The Case Strengthens For “Planet 9”

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Caption: An artist’s conception of Planet X, courtesy of Robin Dienel.
An artist’s conception of Planet 9, or Planet X, which scientists theorize orbits in the distant solar system.  (Robin Dienel/ Carnegie Institution of Washington)

The race is on to find the giant planet that several teams of astronomers are convinced orbits far out beyond Pluto, but is nonetheless still part of our solar system.  Proving the existence of what has become known as Planet X, or Planet 9, would be a discovery for the textbooks and would inevitably change our understanding of how our solar system was formed.

The technology and luck needed to image the planet (if it truly is there) has thus far fallen short, but the discovery of another set of distant solar system objects traveling in surprising orbits has added to the indirect findings that point to a substantial, and perhaps giant planet in the general vicinity.

The new findings come from Scott Sheppard of the Carnegie Institution for Science and Chadwick Trujillo of Northern Arizona University, who two years ago provided some of the first intriguing inklings that this distant planet might exist in our solar system.  That information was subsequently modified and broadened by a California Institute of Technology team, but with the same conclusion that a substantial Planet 9 appeared to be present in the outer solar system.

“What we’ve just released is data on the first extreme objects since the {2014 and 2016} reports of a theorized Planet 9,  and they show the same clustering and orbiting patterns that we think are likely caused by a major planet,” Sheppard said.

“This continues the trend of finding these objects — small dwarf planets or maybe icy objects — that were pushed into similar orbits in ways we think only planets can do.”

“We need more,” he said, “but the evidence is mounting, and at this point I’d say there’s an 80 percent likelihood that the planet is there.”

An illustration of the orbits of 2013 FT28, 2014 SR349, and previously known extremely distant Solar System objects. The clustering of most of their orbits indicates that they are likely be influenced by something massive and very distant, the proposed Planet Nine. Image credit: Robin Dienel.
An illustration of the orbits of newly-discovered small, trans-Neptune objects 2013 FT28, 2014 SR349, and previously known extremely distant Solar System objects. The clustering of most of their orbits indicates that they are likely be influenced by something massive and very distant, the proposed Planet 9 . (Carnegie Institution of Washington, Robin Dienel)

Some call the potential celestial object Planet X and some call it Planet 9 — as in, the ninth planet in our solar system now that Pluto has been demoted to a dwarf planet.  But by any name, the interest is clearly high. The work underway by Sheppard and colleague Chadwick Trujillo of Northern Arizona University on distant solar system dynamics and orbits is supported by NASA, the National Science Foundation and the Department of Energy, as well as the Carnegie Institution.

Shepard spoke to me from Cerro Tololo Inter-American Observatory in the southern region of Chile’s Atacama Desert.   He was about to begin an observing run for more small, distant, but definitely solar system objects, which can range from the size of Earth or Mars to a fraction of the size of Pluto.  They become significant objects when they have the size and mass to become spheres, rather than jagged asteroids.

Scott Shepard, astronomer and trans-Neptune hunter of extreme objects. (Scott Shepard/Carnegie Institution for Science.
Scott Sheppard, astronomer and trans-Neptune hunter of extreme objects. (Scott Shepard/Carnegie Institution for Science.

While the search is now primarily for these small objects, the goal is to find a scientific pathway to the potential Planet 9 — estimated to be between several and 15 Earth masses, and traveling in a very slow orbit that would circle the sun every 10,000 to 20,000 years.

Sheppard said that even if the planet was detected and its orbit determined, it would be so faint that current telescopes would probably wouldn’t be able to pull out important details and characteristics.  But the next generation of much larger ground telescopes, as well as the James Webb Space Telescope, more possibly could.

As interesting to astronomers as the presence and characteristics of a possible Planet 9 might be, it’s also of great interest because of the story it might tell about our early solar system.

Sheppard explained:  “If we find a very large planet, we’ll know it couldn’t have formed way out there – there just isn’t enough material available to form a big object.  Most solar system objects should form in the region of Venus to Earth or Jupiter to Saturn.

“But if another big planet once inhabited those spaces,  it would have created in an extremely chaotic environment.  Inner planets would have all kinds of eccentric orbits because they would be pushed and pulled by the gravity of the big planet, or planets.  So under this theory, at some point the Planet 9 had to get kicked out and over time migrate to its faraway orbit.

“The result is that the orbits in our solar system are now smooth, and the system has stability.  I think it’s fair to say that if that a big Planet 9 had been kicked into the inner solar system rather than the outer solar system, then there’s very little chance that life could have begun and evolved on Earth — too much orbital chaos.  In others, it was a crap shoot, and we may well have been very lucky.”

But there’s another possibility of how a Planet 9 might have arrived at its theorized general location, estimated to be at least 200 times further from the sun than Earth is.  The alternative involves a Planet 9 exoplanet that had been been kicked out of another nearby solar system that formed in the general vicinity of ours.  Such things are known to happen.

“If this turned out to be the case, then we’d know that there were other suns being formed nearby our sun,” Shepard said. “It would have to be a very dense solar environment, and that would also tell us a lot about the formation of our solar system.”

 Object V774104 was discovered in late October, 2015, and is one of the most distant objects ever detected in the solar system. It appears to be about half the size of Pluto, but with an orbit two to three times larger than Pluto's. Credit: Scott Sheppard, Chad Trujillo and Dave Tholen: Subaru Telescope

Object V774104 was discovered in late October, 2015, and is one of the most distant objects ever detected in the solar system. It appears to be about half the size of Pluto, but with an orbit two to three times wider than Pluto’s. (Scott Sheppard, Chad Trujillo and Dave Tholen: Subaru Telescope)

Sheppard’s team is conducting the deepest survey so far for objects beyond Neptune and the Kuiper Belt, a circumstellar disk that lies some 30 to 50 times as far as the Earth is from the sun. It is filled with dwarf planets asteroids, comets, and balls of frozen compounds — remnants of the earliest days of the evolution of the solar system.  The Kuiper Belt is the region that includes Pluto, the now dwarf planet demoted several years ago.

The team has observed nearly 10 percent of the sky using some of the largest and most advanced telescopes and cameras in the world. As they find and confirm these distant and faint objects, they analyze whether their discoveries fit into the larger theories about how interactions with a massive distant planet could have shaped the outer Solar System.

“Right now we are dealing with very low-number statistics, so we don’t really understand what is happening in the outer Solar System,” Sheppard said. “Greater numbers of extreme trans-Neptunian objects must be found to fully determine the structure of our outer Solar System.”

He said that although astronomers believe there are thousands of these small objects,  only about 15 have been positively identified. One discovered by Sheppard and Trujillo in 2014 — designated 2012 VP113 but nicknamed “Biden” — has the most distant known orbit in our solar system.

At the same time, Sheppard and Trujillo noticed that the handful of known extreme trans-Neptunian objects all clustered together and moved at similar orbital angles. These unusual dynamics lead the astronomers to propose that a substantial planet might be shepherding the smaller objects through its gravitational pull.

Cerra Tollolo
Shepard’s team has been using the Dark Energy Camera on the 4-meter Blanco telescope at the Cerro Tollolo Inter-American Observatory in the southern Atacama region of Chile. (above.)  They have also collected data on distant solar system objects with the Japanese Hyper Surpime Camera on the 8-meter Subaru telescope in Hawaii. (National Optical Astronomical Observatory)

The search for Planet 9 is not the first to use the orbits of other bodies as a signpost to another planet.  Indeed, Sheppard said that “we are now in a similar situation as in the mid-19th century when Alexis Bouvard noticed Uranus’ orbital motion was peculiar, which eventually led to the discovery of Neptune.”

The other team most deeply involved with the Planet 9 hunt is led by Mike Brown and Konstantin Batygin of the California Institute of Technology.  They are the ones who made a big splash earlier this year with their predictions of a Planet 9, again based on the orbits of smaller objects.

In an email, we said that the newly detected object “fit perfectly into our Planet 9 hypothesis, so we remain pretty confident that the planet we predicted is indeed the right planet.”

Other groups searching the trans-Neptunian region for planets and information about the early solar system include the Canadian Outer Solar System Origins Survey and the international Dark Energy Survey.

Sheppard said that while the teams searching for Planet 9 are definitely in competition — a discovery would, after all, re-write the textbooks — they are also cooperating in terms of reporting  back to each other if a region of the sky they study comes up with nothing to report.  That way, he said, the teams won’t duplicate efforts where there is no promise of reward.

While Sheppard’s and Brown’s teams have the advantage of access to more sophisticated instruments to work with, it is certainly possible that one of the others will make breakthroughs, and possibly THE breakthrough.

“Actually, it’s quite possible that the planet has already been in some way imaged,”  Sheppard said.  “That happened with Uranus, Neptune and Pluto — they were observed  but not understood before they were actually detected.  Who knows, proof of Planet X {or Planet 9} may already exist in some observatory archive.”

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How Planet 9 Would Make Ours a More Typical Solar System

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The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. The new report shows a planet with 10 times the mass of the earth in a distant eccentric orbit anti-aligned with the other six objects (orange) is required to maintain this configuration. Image: Caltech/R. Hurt (IPAC)"
The six most distant known objects in our solar system with orbits (magenta) exclusively beyond Neptune all mysteriously line up in a single direction. A new report identifies the potential presence of a distant solar system planet — with 10 times the mass of the Earth and in a distant and eccentric orbit (orange) — as the reason why.  (JPL/Caltech; R. Hurt)

There’s been a ton of justifiable excitement these days about the possible discovery of a ninth planet in our solar system — an object ten time the mass  of Earth and 200 times further from the sun.  Especially in the context of the recent demotion of Pluto from a planet to a dwarf planet, the announcement of a potential replacement seems almost karmic, stage managed, in its take-and-give.  This is especially so since the astronomer probably most responsible for the diminished position of Pluto is also the one who now asserts the very far away presence of a different Planet 9 — planetary astronomer Michael Brown of the California Institute of Technology.

The validity of the possible detection of a Planet 9 has set off hot debates — with NASA officials, for instance, making clear that the agency sees the “discovery” as an exciting but early step towards establishing the existence of possible new planet.  We are all drawn to discovery and controversy, so the presence, or non-presence, of the planet has been the focus of attention.

But another most intriguing aspect of the finding has been largely ignored — the way  that such a Planet 9 would make our solar system surprisingly more similar to the many more eccentric exoplanet solar systems now known to be out there.  Our solar system would also suddenly have a range of planets sized more like the galactic norm.

What’s more, there’s reason to consider that a Planet 9 could have been spun off another solar system rather than having been ejected from the inner solar system, as proposed by Brown and colleague Konstantin Batygin.

In other words, Planet 9 may be an “exoplanet” in origin.  And if not, a finding that it was ejected long ago from our inner solar system would answer some questions about why our system seems to be so different from many of the other exoplanetary systems discovered so far.

Mike Brown and Konstanytin Batyglin of Caltech
Astronomers Mike Brown and Konstantin Batygin of Caltech.  They took research by Scott Shepard of the Carnegie Institution for  Science and Chad Trujillo of the Gemini Observatory in Hawaii regarding the unusual paths of objects orbiting beyond Pluto and carried it further to conclude there is a Planet 9 in the distant solar neighborhood.  (Lance Hayshida/Caltech)

“Our Planet 9 has a very eccentric orbit like planets in many other solar systems, and it’s a size of planet not found in our solar system but is the most common in other solar systems,”  said Brown.  “Seems odd to say, but it would make our solar system more normal.”

More specifically, here are the reasons why:

  • The most commonly sized exoplanet detected so far is larger than Earth and smaller than the next largest planet in our solar system, Neptune.  Since the difference in size is substantial — Neptune’s diameter is 4 times greater than Earth’s and its mass is 17 times greater — that leaves a lot of exoplanets of a size category different from anything in our solar system.  This absence has been a puzzle and would be reduced if Planet 9, some 10 times more massive than Earth, was determined to be real.
  • Many, if not most, solar systems identified so far are home to planets with very eccentric orbits.  In our solar system, the eight planets orbit on a generally singular plane, and most orbits are more circular than not.  The proposed Planet 9 would orbit on a very different plane — thirty degrees off the rest of the solar system’s planetary plane — and it circles the Earth in a most peculiar 10,000 to 20,000-year orbit.
  • Astronomers theorize that planets are ejected from their solar systems all the time, and roam through space without an orbit.  But in theory, they can easily move into a solar system where a sun and other planets pull it into an orbit around them.  This kind of planet capture has been successfully modeled many times, and has even once been identified.

 

Artist rendering of possible Planet 9, described in a recent edition of the Astronomical Journal. The authors estimate that the planet comes as close to the sun as 100-200 astronomical units (the distance from the Earth to the Sun) and travels as far away as 1200 AUs. (Caltech/R. Hunt)
Artist rendering of possible Planet 9, described in a recent edition of the Astronomical Journal. The authors estimate that the planet comes as close to the sun as 100-200 astronomical units (the distance from the Earth to the Sun) and travels as far away as 1200 AUs. (Caltech/R. Hunt)

The question of whether the object identified came from the inner solar system (as deemed likely  by Brown) or from elsewhere is a complicated one with a special interest for exoplanet researchers.

As reported by Brown and Batygin, the best theory to explain the faraway presence of Planet 9 is that it was ejected long ago from the region around Jupiter to Neptune.  Such solar system ejections are understood to happen all the time, and it would be a logical explanation given the relative closeness of our solar system planets.  The planet could have gotten knocked off course by coming too close to Jupiter, with its strong gravitational pull.

As theorized by the two authors, the planet could have then come to an orbital rest after being slowed down by gases.  But that wouldn’t occur until it was well past the solar system we know:  each orbit around the sun would take an estimated 15,000 years.

But Hagai Perets, an astrophysicist formerly at the Harvard-Smithsonian Center for Astrophysics and now at the Israel Institute of Technology, says it is equally or perhaps more plausible that Planet 9 (if it exists) came from another solar system entirely.  Having studied “roaming planets” kicked out of their solar systems, he says he is convinced that it could happen.

“Solar systems,” he said, “throw around their planets like we toss balls.

“We know there are planets with very wide orbits, thousands of astronomical units {the distance from the sun to Earth} from their suns.  We need a mechanism to explain this phenomenon, since the planets could not be formed in that region.

Hagai Perets, an astrophysicist at Technion- Israel Institute of Technology. He has studied rogue planets kicked out of their solar systems, and argues that the possible Planet 9 could have arrived from somewhere other than our solar system.
Hagai Perets, an astrophysicist at Technion- Israel Institute of Technology. He has studied rogue planets kicked out of their solar systems, and argues that the possible Planet 9 could have arrived from somewhere other than our solar system.

“That’s where stellar clusters come in, because most stars are formed in these clusters.  With so much activity going on as stars and solar systems are formed, it makes sense that there would be a great scattering of planets in their early epochs, and some of those planets would be ejected completely.

“They become free-floating, rogue planet,” he said.  “We have observational evidence that they exist, as well as our theoretical models.”

Brown agrees that Planet 9 could have come from another solar system, but he believes that an ejection from our inner solar system is the most plausible explanation.

Proposed orbit for a Planet 9 -- eccentric and distant from the sun, like many exoplanets and their host stars. For more information about planets that orbit far, far from their host stars, check out this recent discovery: http://planetquest.jpl.nasa.gov/news/247
Proposed orbit for a Planet 9 — eccentric and distant from the sun, like many exoplanets and their host stars. For more information about planets that orbit far, far from their host stars, check out this recent discovery: http://planetquest.jpl.nasa.gov/news/247

The potential discovery of a Planet 9 was made the way that Neptune was first identified — by detecting its gravitational effects on other objects.  (In the case of Neptune, that meant the effects on Uranus.)  This indirect process of discovery is not dissimilar from the first, and still widely used, method of finding exoplanets — by detecting through radial velocity the gravitational “wobble” that exoplanets cause in their host stars.

Brown and Batygin found evidence for the planet’s existence in the peculiar orbits of objects well beyond Neptune detailed in a previously published study by Scott Shepard of the Carnegie Institution for Science.  The authors analyzed six of the objects and found that they moved in their elliptical orbits while pointing in the same direction and while tilted at similar 30 degrees angles.

“It’s almost like having six hands on a clock all moving at different rates, and when you happen to look up, they’re all in exactly the same place,” Brown said in a statement. “Basically it shouldn’t happen randomly. So we thought something else must be shaping these orbits.”

Brown has a long history of studying the vast Kuiper Belt well beyond Neptune and its untold objects large and small.  It was in the course of his research of these “trans-Neptune objects” that he came to the conclusion that Pluto didn’t meet the accepted standards for what defines a planet.  It was just too small and its presence has little or no effect on surrounding objects.  Having reached that conclusion, he became a leader in the effort to have the planet demoted.

So having been involved in the undoing of the original Planet 9, he is now convinced there is another — very different — Planet 9.  Brown specifically calls it “Planet 9” rather than the long-discussed “Planet X” because, he said, there have been so many false claims made about a possible “Planet X.”

As he explained it:  “We wanted to highlight the strong science behind the finding.”

Now that stronger evidence for the distant world has been discovered, Brown thinks that within five years the planet can be directly imaged by astronomers — or perhaps will be discounted as unable to be confirmed.

Ironically, the naming of a “Planet 9” has already hit some headwinds — well before its existence is confirmed or rejected.  As with the change of the name of Pluto from a “planet” to a “dwarf planet,” there is interesting science behind the objections.

Alan Stern, principal investigator for the New Horizons mission to Pluto, has dismissed the name “Planet 9” due to his firm belief that there are many objects orbiting out beyond Pluto that are potentially planet size.

“I think the number of planets in our own solar system is going to explode, and that this is going to be one of the important lessons of 21st century astronomy.  I think people will get over worrying about their names pretty quickly.”

The reported Planet 9 inhabits the icy realm of the Kuiper Belt. (NASA)
The reported Planet 9 inhabits the icy realm of the Kuiper Belt. (NASA)

Stern pointed to research suggesting the early presence in our solar system of large planets that were later ejected to places unknown.  Some of those planets likely stuck around in far-off orbits like the proposed Planet X (or Planet 9.)

If this turns out to be the case,  Stern said, their existence would confirm his (and others’) long-held belief “that the majority of the planets in our solar system orbit far beyond the classical ones we grew up with.”

In addition to being compelling science, such detections would also support the view that the primary difference between planets in our solar system and exoplanets beyond is simply where they orbit.  Consequently, just as the study of our solar system informs the exploration and characterizing of exoplanets and their systems, so too does the science of exoplanets help better understand our solar system.

Together, they also tell us that our understanding of the vast menagerie of planets out there remains quite limited, with far less known than unknown.

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