Wed, 02 Mar 2011 23:47 UTC
On February 14th, the UK's Daily Mail reported the possible discovery of a planet four times bigger than Jupiter and lurking in the outer solar system. From there, the story quickly spread like a wildfire on the Internet, seeing coverage by mainstream outlets including the Huffington Post and TIME online. The tone of various news stories varied from "Tyche, Giant Hidden Planet, May Exist In Our Solar System" (The Huffington Post) to "Astronomers Question Existence of Solar System's Mystery Planet Tyche" (Fox News). So, is there really a new planet lying out there?
The original research article was published in the February issue of the international planetary science research journal Icarus by John Matese and Daniel Whitmire of the University of Louisiana at Lafayette. It was titled "Persistent evidence of a jovian mass solar companion in the Oort cloud."
Out of the Oort cloud
Matese and Whitmire studied the orbits of 102 comets with extremely long periods that are included in the 17th edition of the Catalogue of Cometary Orbits, which lists 2844 comets in all. The 102 analyzed by Matese and Whitmire are special in that the comets originate particularly far from the sun, at least 10,000 AUs away.
Comets are believed to originate in the Oort Cloud, which is theorized to exist at the outer fringe of solar system. The Oort Cloud is expected to harbor many small, primitive, icy bodies left over from when the solar system originally formed. When an Oort Cloud object is nudged out of its orbit toward the sun, it starts falling toward the inner region of the solar system and eventually becomes a comet.
The 102 comets studied by Matese and Whitmire are special in that their point of origin can be traced so far out that they appear to have come directly from the Outer Oort Cloud, which is thought to have a uniform spherical shell shape centered at the sun.
Matese and Whitmire's analysis of these Outer Oort Cloud comets is consistent with the idea that the galactic tide plays a dominant role in flinging comets toward the sun. As our solar system completes its orbit around the center of the galaxy every quarter billion years, it periodically crosses the galactic plane. The varying gravitational forces perpendicular to the galactic plane plays a large role in nudging the Outer Oort Cloud comets toward the sun. As this force predomoinantly acts in a direction perpendicular to the galactic disk, there should be relatively few comets with their orbits parallel to the galactic plane; the cometary orbits analyzed by Matese and Whitmire clearly show this effect.
However, even after the effects of the galactic tides are accounted for, they found a small bias in the distribution of cometary orbits. If the galactic tide is the only force flinging these comets from a randomly distributed source in the spherical Outer Oort Cloud, we would expect that their average orbital tilt with respect to the galactic plane should be exactly 45 degrees; however, the 102 cometary orbits analyzed in the paper show a small bias toward a shallower orbital tilt.
A signal in the data
Matese and Whitmire further analyze the orientation of aphelion (the point in each comet's orbit where it is the furthest from the sun). They note that there is a statistically significant signal in the data that suggests there is a common plane from which many of these comets originated.
They propose that the bias in the comets' orbital tilt and a common plane of origin was caused by a large object, perhaps about the mass of Jupiter, orbiting in the Outer Oort Cloud. They also present gravitational calculations to show that such an object can act as a source of perturbations that cause the observed bias in the distribution of these comets' orbits.
The idea that there is a large object in the Outer Oort Cloud flinging comets toward the sun is not new. In fact, Whitmire was the leading author of a 1984 Nature paper that proposed the existence of a large object in the Outer Oort Cloud that's responsible for periodically causing comet storms that cause mass extinctions on Earth - that became known as the Nemesis hypothesis. In the new paper, Matese and Whitmire identify their hypothesized new planet as Tyche. This name for a large Outer Oort Cloud object was originally used by Davy Kirpatrick at the Infrared Processing and Analysis Center of the California Institute of Technology. Tyche is the good sister of Nemesis in the Greek mythology.
What is remarkable in the new paper is that, this time around, Matese and Whitmire proposed a specific way to test their hypothesis. Based on the mass necessary to kick Outer Oort Cloud objects toward the sun, they calculated how much infrared radiation such an object should emit, and showed that it may be detectable in the data acquired by NASA's Wide-field Infrared Survey Explorer (WISE) satellite.
WISE's mission is to map the entire sky in the infrared wavelengths with unprecedented sensitivity and resolution. Its original mission was more focused on distant cosmic objects, but it has proven extremely effective at discovering asteroids in the solar system. As a result, its mission was extended with the primary focus of finding as many Near-Earth Objects as it can - the extended mission was named, appropriately, NEOWISE.
The effectiveness of WISE at spotting asteroids is evident in a video produced by the WISE team - when WISE comes online at 3 minutes 3 seconds into the video, the asteroid detection rate dramatically increases. The WISE hardware recently concluded its image-gathering mission on February 17th this year, but the mission team is still continuing to analyze the data collected by the satellite and is in the process of completing the archiving of the data.
We had a chance to interview Edward "Ned" Wright at the University of California Los Angeles, the Principle Investigator of the WISE mission. He notes that, if the brightness of Tyche is close to the bright end of the prediction by Matese and Whitmire, it should have had a good chance to be discovered by now. If it's at the darker end, even WISE may not be able to detect it, and the case may remain open for some time to come. Wright's hope is that it will take a couple of years to complete the mapping of WISE data at the level of sensitivity and precision sufficient to detect something like Tyche, but he also adds "one never knows how long a novel task will take." So, the bottom line is that we will have to wait for a while just to hear if WISE has seen anything.
One thing worth noting about the analysis done by Matese and Whitmire is the relatively small number of comets studied in their report. To be sure, they did study all the comets from the Outer Oort Cloud known to us, but 102 comets are just not enough to make a statistical signal stand out significantly above the noise. For example, the bias found in the study means that they found only about 5 too many comets with low orbital tilts when the 102 comets are sorted into 5 bins between 0 and 90 degree inclinations. The study also indicates that there are several possible fits to the common orbital plane shared by these Outer Oort Cloud comets, so the orbital constraints for this new planet are also not very tight.
The new study is definitely sound science, and it is an extremely careful analysis of cometary orbits that shows interesting hints of something at play; however, the evidence for a new planet is not very strong, and the authors are unable to pinpoint where in the sky Tyche is.
At the close of the interview, Ned Wright of WISE summed up the current situation in one sentence; "No, we have not found a new planet."
Icarus, February 2011: DOI:10.1016/j.icarus.2010.11.009