Deeply ingrained in popular culture, it has been the object of obsession for scientists and conspiracy theorists alike: a hypothesized hidden planet lurking beyond the orbit of Pluto, which was long-thought to be the cause of observed perturbations in the orbits of the outer giant planets, past mass extinction events on Earth, and other modern-day doomsday, apocalyptic scenarios. Detailed all-sky surveys in recent years made with NASA’s Wide-field Infrared Survey Explorer, or WISE, although revealing the presence of thousands of stellar and substellar objects within the Sun’s wider neighborhood, have shown that the long-sought-for “Planet X” is nowhere to be found.
The notion of a massive, unobserved planet orbiting the Sun beyond the known reaches of the Solar System goes back to the 19th century, even before the discovery of Neptune in 1846. The discovery of the blue-tinted ice giant planet itself came as a result of a series of efforts by 19th-century astronomers to explain the discrepancies between the predicted and observed orbit of Uranus that were caused by Neptune’s gravitational tug. When astronomers realised that Neptune was experiencing similar perturbations in its orbit, they suspected that the most likely cause was the presence of a yet another unobserved planet, even farther out from the Sun.
American astronomer Percival Lowell was central in the hunt for this hypothesized trans-Neptunian massive planet during the early 20th century, which he dubbed “Planet X.” With the discovery of Pluto by Clyde Tombaugh in 1930 the matter was initially thought to be resolved. When astronomers finally realised that Pluto’s mass was too small and insignificant to have any effect on the orbits of Uranus and Neptune, the search for Planet X was rekindled. It took the detailed observations of NASA’s Voyager 2 spacecraft during its fly by of Neptune in 1989 for astronomers to realise that the observed orbital discrepancies were due to errors made in the calculation of Neptune’s mass by their 19th-century counterparts. With the help from the Voyager 2 data, more precise measurements of the ice giant mass were made, accounting for the observed orbital anomalies.
Nonetheless, the prospect of a distant world avoiding detection while secretly revolving around the Sun has remained very popular among the general public to this day, fueling countless conspiracy theories at the same time that morphed Planet X to the menacing Nibiru, a hypothetical rogue planet that has been many times prophesied by conspiracy theorists to sweep through the Solar System and lead to the complete obliteration of Earth. The latest in this long series of unfounded eschatological predictions was the infamous 2012 Mayan Apocalypse, supposedly occurring on Dec. 21, 2012, bringing an end to life as we know it.
Despite all the doom and gloom of such apocalyptic scenarios that are so prevalent in our modern-day society, the already slim chances for the existence of this far-off world within the frozen expanses of the outer Solar System were further reduced following the launch of NASA’s WISE spacecraft in December 2009. A low-cost Explorer-class mission, WISE conducted two all-sky surveys in infrared wavelengths in 2010 before running out of its hydrogen coolant, with a sensitivity 1,000 times greater than that of the Infrared Astronomical Satellite, or IRAS, that had performed a similar mission 25 years earlier. Sweeping the sky continuously for 10 months at four different infrared wavelengths—3.3, 4.7, 12, and 23 micrometers—WISE’s observations were of unprecedented detail. By analysing this wealth of data, astronomers were able to discover millions of black hole candidates in distant galaxies, thousands of new asteroids in the Solar System, a new ultra-cool class of brown dwarfs, and dozens of comets.
Contrary to observations made in visible wavelengths where many deep-sky objects are hidden from view, either enveloped in dense regions of gas and dust or bathed in intense surrounding starlight, by observing in the infrared the internal heat that these objects radiate becomes readily visible to astronomers, allowing them to study even very dim and cool celestial objects like planets, asteroids, and brown dwarfs that would otherwise remain invisible.
The sensitivity of the WISE spacecraft was such that it could detect a Jupiter-sized planet out to 26,000 Astronomical Units from the Sun (more than a third of a light-year away) and a Saturn-sized planet out to 10,000 A.U. For comparison, Pluto lies at a mean distance of 40 A.U. from the Sun. Yet, it found none.
“The outer solar system probably does not contain a large gas giant planet, or a small, companion star,” says Kevin Luhman, Associate Astronomy Professor at the Penn State University’s Department of Astronomy and Astrophysics and lead author of a study published in the Astrophysical Journal, detailing the WISE observations.
The absence of a massive gas giant planet lurking at these distances doesn’t mean that one couldn’t exist farther away from the Sun. A Jupiter-sized planet located at a distance of 1 light-year away would have gone undetected by WISE’s instruments. Still, these latest results cast much doubt on earlier hypotheses concerning the existence of a distant planetary companion to the Sun named Tyche and a stellar companion named Nemesis. The latter, in particular, was thought to be a companion red dwarf star to the Sun, or possibly a brown dwarf. Although many times more massive than Jupiter, brown dwarfs are substellar objects, not massive enough to undergo nuclear fusion reactions in their cores. It was postulated that Tyche and Nemesis would have highly elliptical orbits disrupting comets within the Oort Cloud, the hypothetical reservoir of icy bodies that is thought to extend out to 1 light-year from the Sun and flinging them toward the inner Solar System. According to that hypothesis, the past mass extinction events recorded in the Earth’s geological record were caused in part by such extraterrestrial impacts.
Objects like Tyche and Nemesis would have been easily spotted by WISE if indeed they were there. Yet, even though WISE discovered many dozens of brown dwarfs out to a distance of 30 light-years away, it found no traces of such distant companions to the Sun. Results from the infrared telescope also showed that even the number of brown dwarfs discovered near the Sun was much lower than anticipated. “This is a really illuminating result,” says Davy Kirkpatrick, member of the WISE science team at NASA’s Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. “Now that we’re finally seeing the solar neighborhood with keener, infrared vision, the little guys aren’t as prevalent as we once thought. This is how science progresses as we obtain better and better data. With WISE, we were able to test our predictions and show they were wrong. We had made extrapolations based on discoveries from projects like the Two-Micron All-Sky Survey, but WISE is giving us our first look at the coldest brown dwarfs we’re only now able to detect.”
Nevertheless, astronomers were able to detect the two closest brown dwarfs to Earth, both members of the star system WISE J104915.57-531906, lying just 6.5 light-years away. “One major goal when proposing WISE was to find the closest stars to the Sun,” says Edward L. Wright, principal investigator for WISE, from the University of California, LA. “WISE J1049-531906 is by far the closest [brown dwarfs] found to date using the WISE data.”
Following its 10-month all-sky survey in late 2010, and having depleted its hydrogen coolant responsible for keeping the space telescope at the needed operating temperature of minus 429 degrees Fahrenheit, WISE was put into hibernation the following year. NASA reactivated the mothballed orbiting observatory in September 2013, while renaming it NEOWISE and giving it a new three-year mission to discover and characterise Near-Earth Asteroids whose orbits bring them close to the Earth. “WISE is the spacecraft that keeps on giving,” says Wright.
Despite the lack of any evidence for the existence of Planet X or other similar stellar interlopers from the edges of the Solar System, NASA’s WISE mission has provided a wealth of data that will keep astronomers and planetary scientists occupied for years to come. “We’re finding objects that were totally overlooked before,” says Kirkpatrick, who also led a recent study published in the Astrophysical Journal detailing the discovery of 3,525 stars and brown dwarfs within 500 light-years from the Sun, using data from WISE. “We think there are even more stars out there left to find with WISE. We don’t know our own Sun’s backyard as well as you might think,” adds Wright.
Even though these latest results to come from WISE likely won’t help to reduce the number of the pseudoscientific, crackpot conspiracy theories that permeate contemporary popular culture, centered around the always imminent End of Days, the space telescope can nevertheless help us gain a new appreciation for the wondrous Universe that lies above our heads.