"Long before it's in the papers"
January 20, 2016

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“Solid evidence” of ninth planet claimed

Jan. 20, 2016
Courtesy of Caltech
and World Science staff

Re­search­ers are re­port­ing ev­i­dence of an un­seen, gi­ant plan­et trac­ing a bi­zarre, stretched-out or­bit in the out­er so­lar sys­tem. The sci­en­tists are de­scrib­ing the find­ings in the cur­rent is­sue of the As­tro­nom­i­cal Jour­nal.

“This would be a real ninth plan­et,” said Mike Brown, an as­tron­o­mer at the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy and co-author of the stu­dy. “There have only been two true plan­ets dis­cov­ered since an­cient times, and this would be a third.”

This artist's ren­der­ing shows the dis­tant view from Plan­et Nine back to­wards the sun. The plan­et is thought to be gas­e­ous, si­m­i­lar to Ura­nus and Nep­tune. Hy­po­thet­i­c light­ning lights up the night side. (Cred­it­Cal­tech/R. Hurt (IPAC))


Plu­to was tra­di­tion­ally con­sid­ered the so­lar sys­tem’s ninth and furthest-out plan­et. But it was de­mot­ed to the sta­tus of dwarf plan­et in 2006 af­ter as­tron­o­mers con­clud­ed it did­n’t really meet the cri­te­ria to be con­sid­ered a plan­et.

“All those peo­ple who were mad that Plu­to is no long­er a plan­et can be thrilled to know that there’s a real plan­et out there,” still to be no­ticed through tele­scopes, Brown said. The plan­et is about 10 times the mass or weight of Earth, he added, mak­ing it slightly smaller than Nep­tune, the most sim­i­lar-weighing among the other Solar System plan­ets.

The ob­ject, which the re­search­ers have nick­named Plan­et Nine, or­bits an es­ti­mat­ed 20 times far­ther from the sun on av­er­age than does the eighth plan­et, Nep­tune, which or­bits the sun at an av­er­age dis­tance of 2.8 bil­lion miles. In fact, it would take this new plan­et be­tween 10,000 and 20,000 years to make just one full trip around the sun.

But it has­n’t been seen di­rect­ly. Brown and co-research­er Kon­stan­tin Baty­gin in­ferred its ex­ist­ence through math­e­mat­i­cal mod­el­ing and com­put­er sim­ula­t­ions.

Brown said the pu­ta­tive ninth plan­et is big enough that there should be no ques­tion it’s a true plan­et. Un­like the class of smaller ob­jects now known as dwarf plan­ets, Plan­et Nine gravita­t­ionally dom­i­nates its neigh­bor­hood of the so­lar sys­tem. In fact, it dom­i­nates a re­gion larg­er than any of the oth­er known plan­ets—a fact that Brown said makes it “the most plan­et-y of the plan­ets in the whole so­lar sys­tem.”

Baty­gin and Brown de­scribe in their pa­per how Plan­et Nine helps ex­plain var­i­ous mys­te­ri­ous fea­tures of the field of icy ob­jects and de­bris be­yond Nep­tune known as the Kuiper Belt.

“Although we were in­i­tially quite skep­ti­cal that this plan­et could ex­ist, as we con­tin­ued to in­ves­t­i­gate its or­bit and what it would mean for the out­er so­lar sys­tem, we be­come in­creas­ingly con­vinced that it is out there,” said Baty­gin. “For the first time in over 150 years, there is sol­id ev­i­dence that the so­lar sys­tem’s plan­etary cen­sus is in­com­plete.”

Two years ago, a form­er post­doc of Brown’s, Chad Tru­jil­lo, and his col­league Scott Shep­pard pub­lished a pa­per not­ing that 13 of the most dis­tant ob­jects in the Kuiper Belt are si­m­i­lar with re­spect to an ob­scure or­bital fea­ture. To ex­plain that si­m­i­lar­ity, they sug­gested the pos­si­ble pres­ence of a small plan­et. Brown thought that un­like­ly, but got cu­ri­ous.

He took the prob­lem down the hall to Baty­gin, and the two started a col­la­bora­t­ion to in­ves­t­i­gate the ob­jects. They soon realized that the six most dis­tant ob­jects from Tru­jil­lo and Shep­herd’s orig­i­nal col­lec­tion all fol­low el­lip­ti­cal or­bits that point in the same di­rec­tion in phys­i­cal space. That’s sur­pris­ing be­cause the out­ermost points of their or­bits move around the so­lar sys­tem, and they trav­el at dif­fer­ent rates.

“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,” said Brown. The odds of hav­ing that hap­pen are some­thing like one in 100, he said. Further, the or­bits of the six ob­jects are al­so all tilted in the same way. The prob­a­bil­ity of that hap­pening is about 0.007 per­cent—“ba­sic­ally it should­n’t hap­pen ran­dom­ly,” Brown said. “So we thought some­thing else must be shap­ing these or­bits.”

It turned out, he added, Plan­et Nine’s ex­ist­ence helps ex­plain more than just the align­ment of the dis­tant Kuiper Belt ob­jects. It al­so ex­plains mys­te­ri­ous or­bits that two of them trace, in­clud­ing Sedna, dis­cov­ered by Brown in 2003. 

Where did Plan­et Nine come from and how did it get where it is? Sci­en­tists have long be­lieved that the early so­lar sys­tem be­gan with four plan­etary co­res that went on to grab all of the gas around them, form­ing the four gas plan­ets—Ju­pi­ter, Sat­urn, Ura­nus, and Nep­tune. Over time, col­li­sions and ejec­tions shaped them and moved them out to their pre­s­ent loca­t­ions. “But there is no rea­son that there could not have been five co­res, rath­er than four,” said Brown. Plan­et Nine could repre­s­ent that fifth co­re, and if it got too close to Ju­pi­ter or Sat­urn, it could have been ejected in­to its dis­tant, stretched or­bit.

Brown said as­tron­o­mers should be able to spot the plan­et in im­ages cap­tured by pre­vi­ous sur­veys. If it is in the most dis­tant part of its or­bit, the world’s larg­est tele­scopes—such as the twin 10-meter tele­scopes at the W. M. Keck Ob­serv­a­to­ry and the Sub­aru Tel­e­scope, all on Mauna Kea in Hawai­i—will be needed to see it. If, how­ev­er, Plan­et Nine is now lo­cat­ed an­ywhere in be­tween, many tele­scopes have a shot at find­ing it.

“I would love to find it,” said Brown. “But I’d al­so be per­fectly hap­py if some­one else found it. That is why we’re pub­lish­ing this pa­per. We hope that oth­er peo­ple are go­ing to get in­spired and start search­ing.”

In terms of un­der­stand­ing more about the so­lar sys­tem’s con­text in the rest of the uni­verse, Baty­gin said that in a cou­ple of ways, this ninth plan­et that seems like such an odd­ball to us would ac­tu­ally make our so­lar sys­tem more si­m­i­lar to the oth­er plan­etary sys­tems that as­tron­o­mers are find­ing around oth­er stars. First, most of the plan­ets around oth­er sun­like stars have no sin­gle or­bital range—that is, some or­bit ex­tremely close to their host stars while oth­ers fol­low ex­cep­tion­ally dis­tant or­bits. Sec­ond, the most com­mon plan­ets around oth­er stars range be­tween one and 10 Earth-masses.

“One of the most startling disco­veries about oth­er plan­etary sys­tems has been that the most com­mon type of plan­et out there has a mass be­tween that of Earth and that of Nep­tune,” said Baty­gin. “Un­til now, we’ve thought that the so­lar sys­tem was lack­ing in this most com­mon type of plan­et. May­be we’re more nor­mal af­ter al­l.”


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Researchers are reporting evidence of an unseen, giant planet tracing a bizarre, stretched-out orbit in the outer solar system. The scientists are describing the findings in the current issue of the Astronomical Journal. “This would be a real ninth planet,” said Mike Brown, an astronomer at the California Institute of Technology and co-author of the study. “There have only been two true planets discovered since ancient times, and this would be a third. It’s a pretty substantial chunk of our solar system that’s still out there to be found, which is pretty exciting.” Pluto was traditionally considered the solar system’s ninth and furthest-out planet. But it was demoted to the status of dwarf planet in 2006 after astronomers concluded it didn’t really meet the criteria to be considered a planet. “All those people who were mad that Pluto is no longer a planet can be thrilled to know that there’s a real planet out there,” still to be noticed through telescopes, Brown said. The planet is about 10 times the mass or weight of Earth, he added, making it slightly smaller than Neptune, the most similar-weighing body among the previously known planets. The object, which the researchers have nicknamed Planet Nine, orbits an estimated 20 times farther from the sun on average than does the eighth planet, Neptune, which orbits the sun at an average distance of 2.8 billion miles. In fact, it would take this new planet between 10,000 and 20,000 years to make just one full trip around the sun. But it hasn’t been seen directly. Brown and co-researcher, Konstantin Batygin inferred its existence through mathematical modeling and computer simulations. Brown said the putative ninth planet is big enough that there should be no question it’s a true planet. Unlike the class of smaller objects now known as dwarf planets, Planet Nine gravitationally dominates its neighborhood of the solar system. In fact, it dominates a region larger than any of the other known planets—a fact that Brown said makes it “the most planet-y of the planets in the whole solar system.” Batygin and Brown describe in their paper how Planet Nine helps explain various mysterious features of the field of icy objects and debris beyond Neptune known as the Kuiper Belt. “Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we become increasingly convinced that it is out there,” said Batygin. “For the first time in over 150 years, there is solid evidence that the solar system’s planetary census is incomplete.” Two years ago, a former postdoc of Brown’s, Chad Trujillo, and his colleague Scott Sheppard published a paper noting that 13 of the most distant objects in the Kuiper Belt are similar with respect to an obscure orbital feature. To explain that similarity, they suggested the possible presence of a small planet. Brown thought that unlikely, but got curious. He took the problem down the hall to Batygin, and the two started what became a year-and-a-half-long collaboration to investigate the distant objects. As an observer and a theorist, respectively, the researchers approached the work from very different perspectives—Brown as someone who looks at the sky and tries to anchor everything in the context of what can be seen, and Batygin as someone who puts himself within the context of dynamics, considering how things might work from a physics standpoint. Those differences allowed the researchers to challenge each other’s ideas and to consider new possibilities. Fairly quickly Batygin and Brown realized that the six most distant objects from Trujillo and Shepherd’s original collection all follow elliptical orbits that point in the same direction in physical space. That is particularly surprising because the outermost points of their orbits move around the solar system, and they travel at different rates. “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,” said Brown. The odds of having that happen are something like 1 in 100, he said. But on top of that, the orbits of the six objects are also all tilted in the same way—pointing about 30 degrees downward in the same direction relative to the plane of the eight known planets. The probability of that happening is about 0.007 percent. “Basically it shouldn’t happen randomly,” Brown said. “So we thought something else must be shaping these orbits.” It turned out, he added, Planet Nine’s existence helps explain more than just the alignment of the distant Kuiper Belt objects. It also explains mysterious orbits that two of them trace, including Sedna, discovered by Brown in 2003. Where did Planet Nine come from and how did it get where it is? Scientists have long believed that the early solar system began with four planetary cores that went on to grab all of the gas around them, forming the four gas planets—Jupiter, Saturn, Uranus, and Neptune. Over time, collisions and ejections shaped them and moved them out to their present locations. “But there is no reason that there could not have been five cores, rather than four,” said Brown. Planet Nine could represent that fifth core, and if it got too close to Jupiter or Saturn, it could have been ejected into its distant, eccentric orbit. Brown said astronomers should be able to spot the planet in images captured by previous surveys. If it is in the most distant part of its orbit, the world’s largest telescopes—such as the twin 10-meter telescopes at the W. M. Keck Observatory and the Subaru Telescope, all on Mauna Kea in Hawaii—will be needed to see it. If, however, Planet Nine is now located anywhere in between, many telescopes have a shot at finding it. “I would love to find it,” said Brown. “But I’d also be perfectly happy if someone else found it. That is why we’re publishing this paper. We hope that other people are going to get inspired and start searching.” In terms of understanding more about the solar system’s context in the rest of the universe, Batygin said that in a couple of ways, this ninth planet that seems like such an oddball to us would actually make our solar system more similar to the other planetary systems that astronomers are finding around other stars. First, most of the planets around other sunlike stars have no single orbital range—that is, some orbit extremely close to their host stars while others follow exceptionally distant orbits. Second, the most common planets around other stars range between 1 and 10 Earth-masses. “One of the most startling discoveries about other planetary systems has been that the most common type of planet out there has a mass between that of Earth and that of Neptune,” said Batygin. “Until now, we’ve thought that the solar system was lacking in this most common type of planet. Maybe we’re more normal after all.”