Michael Brown doesn’t care how much you love Pluto. He loves it too, but that didn’t stop him from leading the charge in 2006 to strip it of its “planet” designation and bust it down to a mere dwarf planet. But now, the Caltech astronomer is making amends in a big way: along with Caltech planetary scientist Konstantin Batygin, he has just released a paper announcing the highly likely discovery of a new planet to replace Pluto—and no one’s ever going to be able to demote this one.
“The existence of another planet has been spoken about 100 times before,” Brown says. “But this is the first time in 150 years that we can say we have convincing evidence that the census of the solar system is incomplete.”
The discovery of the world that Brown and Batygin refer to in The Astronomical Journalsimply as “Planet 9” began in 2003, with the discovery of a far more modest object named Sedna. A dwarf planet even smaller than Pluto, Sedna is a Kuiper Belt object (KBO), like Pluto one of a vast band of icy, rocky objects that surround the solar system beyond the orbit of Neptune. Brown was part of the team that found Sedna too, and if anything made the new world remarkable, it was its extreme distance from the sun—one that has it completing a single orbit in 11,400 years, compared to Pluto’s 248.
But something else was strange about Sedna as well—or at least about the company it keeps. In the years that followed, astronomers discovered five other KBOs whose closest approach to the sun (or perihelion) matches Sedna’s almost perfectly, both in distance and in the angle of the orbit relative to the horizon of the solar system.
This couldn’t be a coincidence. The matching perihelia have only a 0.7% likelihood of occurring by chance; the matching angles have only a 1% chance. The double match among all six objects factors out to a vanishingly small .007% likelihood of being random. “It was very exciting to notice this collection of objects in this super obscure arrangement,” says Brown. “The orbits are physically lined up in space.”
If it wasn’t chance that was behind such a tidy clustering, it had to be gravity—the influence of a passing object that herds the KBOs together, in the same way Saturn’s moons influence the positioning of particles in the rings. Using mathematical modeling and computer simulations, Brown and Batygin concluded that Sedna and the other members of its flock must be herded by an as-yet-unobserved planet with about ten times the mass of Earth, circling the sun at an average distance 20 times greater than that of Neptune—making one orbit every 20,000 years. A body with different mass or a different distance simply couldn’t explain the behavior of the six KBOs.
How Planet 9 found itself in so remote a place is unclear. It’s unlikely that it formed in situ, essentially coalescing out of the primordial cloud of dust and gas that formed the rest of the solar system, since there probably wouldn’t have been enough raw material to form so big a world out that far. Rather, the new planet might have accreted closer in like the other planets, and then been ejected by their gravitational forces.
“Planet 9 is part of the sun’s family,” says Brown. “The birth cluster turned it around so it was placed in a distant orbit. It has been there all along and has witnessed the development of the solar system from afar.”
Now, Brown and Batygin need earthbound witnesses to confirm that Planet 9 exists. Just as Neptune was originally inferred by wobbles spotted in Uranus’s orbit, the existence of Planet 9 can be inferred by the aligned KBOs. But without a verifiable observation, it’s still a theoretical discovery. The pair are hoping to crowdsource that job, getting as many telescopes looking as possible across the globe. They’ve made the challenge easier by mapping Planet 9’s orbit; now skywatchers have to pinpoint where it is on that very long path.
TIME Picks the Best Space Photos of 2015
This photo of Pluto was made during the New Horizons spacecraft's historic flyby of the dwarf planet in July 2015. New Horizons is now sailing into the Kuiper Belt for a rendezvous with another small world. NASA/ReutersThis nebula named "Thor's Helmet" is powered by a central "Wolf Rayet" star whose explosive tantrums blow huge bubbles of gas and make them glow like neon. This star is easily 20 times the mass of the Sun and located 15,000 light years away. Eventually the star's instability will lead it to explode as a supernova. The photo was captured at the Mount Lemmon SkyCenter in Arizona and released on Jan. 3, 2015.Adam Block—Mount Lemmon SkyCenter/University of ArizonaSuper Typhoon Maysak, as seen from aboard the International Space Station on March 31, 2015. NASA Astronaut Terry Virts captured this image and tweeted "The eye of #Maysak typhoon really stands out early in the morning with the shadow being cast deep into the vortex." His ESA crewmate on station also viewed the storm and wrote, "Commands respect even from #space..."Terry Virts—NASAThe total solar eclipse at Svalbard, Norway, on March 20, 2015. A partial solar eclipse was seen in Europe, northern and eastern Asia and northern and western Africa.Olav Jon Nesvold—EPAPluto's haze layer shows its blue color in this picture taken by the New Horizons spacecraft on Oct. 8, 2015 after it passed the dwarf planet. The high-altitude haze is thought to be similar to that seen at Saturn’s moon Titan. The source of both hazes likely involves sunlight-initiated chemical reactions of nitrogen and methane, leading to relatively small, soot-like particles called tholins.NASA/JHUAPL/SwRI
A NASA camera on the Deep Space Climate Observatory (DSCOVR) satellite captures the entire sunlit side of Earth from one million miles away on July 6, 2015.
NASAThe Orbital ATK Antares rocket, with a Cygnus spacecraft onboard, suffered a catastrophic explosion moments after launch on Oct. 28, 2014, at NASA’s Wallops Flight Facility in Virginia. The Cygnus spacecraft was filled with supplies slated for the International Space Station. The never before seen photographs were released by NASA this year. Joel Kowsky—NASAThe stark transition between radiating sand dunes and an old volcanic flow on the surface of Earth was captured by astronaut Kjell Lindgren aboard the International Space Station, on Dec. 5 2015.Kjell Lindgren—NASAThe Lagoon Nebula, a bright cloud of dust and gas 4,000 light years away and 40 light years across, glows brilliantly due to hot energetic young stars forming within. It can even be glimpsed with the unaided eye under dark skies away from city lights. The photo was captured at the Mount Lemmon SkyCenter in Arizona and released on July 15, 2015.Adam Block—Mount Lemmon SkyCenter/University of ArizonaCircinus X-1 is an X-ray binary star known for its erratic variability can be seen in this photo released on June 23, 2015. Within the system, a dense neutron star, the collapsed remnant of a supernova explosion, orbits with a more ordinary stellar companion Circinus X-1 30,700 light-years away.X-ray - NASA/CXC/Univ. Wisconsin-Madison/S.Heinz et al, Optical - DSSThe spectacular cosmic pairing of the star Hen 2-427 and the nebula M1-67 was captured by the Hubble Space Telescope, released on Aug. 21, 2015. The twin formation is located in the constellation of Sagittarius, 15,000 light-years away. The star shines brightly at the very center of the image. Surrounding it are hot clumps of gas being ejected into space at over 93,000 mph. ESA/Hubble/NASA A meteor from the Perseid Meteor shower can be seen in the upper left corner in this long exposure image taken as a wildfire burned in Lake and Napa Counties near the town of Clearlake, Calif. on Aug. 12, 2015.Stuart Palley—Zuma PressThe once in a generation supermoon total lunar eclipse viewed from Glastonbury, England, on Sept. 28, 2015. Three decades had passed since the last time Earth was witness to the triple crown of lunar events — a full moon, a lunar eclipse, and a lunar perigee all at the same time.Matt Cardy—Getty ImagesThis composite image made from five frames shows the International Space Station, with a crew of nine onboard, in silhouette as it transits the sun at roughly five miles per second, on Sept. 6, 2015.Bill Ingalls—NASAA Soyuz spacecraft is seen as it returns home with NASA astronaut Barry Wilmore and Russian cosmonauts Alexander Samokutyaev and Elena Serova of Roscosmos, near the town of Zhezkazgan, Kazakhstan on March 12, 2015. Bill Ingalls—NASAAstronaut Scott Kelly, re-entering the International Space Station after his spacewalk on Oct. 29, 2015.
NASAAstronaut, Kjell Lindgren captures one last stunning view of the Milky Way before his return to Earth on Dec. 10, 2015. Kjell Lindgren—NASAA Vega rocket lifts off from French Guiana on Dec. 3 2015, carrying the European Space Agency's LISA Pathfinder spacecraft, which will test methods for detecting gravity waves.JM Guillon—CNES/EPAThe supermoon, which occurs when the moon is full, with the closest approach to Earth on its elliptical orbit, above the Washington Monument on Sept. 27 2015. Making the phenomenon even more dramatic: a total lunar eclipse also occurred that evening. Aubrey Gemignani—NASACharon, the largest moon of Pluto, captured by the New Horizons spacecraft on July 13, 2015 from a distance of 290,000 miles.NASA/JHUAPL/SwRI/EPAA Soyuz spacecraft returning from the International Space Station lands in Kazakhstan on June 11, 2015, with NASA astronaut Terry Virts, Russian cosmonaut Anton Shkaplerov, and Italian astronaut Samantha Cristoforetti of the European Space Agency aboard. The burst of fire and smoke at the point of impact comes from the braking rockets the Soyuz fires to ease the landing.Bill Ingalls—NASAVolunteers using the web-based Milky Way Project brought star-forming features nicknamed "yellowballs" to the attention of researchers, who later showed that they are a phase of massive star formation. The yellow balls -- which are several hundred to thousands times the size of our solar system -- are pictured here in the center of this image of the W33 Star forming region taken by NASA's Spitzer Space Telescope, released on Jan. 27, 2015.NASA/JPL/CaltechNew Horizons scientists made this false color image of Pluto using a technique called principal component analysis to highlight the many subtle color differences among Pluto's different regions. The picture was taken from a distance of 22,000 miles on July 14, 2015. NASA/JHUAPL/SwRI
The Twin Jet Nebula, or PN M2-9, is a striking example of a bipolar planetary nebula, formed when the central object is not a single star, but a binary system. An earlier image of the Twin Jet Nebula using data gathered by Hubble’s Wide Field Planetary Camera 2 was released in 1997. This newer version released on Aug. 26, 2015 incorporates more recent observations from the telescope’s Space Telescope Imaging Spectrograph.ESA/Hubble/NASA The brilliant palette of Africa captured by astronaut Scott Kelly aboard the International Space Station on July 22, 2015. Scott Kelly—NASAAurora Borealis seen over the Brecon Beacons, Wales, England on Mar. 17, 2015. Increased solar activity has meant that the northern lights were viewable over many areas of the UK, reaching as far south as South Wales.Polly Thomas/Rex/REX USAThe blood moon lunar eclipse sets behind Pikes Peak on April 4, 2015 in Colorado Springs. The reddish hue is the result of Earth moving between the sun and the moon and Earth's atmosphere absorbing blue wavelengths and allowing red ones to pass through.John Leyba—Denver Post/ Getty ImagesNASA's Curiosity Mars rover recorded this view of the sun setting at the close of the mission's 956th Martian day, or sol, on April 15, 2015, from the rover's location in Gale Crater. This was the first sunset observed in color by Curiosity. NASA/JPL—Caltech/MSSS/Texas A&M Univ.The rippled surface of the first Martian sand dune ever studied up close can been seen is this photo taken on Nov. 27, 2015 by NASA Mars Rover Curiosity. The dunes close to Curiosity's current location are part of "Bagnold Dunes," a band along the northwestern flank of Mount Sharp inside the Gale Crater. NASA/JPL-Caltech/MSSS/REX Shutterstock A 3-D computer model (a digital terrain map) of Hale Crater on Mars based on stereo information from two HiRISE observations showing dark, narrow streaks on the Martian slopes that are inferred to be formed by seasonal flow of water on contemporary Mars was released on Sept. 28, 2015.
NASA/JPL/University of ArizonaAstronaut Scott Kelly captured this image of the Bahamas from the International Space Station on July 19, 2015.Scott Kelly—NASAAn image of the wildfires in the Northwest taken from the International Space Station released on Aug. 17, 2015.Kjell Lindgren—NASASand dunes sculpted by wind, captured by astronaut Kjell Lindgren, aboard the International Space Station on Dec. 8 2015. Kjell Lindgren—NASAThis photo shows the new mountain range discovered by NASA’s New Horizons spacecraft on July 14, 2015 on Pluto, in a heart-shaped region named Tombaugh Regio. NASA/Johns Hopkins University Applied Physics Laboratory/ Southwest Research InstituteNASA's Cassini spacecraft captured this parting view showing the rough and icy crescent of Saturn's moon Dione following the spacecraft's last close flyby on Aug. 17, 2015. The image was captured from a distance of 37,000 to 47,000 miles.NASA/JPL-Caltech/SSIComet 67P, photographed by the Rosetta Orbiter from a distance of 79 miles on Nov. 22, 2015.ESA/Rosetta/NAVCAMSaturn's icy moon Enceladus and a small stretch of Saturn's rings, as seen by the Cassini spacecraft on July 29, 2015. Cassini Imaging Team/ SSI/JPL/ESA/NASAThis image released on April 6, 2015, shows the centre of the globular cluster Messier 22, also known as M22, as observed by the Hubble Space Telescope. Globular clusters are spherical collections of densely packed stars, relics of the early years of the Universe, with ages of typically 12 to 13 billion years. ESA/Hubble/NASAThe Hubble Space Telescope revisits one of its most iconic and popular images: the Eagle Nebula’s Pillars of Creation. This image shows the formation as seen in infrared light, which gives the familiar pillars an unfamiliar look. The image was released in January 2015. NASA/ESA/Hubble Space TelescopeMoonlight over Italy, as captured by astronaut Scott Kelly aboard the International Space Station on Sept. 23, 2015. Scott Kelly—NASAUsing NASA's Hubble Space Telescope, astronomers have assembled a bigger and sharper photograph of the iconic Eagle Nebula's "Pillars of Creation," rereleased in January 2015.NASA/ESA/Hubble and the HubbleThe supermoon rises behind Glastonbury Tor on Sept. 27, 2015 in Glastonbury, England. A supermoon occurs when the moon is both full and at the closest approach it makes to Earth during its orbit. Matt Cardy—Getty ImagesThis photograph, taken by the Cassini spacecraft on March 25, 2015 shows the moons Titan (Saturn’s largest moon), Mimas, and Rhea. Titan appears out of focus in the shot because it’s the only natural satellite that’s currently known to have its own dense atmosphere — clouds hover over the surface, and the atmosphere refracts light and causes the crescent to extend further around the moon than the two other atmosphere-less moons in the photo.NASA/JPL-Caltech/Space Science InstituteSpiral galaxies are delicate and subtle things. This one, NGC 488, is 90 million light years away. The photo was captured at the Mount Lemmon SkyCenter in Arizona and released on Oct. 25, 2015. Adam Block—Mount Lemmon SkyCenter/University of ArizonaThis high-resolution image of Pluto was captured by NASA’s New Horizons spacecraft and released on Oct. 15, 2015. The bright expanse is the western lobe of the heart-shaped formation informally known as Tombaugh Regio. NASA/JHUAPL/SwRI
A brilliant aurora as seen from the International Space Station on June 27, 2015.Scott Kelly—NASAThe Sun blew out a coronal mass ejection along with part of a solar filament over a three-hour period on Feb. 24, 2015. While some of the strands fell back into the Sun, a substantial part raced into space in a bright cloud of particles, as observed by the SOHO spacecraft.NASA/Solar Dynamics ObservatoryAstronaut Scott Kelly captures this striking view of earth on Asteroid Day on June 30, 2015. Scott Kelly—NASAIn a picture taken from the International Space Station, the African desert shows some unfamiliar greening on June 14, 2015. Scott Kelly—NASAAn image captured on Halloween, Oct. 31, 2015, from the International Space Station by astronaut Scott Kelly, who described what he saw as "ghostly and dark but beautiful too." Scott Kelly—NASAThe artificial Palm and World Islands off the coast of Dubai as photographed by astronaut Kjell Lindgren aboard the International Space Station on Nov. 20, 2015.Kjell Lindgren—NASAThis image taken by NASA's Mars Reconnaissance Orbiter in May 2015 shows Acidalia Planitia, a location on the red planet that was recreated in the film "The Martian." Nasa/JPL-Caltech/University of Arizona/PA WireThe Saturnian moon Dione hangs in front of Saturn and its icy rings in this view, captured during the Cassini spacecraft's final close flyby of the icy moon on Aug. 17, 2015.
NASA/JPL-Caltech/SSIThe Saturnian moon Tethys, dwarfed by Saturn itself and its rings, appears as an elegant crescent in this image taken by NASA's Cassini Spacecraft on on Aug. 18, 2015.NASA/JPL-Caltech/Space Science InstituteThe moon shines during the Spanish league football match FC Barcelona vs. Malaga CF at the Camp Nou stadium in Barcelona on Aug. 29, 2015.Josep Lago—AFP/Getty Images
The name of the planet will be crowd-sourced too, if the researchers get their way — as opposed to being proposed by the discoverer and then approved by the International Astronomical Union (IAU), which is the usual way of doing things. Brown’s and Batygin’s personal name preference is “George,” a hat-tip to British astronomer William Herschel, who discovered Uranus and wanted to name it Georgium Sidus (the Georgian Planet) after King George III. That might be a hard sell to the IAU—to say nothing of nearly all other stargazers, who tend to like a little more lyricism in their cosmos.
Whatever the planet is eventually called, its very existence will do more than simply add to the population of the solar system. It will also add to its mystery. Even in our tiny corner of the universe, it seems, there can still be big surprises lurking.
