designed to find Earth-like planets around nearby stars.

Earlier this year, the Kepler team released the mission’s first 136 days of data and it has turned out to be a veritable jackpot. In that time Kepler looked at some 150,000 target stars and found evidence for 1,235 potential exoplanets. That’s quite a haul….. Continue reading One-Third of Sun-Like Stars Have Earth-Like Planets In Habitable Zone