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Artist's concept of star with close-in exoplanet. Credit: NASA/JPL-Caltech

Star Stuff: If Earth had nightmares, they would be about this star

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Oct 19, 2017

Star Stuff is a weekly column by rocket scientist & astrophysicist Summer Ash highlighting some amazing things happening every day on and off the planet, especially great science done by and/or for women. She harnesses her science communication powers to smash the patriarchy and advocate for equality and inclusion across all time and space. Throwdowns with pseudoscience may occur.

Humans have been telling stories about the sky ever since we first looked up at the stars and wondered about our origins. Now in a twist, astronomers at Princeton University are co-opting those stories to explain behavior in the heavens rather than on Earth. The team, led by graduate student, Semyeong Oh, have identified a pair of stars, one of which they believe has “eaten” some of it’s planets. They have christened the planets Kronos and Krios. 

In Greek mythology, Kronos is the son of the sky (Uranus) and the earth (Gaia). Family drama was stirred up when Uranus hid away two of his later, younger children from Gaia and she was not having it. So she put a hit out on Uranus, and managed to recruit Kronos to do the dirty deed of castrating his father. It wasn’t until much later that Kronos learned he was destined to be overthrown by his sons just as he had overthrown his father. In his attempt to escape the prophecy, he ate his first five children as soon as they were born. As you might have guessed, his wife Rhea was not a fan and I could go on and on about what happened next, but let's GET BACK TO SCIENCE.

Kronos, the planet, is half of a wide binary star system. Kronos and Krios (its partner), also known as HD 240430 and HD 240429, are sun-like stars gravitationally bound to each other as they orbit in the Milky Way. They are roughly 350 light-years from Earth, and about two light-years apart from each other, orbiting once every 10,000 years. 

Wide binaries like this are of interest to astronomers because how their orbits evolve over time can tell us something about the environments they pass through. In contrast to tight binaries, where the stars are much closer to each other, wide binaries are far enough apart that if one star or the other has an encounter with a massive object, it’s orbit (and consequently the binary) might be altered. The way in which the orbit is altered depends on whether it had a brush with another star or a molecular cloud or a clump of dark matter. In essence, astronomers can use wide binaries as a proxy for the distribution of matter in our galaxy.

Oh and her team discovered Kronos and Krios as part of their search for wide binaries in the first data release from the European Space Agency’s (ESA) Gaia mission. Gaia is a spacecraft designed to map the distances and positions of over a billion stars in our galaxy. Launched in December 2013, the first data release of positions and magnitudes for 1.1 billions stars and proper motion and parallaxes for ~2 million stars was made publically available in September 2016. Using this data, Oh found over 13,000 candidate “comoving” (i.e., similar enough velocities/motions to assume moving through space together) pairs of stars.

A subset of these candidates were found to be possible wide binaries, one of which turned out to be Kronos and Krios. In order to find out more about the pair, Oh obtained high resolution spectra taken with the Keck Observatory atop Mauna Kea in Hawaii and that’s when thing started to get interesting. Binary stars are expected to have very similar chemical compositions because they are born at the same time, from the same star-forming nebula. The spectra showed that Kronos was composed of the same elements as Krios, but not the other way around - Kronos had a significantly higher metallicity. (For astronomers, everything other than hydrogen and helium is considered a metal...don’t ask.) Oh found other binaries with differening chemical compositions, but none were nearly as dramatic.

HD 240430 and HD 240429, aka Kronos and Krios, as they appear in the Space Telescope Science Institute’s Digitized Sky Survey. Credit: Oh et al. 

Analysis showed that Kronos had an unusual amount of material normally found in rocky planets: magnesium, aluminum, silicon, iron, chromium and yttrium. And it was unusually low in gaseous materials such as oxygen, carbon, nitrogen, and potassium. The presence of these minerals alone wasn’t enough to raise a red flag, but the combination with the suppression of the volatiles is what caught Oh’s eye. The only way this could happen was if Kronos had engulfed a large amount of rocky material that was likely once in the form of planets or planetesimals in orbit around it. How large an amount? Likely 15 Earths worth of material. Nom nom nom.

While we have yet to find an exoplanet system with 15 planets, let alone 15 Earth-sized planets, that doesn’t mean this is abnormal. Plenty of planetary systems have that much mass distributed across fewer planets and we’re discovering more and more “super-Earths” which are large, but not large enough to turn into gas giants instead of terrestrial worlds.  The star HD 69830 has a planet discovered in 2006 via microlensing is 10 Earth masses. In 2008, the star HD 40307, a star only slightly less massive than our Sun, was found to have three super-Earths roughly 4.2, 6.7, and 9.4 Earth masses, respectively. And in 2012, the star Gliese 163 was discovered to have a planet just under 7 Earth masses. So a snack of 15 Earth masses is well within the realm of possibility.

If there are more Kronos-like stars out there, it could mean re-examining of how astronomers think about the chemical evolution of stars. It is currently thought that stars mostly maintain the chemical abundances they are born with (barring any catastrophic mergers), but Kronos flies in the face of that. Additionally, the process of planet formation is still poorly understood, especially the early stages when young stars are obscured by massive amounts of dust and the composition of the star can’t be distinguished from the composition of the dusty disk. So finding more Kronos-like stars would help to better constrain the abundances stars begin with versus the abundances they might get from all that snacking. 

It's still not clear what caused Kronos to engulf its planets in the first place. It could have been anything from a passing star or gravitional interactions with potential gas giant planets that might still exisit. If all goes according to plane, Gaia’s second data release will go public in April of 2018 with improved distances, parallaxes, proper motions, etc. and the team will go digging for signs of any large, lurking giants that may have triggered Kronos's inner plannets to fall into the star. At the same time, the researchers are also embarking on a radial velocity observing campaign for both stars that should also reveal the presence of any remaining planets. Stay tuned!