A planet in an unlikely orbit around a double star 336 light-years away may offer a clue to a mystery much closer to home: a hypothesized, distant body in our solar system dubbed "Planet Nine."

This is the first time that astronomers have been able to measure the motion of a massive Jupiter-like planet that is orbiting very far away from its host stars and visible debris disk. This disk is similar to our Kuiper Belt of small, icy bodies beyond Neptune. In our own solar system, the suspected Planet Nine would also lie far outside of the Kuiper Belt on a similarly strange orbit. Though the search for a Planet Nine continues, this exoplanet discovery is evidence that such oddball orbits are possible.

"This system draws a potentially unique comparison with our solar system," explained the paper's lead author, Meiji Nguyen of the University of California, Berkeley. "It's very widely separated from its host stars on an eccentric and highly misaligned orbit, just like the prediction for Planet Nine. This begs the question of how these planets formed and evolved to end up in their current configuration."

The system where this gas giant resides is only 15 million years old. This suggests that our Planet Nine—if it does exist—could have formed very early on in the evolution of our 4.6-billion-year-old solar system.

An Extreme Orbit

The 11-Jupiter-mass exoplanet called HD 106906 b was discovered in 2013 with the Magellan Telescopes at the Las Campanas Observatory in the Atacama Desert of Chile. However, astronomers did not know anything about the planet's orbit. This required something only the Hubble Space Telescope could do: collect very accurate measurements of the vagabond's motion over 14 years with extraordinary precision. The team used data from the Hubble archive that provided evidence for this motion.

The exoplanet resides extremely far from its host pair of bright, young stars—more than 730 times the distance of the Earth from the Sun, or nearly 68 billion miles. This wide separation made it enormously challenging to determine the 15,000-year-long orbit in such a relatively short time span of Hubble observations. The planet is creeping very slowly along its orbit, given the weak gravitational pull of its very distant parent stars.

The Hubble team was surprised to find that the remote world has an extreme orbit that is very misaligned, elongated and external to the debris disk that surrounds the exoplanet's twin host stars. The debris disk itself is very unusual-looking, perhaps due to the gravitational tug of the wayward planet.

How Did It Get There?

So how did the exoplanet arrive at such a distant and strangely inclined orbit? The prevailing theory is that it formed much closer to its stars, about three times the distance that the Earth is from the Sun. But drag within the system's gas disk caused the planet's orbit to decay, forcing it to migrate inward toward its stellar pair. The gravitational effects from the whirling twin stars then kicked it out onto an eccentric orbit that almost threw it out of the system and into the void of interstellar space. Then a passing star from outside the system stabilized the exoplanet's orbit and prevented it from leaving its home system.

Using precise distance and motion measurements from the European Space Agency's Gaia survey satellite, candidate passing stars were identified in 2019 by team members Robert De Rosa of the European Southern Observatory in Santiago, Chile and Paul Kalas of the University of California.

A Messy Disk

In a study published in 2015, Kalas led a team that found circumstantial evidence for the runaway planet's behavior: the system's debris disk is strongly asymmetric, rather than being a circular "pizza pie" distribution of material. One side of the disk is truncated relative to the opposite side, and it is also disturbed vertically rather than being restricted to a narrow plane as seen on the opposite side of the stars.

"The idea is that every time the planet comes to its closest approach to the binary star, it stirs up the material in the disk," explains De Rosa. "So every time the planet comes through, it truncates the disk and pushes it up on one side. This scenario has been tested with simulations of this system with the planet on a similar orbit—this was before we knew what the orbit of the planet was."

"It's like arriving at the scene of a car crash, and you're trying to reconstruct what happened," explained Kalas. "Is it passing stars that perturbed the planet, then the planet perturbed the disk? Is it the binary in the middle that first perturbed the planet, and then it perturbed the disk? Or did passing stars disturb both the planet and disk at the same time? This is astronomy detective work, gathering the evidence we need to come up with some plausible storylines about what happened here."

A Planet Nine Proxy?

This scenario for HD 106906 b’s bizarre orbit is similar in some ways to what may have caused the hypothetical Planet Nine to end up in the outer reaches of our own solar system, well beyond the orbit of the other planets and beyond the Kuiper Belt. Planet Nine could have formed in the inner solar system and been kicked out by interactions with Jupiter. However, Jupiter—the proverbial 800-pound gorilla in our solar system—would very likely have flung Planet Nine far beyond Pluto. Passing stars may have stabilized the orbit of the kicked-out planet by pushing the orbit path away from Jupiter and the other planets in the inner solar system.

"It's as if we have a time machine for our own planetary system going back 4.6 billion years to see what may have happened when our young solar system was dynamically active and everything was being jostled around and rearranged," said Kalas.

To date, astronomers only have circumstantial evidence for Planet Nine. They've found a cluster of small celestial bodies beyond Neptune that move in unusual orbits compared with the rest of the solar system. This configuration, some astronomers say, suggests these objects were shepherded together by the gravitational pull of a huge, unseen planet. An alternative theory is that there is not one giant perturbing planet, but instead the imbalance is due to the combined gravitational influence of multiple, much smaller objects. Another theory is that Planet Nine does not exist at all and the clustering of smaller bodies may be just a statistical anomaly.

A Target for the Webb Telescope

Scientists using NASA's upcoming James Webb Space Telescope plan to get data on HD 106906 b to understand the planet in detail. "One question you could ask is: Does the planet have its own debris system around it? Does it capture material every time it goes close to the host stars? And you'd be able to measure that with the thermal infrared data from Webb," said De Rosa. "Also, in terms of helping to understand the orbit, I think Webb would be useful for helping to confirm our result."

Because Webb is sensitive to smaller, Saturn-mass planets, it may be able to detect other exoplanets that have been ejected from this and other inner planetary systems. "With Webb, we can start to look for planets that are both a little bit older and a little bit fainter," explained Nguyen. The unique sensitivity and imaging capabilities of Webb will open up new possibilities for detecting and studying these unconventional planets and systems.

The team's findings appear in the December 10, 2020 edition of The Astronomical Journal.

Quelle: NASA

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Exoplanet around distant star resembles our reputed ‘Planet Nine’

 

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

Planet Nine, potentially 10 times the size of Earth and orbiting far beyond Neptune in a highly eccentric orbit about the sun, was proposed in 2012 to explain perturbations in the orbits of dwarf planets just beyond Neptune’s orbit, so-called detached Kuiper Belt objects. It has yet to be found, if it exists.

A similarly weird extrasolar planet was discovered far from the star HD 106906 in 2013, the only such wide-separation planet known. While 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 sitting in a very unexpected location, far above the dust plane of the planetary system and tilted at an angle of about 21 degrees.

The big question, until now, has been whether the planet, called HD 106906 b, is in an orbit perpetually bound to the binary star — which is a mere 15 million years old compared to the 4.5 billion-year age of our sun — or whether it’s on its way out of the planetary system, never to return.

In a paper appearing Dec. 10 in The Astronomical Journal, astronomers finally answer that question. By precisely tracking the planet’s position over 14 years, they determined that it is likely bound to the star in a 15,000-year, highly eccentric orbit, making it a distant cousin of Planet Nine.

If it is in a highly eccentric orbit around the binary, “This raises the question of how did these planets get out there to such large separations,” said Meiji Nguyen, a recent UC Berkeley graduate and first author of the paper. “Were they scattered from the inner solar system? Or, did they form out there?”

According to senior author Paul Kalas, University of California, Berkeley, adjunct professor of astronomy, the resemblance to the orbit of the proposed Planet Nine shows that such distant planets can really exist, and that they may form within the first tens of millions of years of a star’s life. And based on the team’s other recent discoveries about HD 106906, the planet seems to favor a scenario where passing stars also play a role.

“Something happens very early that starts kicking 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 that relates to the puzzling aspects of our own solar system.”

A young, dusty star with a weird planet

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

Surprisingly, images of the star taken in 2013 by the Magellan Telescopes in Chile revealed a planet glowing from its own internal heat and sitting at an unusually large distance from the binary: 737 times farther from the binary than Earth is from the sun (737 astronomical 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 obtain the first images of the star’s debris disk. In 2015, these observations provided evidence that led theorists to propose that the planet formed close to the binary star and was kicked out because of gravitational interactions with the binary. The evidence: The stars’ outer dust disk and inner comet belt are lopsided, suggesting that something — the planet — perturbed the symmetry.

“The idea is that every time the planet comes to its closest approach to the binary star, it stirs up the material in the disk,” said team member Robert De Rosa of the European Southern Observatory in Santiago, Chile, who is a former UC Berkeley postdoctoral fellow. “So, every time the planet comes through, it truncates the disk and pushes it up on one side. This scenario has been tested with simulations of this system with the planet on a similar orbit — this was before we knew what the orbit of the planet was.”

The problem, as pointed out by those simulating such planet interactions, is that a planet would normally be kicked out of the system entirely, becoming a rogue planet. Some other interaction, perhaps with a passing star, would be necessary 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 kicked it out of the inner solar system, after which passing stars in our local cluster stabilized its orbit.

Kalas went looking for such a fly-by star for HD 106906 b, and last year he and De Rosa, then at Stanford University, reported finding several nearby stars that would have zipped by the planetary system 3 million years earlier, perhaps providing the nudge 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 very elliptical, orbit around its binary star.

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

The science team’s orbital measurements came 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 obscure the glare from the binary star to see the dimmer debris disk, astronomers were unable to determine the exact position of the star relative to HD 106906 b. Gaia data allowed the team to determine the binary’s position more precisely, and thus chart the movement of the planet relative to the binary between 2004 and 2018, less than one-thousandth of its orbital period.

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

In addition to confirming the planet’s 15,000-year orbit, the team found that the orbit is actually tilted much more severely to the plane of the disk: between 36 and 44 degrees. At its closest approach to the binary, its elliptical orbit would take it no closer than about 500 AU from the stars, implying that it has no effect on inner planets also suspected to be part of the system. That is also the case with Planet Nine, which has no observed effect on any of the sun’s eight planets.

“What I really think makes HD 106906 unique is that it is the only exoplanet that we know that is directly imaged, surrounded by a debris disk, misaligned relative to its system and is widely separated,” Nguyen said. “This is what makes it the sole candidate we have found thus 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 NASA’s Science Mission Directorate.

Quelle: Berkeley University