Astronomers confirm the bound orbit of a distant planet from its star, showing that distant planets exist

Astronomers are still searching for a hypothetical “Planet Nine” in the far reaches of our solar system, but an exoplanet 336 light-years from Earth is looking more and more like Planet Nine in its star system.

Planet Nine, potentially 10 times the size of Earth and orbiting well beyond Neptune in a highly eccentric orbit around the sun, was proposed in 2012 to explain disruptions in the orbits of dwarf planets just beyond the Sun. orbit of Neptune, so-called detached Kuiper belt objects. It remains to find it, if it exists.

An equally bizarre extrasolar planet was discovered far from the star HD 106906 in 2013, the only known planet at such a distance. Although much heavier than the predicted mass of Planet Nine – perhaps 11 times the mass of Jupiter, or 3,500 times the mass of Earth – it too was in a very unexpected place, well above the plane planetary system dust. and tilted at an angle of approximately 21 degrees.

The big question, so far, has been whether the planet, called HD 106906 b, is in an orbit perpetually tied to the binary star – which is only 15 million years old relative to the age of 4.5 billion years from our sun — or if it’s going out of the planetary system, never to return.

In an article published on December 10 in the Astronomical Diary, astronomers finally answer this question. By accurately tracking the planet’s position over 14 years, they determined that it is likely bound to the star in a highly eccentric 15,000-year orbit, making it a distant cousin to Planet Nine.

If it’s in a highly eccentric orbit around the binary, “it begs the question of how these planets got to such great separations,” said Meiji Nguyen, a recent UC Berkeley graduate and first author of the paper. item. “Were they dispersed from the inner solar system? Where did they form there?

According to lead author Paul Kalas, University of California, Berkeley, assistant professor of astronomy, the resemblance to the orbit of the proposed planet nine shows that such distant planets can actually exist and that they can form during the first tens of millions of years. of a star’s life. And based on the team’s other recent findings on HD 106906, the planet appears to favor a scenario where passing stars also play a role.

“Something happens very early on that starts throwing planets and comets outward, and then you have passing stars that stabilize their orbits,” he said. “We are slowly accumulating the evidence needed to understand the diversity of extrasolar planets and how this relates to puzzling aspects of our own solar system.”

A dusty young star with a strange planet

HD 106906 is a binary star system located in the direction of the constellation Crux. Astronomers have studied it extensively over the past 15 years because of its prominent dust disk, which could give rise to planets. Our solar system may have looked like HD 106906 about 4.5 billion years ago when the planets formed in the swirling disk of debris left over from the formation of the sun.

Amazingly, images of the star taken in 2013 by Magellan Telescopes in Chile revealed a planet radiating its own internal heat and located at an unusually large distance from the binary: 737 times farther from the binary than Earth is. from the sun (737 astronomical distances). units, or AU). That’s 25 times farther from the star than Neptune is from the sun.

Kalas, who searches for planets and dust disks around young stars, co-led a team that used the Gemini Planet Imager on the Gemini South Telescope to get the first images of the star’s debris disk. In 2015, these observations provided evidence that led theorists to propose that the planet formed near the binary star and was expelled due to gravitational interactions with the binary. The evidence: the stars’ outer dust disk and inner cometary belt are out of balance, suggesting that something – the planet – has disrupted their symmetry.

“The idea is that each time the planet gets closest to the binary star, it stirs up the material from the disk,” said Robert De Rosa, a member of the European Southern Observatory team in Santiago. , in Chile, who is an alumnus of UC. Postdoctoral fellow at Berkeley. “So every time the planet passes, it truncates the disk and pushes it to one side. This scenario was tested with simulations of this system with the planet in a similar orbit — this was before we knew what was the planet’s orbit was.”

The problem, as those who simulate such planetary interactions have pointed out, is that a planet would normally be kicked out of the system entirely, becoming a rogue planet. Another interaction, perhaps with a passing star, would be needed to stabilize the orbit of an eccentric planet like HD 106906 b.

A similar scenario has been proposed for the formation of Planet Nine: that its interaction with our giant planets early in our solar system’s history expelled it from the inner solar system, after which stars passing through our local cluster stabilized its orbit.

Kalas went in search of such a flying star for HD 106906 b, and last year he and De Rosa, then at Stanford University, reported finding several nearby stars that would have passed close to the planetary system. 3 million years earlier, possibly providing the boost needed to stabilize the planet’s orbit.

Now, with precise measurements of the planet’s orbit between 2004 and 2018, Nguyen, de Rosa and Kalas present evidence that the planet is most likely in a stable, but highly elliptical, orbit around its binary star.

“Although it’s only been 14 years of observations, we were still surprisingly able to get a constraint on the orbit for the first time, confirming our suspicion that it was very misaligned and also that the planet is in an orbit of about 15,000 years.” Nguyen said. “The fact that our results are consistent with predictions is, I think, strong evidence that this planet is, indeed, bound. In the future, a radial velocity measurement will be needed to confirm our findings.”

The science team’s orbital measurements come from comparing astrometric data from the European Space Agency’s Gaia Observatory, which accurately maps the positions of billions of stars, and images from the Hubble Space Telescope. Because Hubble must mask the binary star’s glare to see the dimmer debris disk, astronomers were unable to determine the star’s exact position relative to HD 106906 b. Gaia’s data allowed the team to more accurately determine the binary’s position, and thus plot the planet’s motion relative to the binary between 2004 and 2018, less than a thousandth of its orbital period.

“We can exploit Gaia’s extremely precise astrometry to infer where the main star should be in our Hubble images, and then measuring the position of the companion is rather trivial,” Nguyen said.

As well as confirming the planet’s 15,000-year-old orbit, the team discovered that the orbit is actually much more steeply inclined to the plane of the disk: between 36 and 44 degrees. At its closest approach to binary, its elliptical orbit would bring it no closer than about 500 AU to the stars, implying that it has no effect on the inner planets also believed to be part of the system. This is also the case with Planet Nine, which has no observed effect on any of the eight planets from the sun.

“What I really think makes HD 106906 unique is that it is the only exoplanet we know of that is directly imaged, surrounded by a disk of debris, misaligned to its system, and widely separated “, Nguyen said. “This makes it the only candidate we have found so far whose orbit is analogous to the hypothetical Planet Nine.”

The work was supported by the National Science Foundation (AST-1518332) and the National Aeronautics and Space Administration (NNX15AC89G, NNX15AD95G, HST-GO-14670/NAS5-26555). This work benefited from NASA’s Nexus for Exoplanet System Science (NExSS) Research Coordination Network sponsored by the NASA Science Mission Directorate.

Arline J. Mercier