For decades astronomers have searched for a possible “Planet X” in the far outer reaches of our solar system, speculating that something big and dark may be lurking out there, its gravitational influence occasionally stirring up trouble in the orbits of the objects that we do see. There are major incentives to look: When astronomers sought a Planet X beyond Uranus in 1846, they discovered Neptune; when they looked for one beyond Neptune in 1930, they found Pluto. Since then, the search for a Planet X beyond Pluto has almost been too successful—astronomers have found so many new and Plutolike “trans-Neptunian objects” (TNOs) that it became more sensible to demote Pluto from planethood rather than swell the solar system’s planetary population into the hundreds. After all, even the largest of the newfound TNOs were just about Pluto’s size—astronomers knew of nothing out there worthy of the “Planet X” name.
That is, perhaps, until now. On December 8 researchers from Sweden and Mexico quietly submitted two papers to the prestigious journal Astronomy & Astrophysics, announcing their discovery of not one but two possible Planet X candidates. The quiet did not last for long. Even though neither paper has yet been accepted for peer-review and publication, the researchers uploaded both to the arXiv, a public online repository for preprint papers, where they appeared last night. Today, as claims of newfound planets in our solar system reverberate around the world in news stories and blog posts, other astronomers are reviewing the papers and reacting mostly with skepticism. The ensuing discussions between experts in public forums like Twitter and Facebook offer a rare, real-time glimpse of the sometimes messy scientific process as it unfolds.
“Normally I prefer to only upload accepted papers,” says Wouter Vlemmings, an astronomer at Chalmers University of Technology in Sweden and co-author on both studies. “This time, however, we had exhausted our ideas. … With the arXiv upload we specifically wanted to reach the community that could tell us if we overlooked something, in which case we fully intend to withdraw the papers…. What I personally did not count on was the impact it has had outside the astronomy community.”
One of the candidates, nicknamed “Gna” (after a fast-moving “Nordic messenger goddess,” Vlemmings says) showed up in the sky next to the star W Aquilae whereas the other, as-yet-unnamed, appeared adjacent to our nearest neighboring star system Alpha Centauri. Astronomers detected both objects using the Atacama Large Millimeter/submillimeter Array (ALMA), a massive group of radio dishes perched in the high desert of the Chilean Andes, and thought at first that the bodies were faint glows from far-distant background galaxies. But in separate pairs of snapshots taken over a period of months, both objects seemed to move swiftly against the “fixed” background stars, suggesting a relatively close cosmic proximity to our solar system. Considerable uncertainty exists about the properties of both objects because each was observed only twice, and bodies with a wide range of sizes, compositions and distances from us could explain the measured brightness.
Gna, the researchers say, is quite likely to be something like a 200-kilometer-wide asteroid floating between Saturn and Uranus, but it could also be a free-floating Neptune-size planet drifting a hundred times farther out or a failed star—a Jupiter-size brown dwarf—passing by in nearby interstellar space. Similarly, the object seen in the direction of Alpha Centauri could conceivably be a nearby brown dwarf, a super-Earth midway in size between our planet and Neptune some six times farther out than Pluto or an impressively-sized hunk of ice much, much closer in.
Alternatively, both objects could be illusory, random blips of noise echoing through the world’s most complex and ambitious array of radio telescopes. According to Scott Sheppard, a planetary scientist at the Carnegie Institution for Science involved with surveys of the outer solar system, the fact that only two observations apiece underpin both discovery claims makes them hard to swallow. “Anything could create two random detections, and you can always fit a straight line through any two points,” Sheppard says. Demonstrating that either object was real, he says, would likely require a third detection, one that shows the object’s clear, linear movement at a consistent speed.
What these objects are, and whether they exist at all, are open questions. What is certain, however, is that earlier searches have placed limits on the possibilities for any Planet X. An all-sky search by NASA’s Wide-field Infrared Survey Explorer space telescope previously found no signs of any additional planets in our solar system, ruling out anything Jupiter-size within about three trillion kilometers of the sun, and anything Saturn-size within half that distance. Something smaller and dimmer like a super-Earth could still lurk out there, unseen, but to find it with such easy serendipity in routine ALMA measurements seems statistically unlikely, astronomers say.
Mike Brown, a prominent California Institute of Technology astronomer and self-described “Pluto killer” who discovered several large TNOs that dethroned the former planet, unleashed another statistical argument against the claimed new planets on Twitter. “If it is true that ALMA accidentally discovered a massive outer solar system object in its tiny, tiny, tiny, field of view,” Brown tweeted, “that would suggest that there are something like 200,000 Earth-sized planets in the outer solar system. Which, um, no.”
“Even better,” he added later, “I just realized that this many Earth-sized planets existing would destabilize the entire solar system and we would all die.” That said, Brown notes, “the idea that there might be large planets lurking in the outer solar system is perfectly plausible.”
Many of the most cutting reactions came from astronomers discussing the results on a public Facebook group devoted to imaging exoplanets—that is, planets around other stars. (Update: the group has since been made private.) After tweeting that the two papers “will launch 1,000 undoubtedly wrong blogs and news releases,” University of Rochester astronomer Eric Mamajek detailed what he believes to be serious inconsistencies in the measurements of motion and brightness for both objects. “‘Gna’ presumably stands for ‘Goofy Non-Asteroid,’” Mamajek quipped, before suggesting that the objects could perhaps be activity in faraway galaxies, simply misconstrued as being much closer to Earth. “Please pass whatever they are smoking in Onsala,” he added.
In the same group, astronomer Bruce Macintosh at Stanford University noted the “astonishing coincidence” that the first two trans-Neptunian objects discovered by ALMA would be found right next to bright stars. More likely, Macintosh guessed, is that the putative objects are actually “some residual artifact”—mirages produced in the data by quirks in ALMA’s complex calibration methods.
Vlemmings insists that he and his colleagues have already carefully checked these and several other scenarios, but to no avail. Whatever they are, the objects simply seemed to be too bright and pointlike to be explained away as far-off galaxies, and their proximity to bright stars, he says, actually helped the data calibration and reduced the likelihood of observational errors. “Still, we are certainly open to such options and have several times sent out queries to ALMA colleagues [asking] if they could conceive of how such point sources could be artificially created,” Vlemmings says. “None have yet said they think it could be done.” The trial of these claims in the court of public opinion has not come without its perks, Vlemmings adds. Although the sudden publicity was unwanted, “the most helpful feedback so far has been numerous offers to observe with other instruments.” With a little help from the rest of the astronomy community, evidence for—or against—the next Planet X may not be so far-off after all.