Asteroid Remains Around Dead Stars Reveal the Likely Fate of Our Solar System

Artist concept of an asteroid breaking up. (NASA/JPL-Caltech)

(This column was written by my colleague Elizabeth Tasker, now at the Japan Aerospace Exploration Agency (JAXA), Institute of Space and Astronautical Sciences (ISAS).  Trained as an astrophysicist, she researches planet and galaxy formation and also writes on space science topics.  Her book, “The Planet Factory,” came out last year.)

June 30th has been designated “Asteroid Day” to promote awareness of these small members of our solar system. But while asteroids are often discussed in the context of the risk they might pose to the Earth, their chewed up remains around other stars may also reveal the fate of our solar system.

It is 6.5 billion years into our future. The sun has fused hydrogen into a core of heavier helium. Compressed by its own gravity, the helium core releases heat and the sun begins to swell. It is the end of our star’s life, but what will happen to the solar system?

While very massive stars end their element-fusing days in a colossal explosion known as a supernovae, the majority of stars in our galaxy will take a less dramatic exit.

Our sun’s helium core will fuse to form carbon but there is not enough mass to achieve the crushing compression needed for the creation of heavier elements. Instead, the outer layers of the dying star will be blown away to leave a dense remnant with half the mass of our current sun, but squeezed down to the size of the Earth. This is a white dwarf; the most common of all stellar ends.


The life cycle of our sun

The white dwarf rapidly cools to become a dim twinkle in the sky. Within a few million years, our white dwarf will be less luminous that the sun today. Within 100 million years, it will be dimmer by a factor of 100. But examination of white dwarfs in our galaxy reveals this gentle dimming of the lights is not as peaceful as first appears.

The remnants of stars too light to fuse carbon, white dwarfs have atmospheres that should be thin shells of residue hydrogen and helium. Instead, observations have detected 20 different heavy elements in this envelope of gases that include rock-forming elements such as silicon and iron and volatiles such as carbon and nitrogen.

Infrared observations of over forty white dwarfs have additionally revealed compact dusty discs circling the dead stars. Sitting within the radius of a regular star, these could not have formed before the star shrank into a white dwarf. These must be the remains of what occurred as the star morphed from a regular fusion burner into a white dwarf.

This grizzly tale begins with the star’s expansion. Inflated by the heat from the helium core, our sun will increase to 230 times its current size. The outer layers will cool to emit a red hue that earns this bloated dying star the name “red giant”.

The outer layers of our red giant will sweep outwards and engulf Mercury and Venus, possibly stopping just short of the Earth’s position. But for any life remaining on our planet’s surface, the difference between envelopment and near-envelopment is rather moot.

The sun’s luminosity will peak at about 4000 times its current value, roasting Mars and triggering a whole new set of chemical reactions in Jupiter’s huge atmosphere. As the outer layers blow away and the red giant shrinks in mass, the surviving planets will drift outwards onto longer orbits, circling the white dwarf remnant at around twice their current distance from the sun.

The asteroids in our solar system discovered between 1980 – 2015. (Scott Manley)

But if the surviving planets are pushed outwards and the innermost worlds engulfed and vaporized, what is the origin of the compact disc and rocky pollutants? The answer, explains Dimitri Veras, explains Dimitri Veras, a planetary scientist at the University of Warwick in the UK, is asteroids.

Sitting between Mars and Jupiter, the asteroid belt is a band of rocky rubble left over from the planet formation process.

Occasionally, a kick from Jupiter’s gravity can send these space rocks skittering towards the Earth. These become known as “Near-Earth Objects” (NEOs) and are studied both for the potential threat to our planet should they collide, and also for their scientific value as time capsules from the earliest stages of planet formation.

At the moment, two missions are en-route to bring a sample from two different asteroids back to Earth. Japan’s Hayabusa2 mission has just arrived at asteroid Ryugu, returning stunning images of the asteroid to Earth. The NASA OSIRIS-REx mission is traveling to asteroid Bennu, and will arrive later this year.


Asteroid Ryugu images by the ONC-T camera onboard Hayabusa2 between June 18 – 20, 2018.  (JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST)


But sitting further out than Mars, should not the majority of these small celestial bodies be unaffected by the sun’s demise? The problem turns out to be radiation.

Walk outside on a sunny afternoon and you are likely to notice that the ground beneath your feet is hottest at around 2pm in the afternoon, several hours after the sun has moved from directly overhead. This is because it takes time for the pavement to warm and re-emit the solar radiation as heat.

During that time, the Earth has rotated so that this heat radiation is released in a different direction to the absorbed radiation. Like catching a ball and throwing it away at an angle, this difference in direction gives the planet a small kick.

This kick is too small to make a difference to the Earth, but it can have a much more significant result on the evolution of an asteroid. The result is known as the YORP effect (standing for the Yarkovsky-O’Keefe-Radviesvki-Paddock effect, after the mouthful of researchers who developed the theory) and the related phenomenon named after the same first researcher, the Yarkovsky effect. Stemming from the push due to the uneven absorption and emission of radiation, the YORP effect causes a turning torque on asymmetric bodies while the Yarkovsky effect results in a push.


The Yarkovsky Effect describes how outgoing infrared radiation on an asteroid can speed up or slow down its motion, and in time change its orbit.  (A. Angelich, NRAO/AUI/NSF)


As radiation absorption and emission depends on the individual asteroid’s composition and topology, these forces are immensely hard to predict. This point was driven home in February 2013, when the world was primed for the close approach of asteroid Duende.

While everyone watched the sky in one direction, a second asteroid shot towards the Earth and exploded above Russia. This was the Chelyabinsk meteorite whose collisional path had not been anticipated. Studying the changes in an asteroid’s path due to radiation is therefore one of the primary goals of the OSIRIS-REx mission.

Given these challenges at the sun’s current level of radiation, it perhaps is not surprising that the red giant phase has more violent consequences.

Too small for gravity to pull them into a sphere, asteroids are typically lumpy rocks resembling potatoes or dumplings, like the rocky destination of Hayabusa2 and its predecessor which visited asteroid Itokawa. This asymmetry causes differences in the radiative force across the asteroid and creates a torque. This is the YORP effect and it spins the asteroid. As these small bodies typically have a weak tensile strength, the asteroid can self-destruct by spinning itself to pieces.

This effect is seen in our solar system as there is a sharp cut-off in the population for asteroids around 250m in size with rotation periods shorter than 2.33 hours.

As the radiation from our swollen red giant beats down on the asteroid belt, these space rocks will start to spin and fission. The pieces will form a disc of dust around the dying star as it becomes a white dwarf, slowly accreting onto the dead remnant to pollute its atmosphere .

So is this now the end of our tale? A white dwarf surrounded by the fissioned remains of the asteroid belt, orbited by our more distant planets on wide orbits? It could be, depending on the existence of Planet 9.

Proposed by Mike Brown and Konstantin Batygin at the California Institute of Technology, Planet 9 is a possible addition to our solar system that sits on a very distant orbit beyond Neptune. Its presence is suggested by the alignment of six small objects in the Kuiper belt, a second outer band of rocky rubble that includes the dwarf planet, Pluto.

How Planet 9 might have formed remains a subject of debate. A likely scenario is that the planet formed in the neighborhood of the gas giants, but was thrown outwards in a game of gravitational pinball during a chaotic period as our planet-forming disc was evaporating. If this is true, the planet may be able to enact a terrible revenge.


The six most distant objects in the solar system with orbits exclusively beyond Neptune (magenta) all line up in a single direction, indicating the presence of an outside force from an unseen Planet 9. (Caltech/R. Hurt; IPAC)

Running a set of 300 simulations, Veras discovered that the fate of Planet 9 will depend on the planet mass, the distance of its current orbit and how rapidly the sun loses its mass. In the most benign outcome, Planet 9 meets the same fate as the gas giants and drifts outwards onto an even longer orbit. However, there are two situations in which this expansion causes the orbit of Planet 9 to bend.

If a star loses mass gradually, then the orbiting planets will gently spiral outwards and keep their nearly circular paths. But if the stellar mass loss is more rapid, then very distant planets that are more loosely held by the star’s gravity may undergo a runaway expansion of their orbits. As the planet shoots away, its orbit can become bent into an ellipse.

Dimitri Veras is an astrophysicist who researches the contents of planetary systems, including our own, at the University of Warwick, United Kingdom.

Such distant worlds also risk becoming susceptible to the gravitational tug of the surrounding stars in the galaxy. Known as the “galactic tide”, this force is much too weak to affect the planets in their current positions. Yet if Planet 9 drifts too far outwards, then the tidal forces could become strong enough to bend the planet’s orbit.

On an elliptical path, Planet 9 could move from its distant location to swing into the neighborhood of the gas giants. If the planet is massive enough, this could result in either Uranus or Neptune being ejected from the solar system to become rogue worlds: a fitting, final revenge for Planet 9.

Veras’s calculations suggest the most risky discovery for internal harmony would be a Jupiter-sized Planet 9 on an orbit beyond 300 AU, or 300 times the current distance between the Earth and the sun. For comparison, Neptune sits at 30 AU and the dwarf planet Sedna is three times as far, at about 86 AU. Alternatively, a smaller super-Earth Planet 9 could pose a risk if it was further out than 3000 AU.

Observing the gory remains of this process in other star systems provides us with more than just an eerie snapshot of our future. The crushed up asteroids in the atmosphere of white dwarfs reveal the composition of that planetary system.

“There’s no other way of performing an exoplanet autopsy,” explains Veras.

The results can reveal whether the asteroids and planets that orbited the star have a similar composition to our own or something more exotic. So-called “carbon worlds” have been proposed to orbit stars more carbon-rich than our own, whose rocky base may contain graphite and diamond rather than silicates.

So far, the planet autopsy has shown Earth-like remnants, but this is one area in which we would love to see more dead remains.



The Case Strengthens For “Planet 9”

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.”


How Planet 9 Would Make Ours a More Typical Solar System



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:
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:

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.