Exoplanet Friday

Kepler Confirms Thousands of New Planets

Yay, statistics! (No, really…) On Tuesday, May 10, NASA announced that a new statistical method, called probabilistic validation, has resulted in the biggest haul of confirmed extrasolar planets—ever! More than a thousand new exoplanet discoveries showed up in a re-analysis of data from the planet-hunting Kepler mission. From 2009–13, Kepler observed 150,000 stars, searching for tiny decreases in their light output caused by planets moving in front of them (similar to May 9’s transit of Mercury, only happening a lot farther away). During that time, the spacecraft collected so much data that it couldn’t be analyzed all at once and will instead be released in batches, with the final one expected in September 2017.

The new analytical method looks at large numbers of objects at once instead of laboriously crunching the numbers for one object at a time. It was applied to the seventh catalog of Kepler Objects of Interest, which was collected in July 2015. From that dataset of 4,302 potential candidates, 1,284 objects are being given a 99 percent probability of being planets, with nearly 550 of them possibly rocky in composition and nine of those located in the habitable zones of their host stars, where the temperature would allow water to exist in liquid form on an Earth-ish-sized planet. From that same sample, 1,327 are deemed likely planets, but with less than a 99 percent certainty: these will require further study. In addition, 984 previously-confirmed exoplanets were re-validated, and 707 bodies were identified as phenomena other than planets, such as eclipsing binary star systems, in which a star’s light is being blocked by another star rather than a planet (you can have fun simulating that here).

The result of this new analysis raises the total of confirmed extrasolar planets to more than 3,200, with 2,325 of them discovered by Kepler alone. Its primary mission now complete, Kepler has been repurposed to continue observations in other areas, and it will be succeeded by the Transiting Exoplanet Survey Satellite (TESS). After its planned launch in 2018, TESS will build on Kepler’s successes in continuing the search for Earth-size planets. –Bing Quock

Resonant Exoplanets

The Kepler-223 system, 4,458 light years away, is quite different from our own solar system. Its four gaseous planets, much larger than Earth and slightly smaller than Neptune, orbit very close to their star in resonance.

Planets are in resonance when, for example, for every one orbit a planet makes around its star, another planet orbits the same star twice. Jupiter’s moons, where the phenomenon first described, display resonance, and Pluto orbits the Sun twice during the same period that Neptune completes three orbits. Outside the Solar System, astronomers have seen systems containing two or three planets in resonance, but until Kepler-223, not four.

“The orbital periods of the four planets of the Kepler-223 system have ratios of exactly three to four, four to six, and six to eight,” says Penn State’s Eric Ford.

“This is the most extreme example of this phenomenon,” confirms the University of Chicago’s Daniel Fabrycky.

And they’ve been locked in this resonance for billions of years, the team reports this week in Nature.

Studying systems like Kepler–223 provides a rare opportunity to test models of planet formation.

Some stages of planet formation can involve violent processes, but during other stages, planets can evolve from gaseous disks in a smooth, gentle way, which is probably what the sub-Neptune planets of Kepler-223 did, says the University of Chicago’s Sean Mills, one of the study’s co-authors.

“We think that two planets migrate through this disk, get stuck and then keep migrating together; find a third planet, get stuck, migrate together; find a fourth planet and get stuck,” Mills explains.

This process differs completely from the one that scientists believe led to the formation of Earth, Mercury, Venus, and Mars, which likely formed in their current orbital locations. Earth formed from Mars- or moon-sized bodies smacking together, Mills continues, a violent and chaotic process. When planets form this way their final orbital periods are not near a resonance.

And the Solar System’s larger, more distant planets of today—Jupiter, Saturn, Uranus, and Neptune—moved around substantially during their formation. They may have been knocked out of resonances that once resembled those of Kepler-223, possibly after interacting with numerous asteroids and small planets (planetesimals).

“Before we discovered exoplanets, we thought that every system must form like ours,” says co-author Howard Isaacson of UC Berkeley. “Thanks to Kepler, we now have hot Jupiters, many planets that are closer to their star than Mercury or in between the size of the Earth and Neptune. Without the discovery of exoplanets, we would not have known that the Earth is something of an outlier.” –Molly Michelson

Yeah, It's Not Aliens

Back on October, we reported on the mysterious star KIC 8462852, a big, bright star about 1,500 light years away in the constellation Cygnus that exhibited multiple, periodic, deep dips in its brightness, inconsistent with planets simply transiting the face of the star. Speculations abounded on what could be causing such unusual readings—everything from a large comet field to a debris disk from a major collision to an alien constructed Dyson Sphere surrounding large sections of the star.

As to be expected, follow up studies have been conducted. The SETI Institute even broke out their Allen Telescope Array to scan the region for radio signals, but found no such evidence of alien transmissions.

Another study, published in January by Louisiana State University, examined archived photographic glass plates (a dataset called Digital Access to a Sky Century @ Harvard or DASCH) of the sky taken by Harvard astronomers between 1885-1993 and used it to conclude that KIC 8462852 has dimmed around 20 percent over that last 100 years. This kind of dimming would be hard to explain with natural phenomena, however it is actually consistent with the Dyson Sphere hypothesis—aliens taking material from the planetary system and to build a mega structure that absorbs increasing amounts of light from the star.

So… aliens? Don’t get your hopes up just yet…

This week, a new report published in the Astrophysical Journal called the dataset, which included images taken from multiple telescopes and cameras over a century, into question. “Whenever you are doing archival research that combines information from a number of different sources, there are bound to be data precision limits that you must take into account,” explains Keivan Stassun of Vanderbilt. “We looked at variations in the brightness of a number of comparable stars in the DASCH database and found that many of them experienced a similar drop in intensity in the 1960’s. That indicates the drops were caused by changes in the instrumentation not by changes in the stars’ brightness.”

One more strike against alien megastructure; however, no single hypothesis can yet explain all of the observations around star KIC 8462852.

“What does this mean for the mystery?” asks Michael Hippke, an amatur astromer working alongside the Vanderbilt team. “Are there no aliens after all? Probably not! Still, the dips found by Keplerare real. Something seems to be transiting in front of this star and we still have no idea what it is!”

Even without aliens, KIC 8462852 remains, according to discoverer Tabetha Boyajian of Yale, “the most mysterious star in the Universe” and continues to inspire astronomers, professional and amateur alike, around the world. –Elise Ricard

​Image: NASA Ames/ W Stenzel