Despite what we've learned about our own solar system, especially over the past few decades, researchers still face many unanswered questions. One of these questions concerns what is known as Planet Nine. The Planet Nine hypothesis states that there is a massive planet in our solar system that orbits a great distance from the sun.
Nobody has ever observed the hypothetical planet. The evidence for this lies in a group of bodies orbiting the sun 250 times farther than the earth. These objects are called e-TNOs for extreme trans-Neptunian objects. It is hypothesized that Planet Nine's gravity is responsible for the unusual cluster orbits of these e-TNOs.
Now astronomers have found a distant solar system with its own planet Nine, and this discovery breathes new life into the hypothesis.
The planet is called HD 106906 b and orbits a binary star 336 light years away. It has a mass of about 11 Jupiters and orbits the stars at a distance of more than 730 times the distance from Earth to the Sun. It's an extremely distant orbit.
The paper with these results is entitled "First detection of orbital motion for HD 106906 b: An exoplanet with great separation in an orbit similar to planet nine". The first author is Meiji M. Nguyen of the University of California at Berkeley. It was published in the Astronomical Journal.
The story of this discovery begins in 2004 when the Hubble HD 106906b first observed it. Little was known about the system at the time. The Hubble observations were follow-up observations aimed at indirect evidence that a warm disk of dust surrounded the star, and astronomers wanted to know more about that disk.
Paul Kalas, also from UC Berkeley, is one of the authors of the new paper. In an email exchange with Universe Today, Kalas explained the interesting background story behind HD 106906 b. "The first Hubble observation was made on July 24, 2004. The aim was to pursue indirect evidence that the central star was surrounded by a disk of dust, similar to the way our solar system has the asteroid belt and the Kuiper belt."
"At the time, we didn't know that the star was actually a binary star, nor did we know that one of the background stars in the field of view was an extrasolar planet instead of a real background star," Kalas explained. "What we did know is that the star HD 106906 was a source of too much infrared radiation, and we concluded that there was a warm disk of dust around it."
Some of the Hubble images from the HD 106906 system from 2004. The upside is an image before the star is moved past the coronographic spot (black circle) and after moving behind the spot. HD 106906 b is displayed as a yellow circle labeled "b". In 2004, HD 106906 b was believed to be a background star. Photo credit: Nguyen et al., 2020.
When astronomers pointed Hubble at the system in 2004, they looked for the dust disk but did not find it, even though they looked closely at it. "The hard drive is so distorted that it is difficult to identify as a hard drive and may have been mistaken for a noise artifact," Kalas said.
They also looked directly at the planet but did not identify it as one. Instead, they thought it was a background star, also because it orbits at such an extreme distance from the stars in the system. "The planet is so far from the binary that one would of course expect to find a background star in this distant location, rather than a planet," Kalas said.
Now the story jumps to 2013, when astronomers used the Magellan telescopes at Las Campanas Observatory in Chile's Atacama Desert to study the system. You have taken several pictures over time that showed movement. Kalas explains, “HD 106906 (the double star) moves across the sky relative to more distant background stars. They discovered that HD 106906b was also moving in the same direction and distance as the binary file. It wasn't a background star at all, but a huge planet physically connected to the binary. "
This is where the Gemini Planet Imager (GPI) comes into play. The GPI is an extremely advanced optical system for the Gemini South Telescope in Chile. It performs both spectroscopic and polarimetric observations. His specialty is detecting gas giants that are relatively close to their stars, which other instruments can hardly do. It is also ideal for studying material discs like the one around the double star HD 106906.
A Gemini Planet Imager image of the disk of dust surrounding HD 106906. Photo credit: GPI / Kalas et al., 2015.
"Then in a research report I led in 2015, we used an advanced ground-based instrument called the Gemini Planet Imager to directly image the disk of dust surrounding the binary," Kalas said. “Looking back at the Hubble data archive, I found that the hard drive was recognized 11 years ago. It was really a distorted planetary system and not a noise artifact. "
In 2017 and 2018, astronomers re-aligned the Hubble with the HD 106906 system to provide a more detailed picture of the system and its hard drive. "I was the lead investigator of the 2017 observation," Kalas said. “In our new work, we measured the movement of the planet relative to its host star using the 14-year Hubble data from 2004 to 2018 for the first time. We found that one year for HD 106906b corresponds approximately 15,000 years later to Earth. "
A 2017 Hubble image of the HD 106906 system. HD 106906 b is marked as a yellow circle. Other stars come from ESA's Gaia Mission Data Release 2, each marked with the last four numbers of their ID. Photo credit: Nguyen et al., 2020.
And this is another observatory and its data: ESA's Gaia mission. Kalas explains: “Without data from another space observatory called Gaia, it would have been impossible to measure its tiny movement over 14 years, which gave us precise positions for the background stars and thus provided a very fine reference grid for measuring position changes over time. "
Astronomers believe that the planet's slow motion orbit is due to its distance from the stars and the weak gravity they exert on it. The orbit is also inclined and elongated, well outside the dusty disk that surrounds the stars. The shape of the debris disk itself is also unusual, which made it difficult to see, and that is likely due to the gravity tug from the planet.
"To highlight why this is strange, we can just look at our own solar system and find that all the planets are roughly in the same plane," said lead author Nguyen in a press release. “It would be bizarre if, for example, Jupiter were only tilted 30 degrees from the plane in which every other planet orbits. This begs all sorts of questions about how HD 106906b ended up so far out on such an inclined orbit. ”
What caused this strangely warped pane? The likely reason is the planet itself. It may have formed much closer to its stars than it does now and then migrated outward. Then it was pulled by the disk of gas surrounding the stars, causing its orbit to disintegrate. So it would actually have come closer to the stars at first.
But twin stars can have complex gravitational forces, and those forces likely threw the planet out of its comfortable position. It was almost expelled from the system and doomed to wander through interstellar space as a rogue planet. Instead, it took an eccentric orbit. All of these activities have probably given the dust disk its unusual shape.
Then, astronomers think, a rogue star passed by. This encounter stabilized the orbit of HD 106906 b, and the system became what we see now. The Gaia Mission also identified candidate stars who may be responsible, which reinforced that statement.
There are parallels between HD 106906 b and the hypothetical planet nine of our own solar system. In the case of Planet Nine, it may have formed near the Sun as well, but was then displaced by interactions with Jupiter. This interaction likely threw Planet Nine into the sub-regions of the solar system, well beyond Pluto's orbit. A passing star may have played the same role it played in HD 106906b's orbit in stabilizing it.
"It's like having a time machine for our own solar system, which was 4.6 billion years ago, to see what might have happened when our young solar system was dynamically active and everything was being pushed around and rearranged," Kalas explained.
One persistent fact, however, remains: there is no direct evidence of Planet Nine. There is only circumstantial evidence.
However, many scientific developments do not start with much more. Neptune was discovered using mathematics long before there were direct observations. And it was discovered in part due to irregularities in its neighbor Uranus' orbit. Orbital irregularities have already led to the discovery of planets.
Six original and eight additional eTNO objects orbit current positions near their perihelion in purple, with a hypothetical planet nine orbit in green. Photo credit: Von Tomruen – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=68955415
We can observe our own solar system much more easily than others, and we know that there are a group of bodies with unusual orbits. Could there be a Planet Nine responsible for them? Could be.
"Despite the lack of discovery of Planet Nine so far, the planet's orbit can be deduced from its effects on the various objects in the outer solar system," said team member Robert De Rosa of the European Southern Observatory in Santiago, Chile who led the analysis of the study. “This suggests that if a planet was actually responsible for what we observe in the orbits of trans-Neptunian objects, it should have an eccentric orbit that is inclined relative to the plane of the solar system. This prediction of Planet Nine's orbit is similar to what we see with HD 106906b. "
The Planet Nine Hypothesis is only one possible explanation. Some astronomers believe that the combined mass of the eTNOs could provide the necessary gravity for their orbits. Others suggest that Planet Nine might actually be a primordial black hole rather than a planet. Others believe that an observational distortion could occur here, and the orbital clustering of eTNOs is only part of the picture.
But the discovery of HD 106906b certainly sheds new light on the Planet Nine hypothesis. We now know that a large planet can end up in an extremely wide orbit. Next we may discover HD 106906 b's own little herd of bodies whose orbits have been shaped by their presence. But that is probably beyond the reach of our observations.
"There are still many unanswered questions about this system."
Robert De Rosa, co-author, ESO.
Better still, maybe we're actually watching Planet Nine. But for that we may have to wait for another leap in the ability watch.
As for the HD 106906 system, we might just be at the beginning of our studies and there are many unanswered questions.
"There are still many unanswered questions about this system," added De Rosa. “For example, we do not know clearly where or how the planet was formed. Although we took the first measurement of orbital motion, there are still great uncertainties about the various orbital parameters. It is likely that both observers and theorists will study HD 106906 in the years to come to unravel the many mysteries of this remarkable planetary system. "