An exoplanet discovered by Hubble looks like the “new planet” in our solar system

Astronomers confirm the linked orbit for the planet far from its star, showing that there are distant planets.

Astronomers are still looking for a hypothetical “Planet Nine” in the far reaches of our solar system, but a exoplanet 336 light years from Earth increasingly resembles planet nine in its star system.

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

An equally strange extrasolar planet was discovered far from 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, 3,500 times the mass of Earth – it too was in a very unexpected location, well above the dust plane of the planetary system and tilted at an angle of about 21 degrees.

The big question, so far, has been whether the planet, called HD 106906 b, is in an orbit perpetually linked to the binary star – which is only 15 million years old compared to its age. 4.5 billion years of our sun – or whether it’s about to leave the planetary system, never to return.

In an article published in December 2020 in The astronomical journal, astronomers are finally answering this question. By accurately tracking the planet’s position over 14 years, they determined that it is likely tied to the star in a very eccentric 15,000-year orbit, making it a distant cousin of planet nine.

Exoplanet HD 106906b

Artist’s impression of the exoplanet HD 106906 b located at a great distance from the central binary star and the disk of dusty matter which surrounds it. Credit: NASA, ESA and M. Kornmesser (ESA / Hubble)

If this is a very eccentric orbit around the binary, “It raises the question of how these planets came to such great separations,” said Meiji Nguyen, a recent UC Berkeley graduate and first author of the article. “Were they scattered from the inner solar system? Or, did they get trained there? “

According to lead author Paul Kalas, University of California, Berkeley, assistant professor of astronomy, the resemblance to the proposed orbit of Planet Nine shows that planets so far apart can actually exist, and that they can form during the first tens of millions of years of the life of a star. 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 stars passing by 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 the 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 due to its prominent dust disk, which could be the birth of planets. Our solar system may have looked like HD 106906 around 4.5 billion years ago, when planets formed in the swirling disc of debris left behind by the formation of the sun.

Exoplanet HD 106906 b Orbit

This image from the Hubble Space Telescope shows a possible orbit (dotted ellipse) of the 11-mass exoplanet Jupiter HD 106906 b. This distant world is largely separated from its host stars, whose brilliant light is here masked to allow the planet to be seen. The planet resides outside of its system’s circumstellar debris disk, which is akin to our own Kuiper Belt of small icy bodies beyond Neptune. The disc itself is asymmetrical and warped, possibly due to the gravitational pull of the wayward planet. The other bright spots in the image are background stars. Credit: NASA, ESA, M. Nguyen (University of California, Berkeley), R. De Rosa (European Southern Observatory) and P. Kalas (University of California, Berkeley and SETI Institute)

Surprisingly, images of the star taken in 2013 by Magellanic telescopes in Chile revealed a planet radiating its own internal heat and located an unusually large distance from the binary: 737 times farther from the binary than Earth is. east of the sun (737 astronomical units, or AU). It is 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 proof: the outer dust disk and inner cometary belt of stars are out of balance, suggesting that something – the planet – has disturbed the symmetry.

“The idea is that whenever the planet comes closest to the binary star, it stirs up matter in the disc,” said Robert De Rosa, team member at the European Southern Observatory in Santiago, in Chile, which is a former UC. Berkeley Postdoctoral Fellow. “So every time the planet passes, it truncates the disc and pushes it to one side. This scenario was tested with simulations of this system with the planet in a similar orbit – that was before we knew what the planet’s orbit was.

The problem, as those who simulate such planetary interactions have pointed out, is that a planet would normally be entirely kicked out of the system, 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 was proposed for the formation of planet nine: its interaction with our giant planets early in the history of our solar system expelled it from the inner solar system, after which the stars passing through our local cluster stabilized its orbit.

Kalas went in search of such a hovered star for HD 106906b, and last year he and De Rosa, then at Stanford University, reported finding several nearby stars that were believed to have passed through the system. planetary 3 million years earlier, perhaps 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.

Exoplanet HD 106906b Evolution

This graphic shows how the exoplanet HD 106906 b could evolve over time, to arrive at its current orbit, largely separated, eccentric and very misaligned. (1) The planet formed much closer to its stars, inside a circumstellar disk of gas and dust. The disc’s drag caused the planet’s orbit to disintegrate, forcing it to spiral inward toward its star pair. (2) The gravitational effects of the host stars then propelled the planet into an unstable orbit that nearly knocked it out of the system and into the void of interstellar space. (3) A star passing from outside the system stabilized HD 106906 b’s orbit and prevented the planet from leaving its home system. Credit: Graphic courtesy of NASA, ESA and L. Hustak / STScI

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

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 The Hubble Space Telescope. Because Hubble must mask the glow of the binary star to see the weaker debris disk, astronomers could not determine the star’s exact position relative to HD 106906b. Gaia’s data allowed the team to more precisely determine the position of the binary, and thus plot the movement of the planet relative to the binary between 2004 and 2018, less than a thousandth of its orbital period.

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

In addition to confirming the planet’s orbit at 15,000 years, the team found that the orbit is actually much more tilted from the plane of the disc: between 36 and 44 degrees. At its closest binary approach, its elliptical orbit would not bring it about 500 AU closer to the stars, implying that it has no effect on 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 sun’s eight planets.

“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 disc of debris, misaligned with its system, and widely separated. “said Nguyen. “This makes it the only candidate we have found so far whose orbit is analogous to the hypothetical Planet Nine.”

Learn more about this research:

Reference: “First Detection of Orbital Motion for HD 106906 b: A Wide-separation Exoplanet on a Planet Nine – like Orbit” by Meiji M. Nguyen, Robert J. De Rosa and Paul Kalas, December 10, 2020, The astronomical journal.
DOI: 10.3847 / 1538-3881 / abc012

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 ">NasaNexus for Exoplanet System Science (NExSS) research coordination network sponsored by the NASA Science Missions Directorate.

Arline J. Mercier

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