[LINK] "Discovery of the Smallest Exoplanets: The Barnard's Star Connection"

Science Daily's report on the discovery of the planetary system of red dwarf KOI-961 explores how that star's similarities to Barnard's Star--a nearby red dwarf, second-closest star to our solar system after the three of Alpha Centauri, itself believed to host planets by earlier generations of planet-hunters--helped astronomers build a model for KOI-961's system.

The team used data from NASA's Kepler mission combined with additional observations of a single star, called KOI-961, to determine that it possesses three planets that range in size from 0.57 to 0.78 times the radius of Earth. This makes them the smallest of the more than 700 exoplanets confirmed to orbit other stars.

In their investigation of KOI-961, which is about 130 light years away in the Cygnus constellation, the astronomers found that it is nearly identical to Barnard's star, which is only six light years away in the constellation Ophiuchus. This similarity allowed them to use information about Barnard's star, which was discovered in 1916 by Vanderbilt astronomer E.E. Barnard, to determine the mass, size and luminosity of the distant star. These values, in turn, were used to determine the size of the three new exoplanets.

"Barnard's star and KOI-961 are both M dwarfs, which are also known as red dwarfs. This is the smallest category of stars. They are popular targets for exoplanet hunters because their small size makes it easier to detect Earth-sized planets," said Keivan Stassun, the professor of astronomy who headed the Vanderbilt contingent. The other Vanderbilt scientists involved were Research Assistant Professors Joshua Pepper and Leslie Hebb.

From the 1960's through the 1980's, astronomers thought that Barnard's star also had a planetary system -- specifically one or two planets larger than Jupiter. If their existence had been verified, it would have been a scientific first, but the evidence was ultimately discredited. Today, advances in telescope technology and image processing allow astronomers to identify stars with exoplanets with considerable confidence.

[. . .]

Once the size of the star was established, the team used the Kepler data to calculate that the three exoplanets range from the size of Mars to slightly more than three-quarters the size of Earth. They also determined that these planets orbit the star with periods ranging from a half day to two days. Such short periods mean that all three orbit so close to their star that they must be too hot for liquid water to exist and life to evolve, the astronomers calculate.

The diminutive dimensions of this planetary system prompted John Johnson, the principal investigator of the research from NASA's Exoplanet Science Institute at Caltech, to comment, "The really amazing thing about this system is that the closest size comparison is to Jupiter and its moons." (KOI-961 is just 70 percent bigger than Jupiter and its exoplanets are comparable in size and have similar orbital periods to the Galilean moons that circle the Jovian planet.)

Sol Station's profile of Barnard's Star, linked above, suggests that a habitable world orbiting that star would need to have an orbit of roughly one to three weeks.

[LINK] More than an exo-Saturn

Via Bad Astronomy news comes about an another astonishing exoplanet discovery. Calling this an "exo-Saturn" is misleading--the rings are much larger than Saturn's, while the planet itself may be so massive that it isn't a planet but a brown dwarf instead. What's remarkable to me is the fine detail that's produced.

The planet was discovered with the SuperWASP (Wide Angle Search for Planets) telescopes — a UK project that employs low-magnification but very sensitive cameras which can observe large areas of the sky at the same time. It orbits a young star called 1SWASP J140747.93-394542.6, which is 420 light years away. The star’s youth — 16 million years — indicates that the rings are probably the leftover remnants from when the planet formed.

The planet and its rings were discovered using the transit method: looking for small dips in starlight as a planet passes directly between us and the star. This is how the vast majority of exoplanets are found. Usually, when you graph the brightness of the star over time, the dip in the plot as the planet transits the star starts suddenly, drops to some minimum, then jumps back up (see here for example). The whole thing is usually over in a matter of hours at most.

But this planet took nearly two months to transit the star! And the dip was weird: there were multiple times the star dimmed then got brighter again, at one point having 95% of its light blocked. Even though the planet wasn’t seen directly, the most obvious explanation is a ring system similar to Saturn’s (though much larger), blocking the light. It must have gaps in the rings, like Saturn’s do, to explain the starlight jumping up again over time. Overall, four rings were detected, and they stretch tens of millions kilometers in diameter!

And, of course, there are the moons that may yet be sighted.

Perhaps most intriguing about all this are those gaps in the rings. The easiest way to explain them is that there are objects there, moons, sweeping out the material in the rings. Saturn’s rings have gaps for this reason. In fact, there are hundreds of gaps in Saturn’s rings! These are caused by resonances: if a ring particle orbits twice for every one time a moon orbits, for example, the moon’s gravity tugs on it every time it swings by, pulling it into a different orbit. Over time, all the particles in that orbit are gone, leaving behind a gap.

If the planet itself is big, how big are those moons? Could one be Earth-sized? It’s an idea that’s been around awhile, but none has ever been seen… yet. All these super-Jupiters being found have a lot of gravity, and it’s possible they have big moons. We’re also getting better at detecting smaller objects, so it wouldn’t surprise me if that announcement is made sometime relatively soon, too!