smaller planets are preferentially found in low-eccentricity orbits.

More Kepler results: From the abstract of a preprint paper published today on the Los Alamos astro-ph website:

The mean eccentricity of the Kepler candidates decreases with decreasing planet size indicating that smaller planets are preferentially found in low-eccentricity orbits.

In other words, the smaller a planet is, the more likely its orbit will be circular like the Earth’s. This result is encouraging news for the search for life on other worlds. Before Kepler, astronomers had found that the orbits of most exoplanets were far more eccentric than the orbits of the planets in our solar system, a condition that scientists thought was unfriendly for the development of life. These new results counter that conclusion. The orbits of the planets in our solar system might not be as unusual as first thought.

The Kepler team today announced the discovery of eleven new solar systems holding twenty-six planets.

Planets galore! The Kepler team today announced the discovery of eleven new solar systems holding twenty-six planets.

The planets orbit close to their host stars and range in size from 1.5 times the radius of Earth to larger than Jupiter. Fifteen of them are between Earth and Neptune in size, and further observations will be required to determine which are rocky like Earth and which have thick gaseous atmospheres like Neptune. The planets orbit their host star once every six to 143 days. All are closer to their host star than Venus is to our sun.

No Earths in the habitable zone quite yet, but we are circling in on our prey.

A Sun-like Star with Three Sub-Neptune Exoplanets and Two Earth-size Candidates

Want to read the actual paper, “A Sun-like star with three sub-Neptune exoplanets and two Earth-size candidates,” describing the discovery announced yesterday of two Earth-sized planet? You can download it here.

The paper’s closing paragraph sums the discovery up nicely:

A striking feature of the Kepler-20 planetary system is the presence of Earth-size rocky planet candidates interspersed between volatile-rich sub-Neptunes at smaller and larger orbital semi-major axes, as also seen in Kepler candidate multi-planet systems. Assuming that both [Kepler-20e] and [Kepler-20f] are planets, the distribution of the Kepler-20 planets in orbital order is as follows: Kepler-20b (3.7 days, 1.9 Earth radii), [Kepler-20e] (6.1 days, 0.9 Earth radii), Kepler-20c (10.9 days, 3.1 Earth radii), [Kepler-20f] (19.6 days, 1.0 Earth radii), and Kepler-20d (77.6 days, 2.8 Earth radii). Given the radii and irradiation fluxes of the two Earth-size planet candidates, they would not retain gas envelopes. The first, second, and fourth planets have high densities indicative of solid planets, while the other two planets have low densities requiring significant volatile content. The volatile-rich third planet, Kepler-20c dominates the inner part of the Kepler-20 system, by holding much more mass than the other three inner planets put together. In the Solar System, the terrestrial planets, gas-giants, and ice giants are neatly segregated in regions with increasing distance from the sun. Planet formation theories were developed to retrodict these Solar System composition trends. In the Kepler-20 system, the locations of the low-density sub-Neptunes that are rich in water and/or gas, and the Earth-size planet candidates does not exhibit a clean ordering with orbital period, challenging the conventional planet formation paradigm. In situ assembly may form multi-planet systems with close-in hot-Neptunes and super-Earths, provided the initial protoplanetary disk contained massive amounts of solids (∼ 50–100 Earth masses) within 1AU of the star.

First Earth-sized planets found

Big news: The first Earth-sized planets have been found by Kepler.

The two planets, dubbed Kepler-20e and 20f, are the smallest planets found to date. They have diameters of 6,900 miles and 8,200 miles – equivalent to 0.87 times Earth (slightly smaller than Venus) and 1.03 times Earth. These worlds are expected to have rocky compositions, so their masses should be less than 1.7 and 3 times Earth’s.

Both worlds circle Kepler-20: a G-type 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 tight orbits, they are heated to temperatures of 1,400 degrees Fahrenheit and 800 degrees F.

Once again, this is only the beginning. The announcement of an Earth in the habitable zone is only a matter of months away.

Kepler team pushes for mission extension

The Kepler team pushes for a mission extension in order to find Earth-sized planets.

The Kepler spacecraft has hit an unexpected obstacle as it patiently watches the heavens for exoplanets: too many rowdy young stars. The orbiting probe detects small dips in the brightness of a star that occur when a planet crosses its face. But an analysis of some 2,500 of the tens of thousands of Sun-like stars detected in Kepler’s field of view has found that the stars themselves flicker more than predicted, with the largest number varying twice as much as the Sun. That makes it harder to detect Earth-sized bodies. As a result, the analysis suggests that Kepler will need more than double its planned mission life of three-and-a-half years to achieve its main goal of determining how common Earth-like planets are in the Milky Way.

While it is important to find those Earth-sized planets, to me the important discovery here is that Kepler is confirming what previous research has suggested: Stars like our Sun are generally far more active and variable than the Sun itself. Which means that either the Sun is unusual, or has been unusually inactive during recorded human history.

Kepler finds 68 Earthsized planets

Exoplanets galore! The Kepler team announced today the discovery of 68 Earth-sized planets, five in the habitable zone. Key quote:

The discoveries are part of several hundred new planet candidates identified in new Kepler mission science data, released on Tuesday, Feb. 1. The findings increase the number of planet candidates identified by Kepler to-date to 1,235. Of these, 68 are approximately Earth-size; 288 are super-Earth-size; 662 are Neptune-size; 165 are the size of Jupiter and 19 are larger than Jupiter. Of the 54 new planet candidates found in the habitable zone, five are near Earth-sized. The remaining 49 habitable zone candidates range from super-Earth size — up to twice the size of Earth — to larger than Jupiter.

New Kepler results!

Scientists released additional Kepler results [pdf] today, this time describing what they are learning about the stars being observed rather than any planets that might be orbiting them. In studying each star’s minute variations of light, the astronomers can track how the star itself is oscillating like a bell ringing. From this they can do a kind of stellar seismology, finding out a great deal about what is going on inside the star. The data has thus:

  • produced the most precise measurements of the size and age of another star beside the Sun. KIC 11026764 has a radius 2.05 times the size of the Sun, and is now believed to be 5.94 billion years old, slightly older than the Sun’s 4.57 billion years. Though larger than the Sun, this star is a G-type star like the Sun. So far, Kepler has observed about 1500 solar-type stars. The astronomers are still analyzing this data, with results to follow.
  • measured the oscillations of a thousand red giant stars, ranging from slightly larger to dozens of times larger than the Sun. The larger the star, the faster the oscillation and the larger the amplitude, which in turn has confirmed the theories about how the nuclear processes in the core of stars evolve over time, shifting from burning hydrogen to helium. Since these red giants are what our Sun will be like when it reaches old age, we are thus learning something about the Sun’s future.
  • and provided the most precise measurements ever of RR Lyrae stars, a class of unusual variable stars that have puzzled astronomers for more than a century. From this data the astronomers hope to find out exactly why these stars fluctuate as they do.

The variability of stars according to Kepler

More data from Kepler! In a paper [pdf] published today on the astro-ph website, scientists outline Kepler’s census of the variability of stars. Key quote from the abstract:

We have separated the sample in 129,000 dwarfs and 17,000 giants, and further sub-divided, the luminosity classes into temperature bins corresponding approximately to the spectral classes A, F, G, K, and M. G-dwarfs are found to be the most stable with < 20% being variable. The variability fraction increases to 30% for the K dwarfs, 40% for the M and F dwarfs, and 70% for the A-dwarfs. At the precision of Kepler, > 95% of K and G giants are variable.

Kepler finds more than 100 Earthlike planets

In its first six weeks of observation, the Kepler mission apparently found almost 150 planets similar in size to the Earth. The results, learned by accident because a talk given by one of the co-investigators was posted on the web, have not yet been officially announced because the project scientists feel a need for additional time to confirm them. Many of these so-called planets might turn out to be false positives, so some caution is in order.

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