Three orbiting two...

Three giant exoplanets found orbiting twin stars

By Ashley Strickland

It's beginning to look a lot like Star Wars in our universe. Ours just happens to have even more Tatooine-like planets attached to multiple stars.

This year, we've already seen discoveries like a planet orbiting two stars, then there was a planet orbiting three stars and now, researchers at the Carnegie Institution for Science have found three giant exoplanets orbiting twin stars. One star hosts two planets, and the second star hosts the third planet.

As far as stars go, these two are rather close together to have such massive, Jupiter-sized planets orbiting them. They are 360 astronomical units (AUs) apart. A single AU represents the distance between the Earth and the sun, or 92.96 million miles. This binary star system and its planets represents the smallest-separation twin binary, where both stars host planets, that has ever been observed, according to lead researcher Johanna Teske.

To put this in perspective, the shortest previous distance between two stars in a binary system hosting planets was 1,000 AU.

The planets and their stars were found 154 light-years away in the constellation Libra. In other words, it would take more of a ship than the Millennium Falcon to make the Kessel Run in less than 12 parsecs -- or in this case, 47 parsecs.

There are also some unique quirks associated with this "weird system," as Teske refers to it. To find one with all of these properties is so rare that the odds total up to a few percent.

Most exoplanet discoveries involve what researchers like to call super-Earths, or planets that can be a few times the size of ours but not as big as Neptune. These three planets are Jupiter-sized, which has only been found around a small percentage of stars.

The two stars they orbit are "metal poor." Rather than being heavy in iron or oxygen, these stars are mostly made up of hydrogen and helium. Most stars hosting giant exoplanets are "metal rich." This system is incredibly rare, being one of only seven ever found like it.

"This is important for planet formation because we think some minimal amount of solid material is needed in a protoplanetary disk, where planets form around a star, to make giant planets quickly enough, before the gas disperses and the giant planet cores can accrete enough gas to become giants," Teske said.

The twin stars are also more fraternal rather than identical, according to Teske. They differ slightly in chemical composition, with one having more elements that are less volatile and don't evaporate easily, such as silicon, iron, magnesium and titanium. But the two stars are almost identical in surface gravity and temperature, which allowed the scientists to measure the difference in chemical composition.

One proposed reason behind this difference? One of the stars could have swallowed "baby planets" early on, changing its composition.

Researchers used the Planet Finding Spectrograph, which was developed by Carnegie scientists, and placed on the Magellan Clay Telescopes at Las Campanas Observatory in Chile. This is the first published result of planet detections to come from that spectrograph, which has been up and running for six years. Teske said that her team needed every bit of that data to confirm that the planets were out there.

The spectrograph is also designed to find large planets with long or elliptical orbits, rather than short or circular orbits of many other planets. These long orbits can span Earth years, rather than days or months.

Even though it's far away, studying these kinds of systems can teach us about how our own solar system formed since it is so different from all of the others that have been found, Teske said.

"To find three planets around stars that are so metal-poor was not expected," Teske said. "Maybe there is something else, or an additional variable, that controls the formation of giant long-period planets [such as Jupiter]. Jupiter and Saturn had a huge influence over the evolution and eventual architecture of our solar system because they were the most massive planets."

The more long-period planets they find, the more answers they may have about why some planets are located where they are as well.

"The planets around [the stars] are moderately eccentric, so they could have dynamically influenced small planets earlier in the lifetime of the system and caused the small planets to be ejected from the system or flung into the star," Teske said. "One hypothesis for the existence of newly discovered 'Planet Nine,' for example, is that it was captured from another star that passed close to the sun early in the formation of the solar system."