Scientists at the Southwest Research Institute (SwRI) have presented the discovery of a binary pair of objects in the Kuiper Belt. They are Trans-Neptunian Objects (TNOs), which means that their orbit is outside of the orbit of Neptune, the outermost planet in our solar system. This binary pair is unusual because they are close together.
The Kuiper Belt is similar to the asteroid belt between Mars and Jupiter. But the Kuiper Belt is much more massive than the asteroid belt. It begins in Neptune's orbit about 20 AU from the Sun and extends up to about 50 AU. The Kuiper Belt is sometimes referred to as the circumstellar disk.
The objects in the Kuiper Belt are remains from the early days of the formation of the solar system. Most Kuiper Belt Objects (KBOs) are predominantly icy objects formed from frozen volatiles such as methane and ammonia.
The Kuiper Belt extends from about 20 AU from the Sun to about 50 AU from the Sun. Photo credit: NASA
The title of the study that explains these results is "Stellar Occultation by the Resonant Trans-Neptunian Object (523764) 2014 WC510 shows a tight binary TNO." The lead authors are Rodrigo Leiva and Marc Buie, both scientists at SwRI. The study was published in the Planetary Science Journal.
The research that found the binary pair came from the Research and Education Collaborative Occultation Network (RECON). RECON is a citizen science initiative that "aims to explore the outer solar system," as stated on their website. RECON employees "conduct coordinated telescope observations to measure the size of objects from a region called the Kuiper Belt."
“For me, this Citizen Science project is at its best. If they didn't, we wouldn't learn anything about these objects. "
Marc Buie, co-author, Southwest Research Institute
As the name of the group makes clear, the observers at RECON use stellar occultations to find objects in the Kuiper Belt. A star occultation is different from a transit, although both involve an object passing in front of a star. While an occultation completely blocks the distant star, a transit only partially blocks it. While the object briefly fades out the star's light, observers can determine its size based on the duration of the occultation.
An occultation occurs when an object passes in front of another object and completely blocks it from view. If the background object is completely blocked, it is called a transit. This graphic shows the occultation of Aldebaran by the moon. Image credit: Stellarium.
This discovery was unusual not only for the proximity of the binary pair to one another, but also because the occult star was also a binary pair.
“In this case it turned out that the occult star is also a binary system. Binary stars are not uncommon and binary objects are not uncommon, ”said Buie. "But it's unusual for us to have a binary TNO obscuring a binary star."
"The properties of this property are also interesting and unusual," said Leiva in a press release. “The two components are pretty close to each other, only 350 kilometers apart. Most binary TNOs are very separate, typically 1,000 kilometers or more. This proximity makes it difficult to detect this type of binary TNO using other methods, which is what RECON was designed for. "
Pan-STARRS1 discovered the binary object in 2011. The binary object has the name "523764 2014 WC510" or simply 2014 WC510. During the observed occultation it was 30.5 au from the sun and 29.6 au from the earth, with an apparent magnitude of V = 22.1. For comparison, the apparent size of Neptune is 7.78. The primary object in the pair has a diameter of D = 181 ± 16 km and the secondary object has a diameter of D = 138 ± 32 km.
"To date," the authors write in their paper, "only 14 TNOs or their satellites (apart from Pluto) have an exact size and albedo that has been published from stellar occultations."
This figure from the study shows the sizes and albedos for the TNOs with precisely defined values. The black symbols stand for S.secondary object that P.Edge object and the two components together. The other symbols are either filled, which means they were determined by coverages, or blank, which means that they were determined by other methods. Photo credit: Leiva et al., 2020.
The shape of the object is also a mystery. "The projected shape of the object components is circular with faint statistical clues to elliptically projected shapes, shapes that could be confirmed in future occultations with higher S / N data, or rotating light curve data," the authors write.
The RECON group played a major role in this finding. RECON consists of a series of civil observation locations along the west coast of North America. There are currently 64 websites in RECON. Part of the funding for RECON comes from the National Science Foundation, and each site has an 11-inch telescope and CCD. When an object in the outer solar system covers a distant star, the shadow it casts over this collection of observation points helps determine the size of the object.
Each of the 64 RECON locations has a telescope and a video camera. They are all directed at the same time to the location of an impending covering. Photo credit: RECON / Google Earth
Buie is very polite to RECON and to the contribution that citizen scientists can make. "For me, this Citizen Science project is at its best," said Buie. "They learn, observe, and help collect data. If they didn't, we wouldn't learn anything about these objects."
Leiva and Buie intend to continue searching for additional TNOs, with a particular interest in tight binaries. They want to know how common and abundant they are. Your results will help model the formation of the solar system.
Astronomers believe that most TNOs started out in binary pairs. The fact that these are so close together could have contributed to their longstanding binary relationship. "Its dual nature is not surprising and supports the expectation that most TNOs were built as binaries and that tighter binaries have a better chance of survival," the authors explain.
Scientists believe that these smaller TNOs, unlike larger TNOs, are true remnants from primeval times in the Solar System. "Due to the inferred diameter of the object components, in 2014 WC510 was placed in the category of binary files of similar size that could have an original origin, as opposed to the large TNOs with small satellites that are believed to have formed after catastrophic impacts" write the researchers.
The largest trans-Neptunian (TNO) objects currently known are likely to be surpassed by future discoveries. (Photo credit: Larry McNish, data: M.Brown)
"Most solar system models indicate that binaries are very common, especially tight binaries like this one," Leiva said in a press release. "If you can measure exactly how frequent they are, you can fine-tune these models."
"Our overall goal is to know how common tight binary TNOs are," added Buie. "Is this item one in a million, or exactly like 90% of them? This strengthens our knowledge to create better models for the formation of the solar system. "
RECON's ongoing efforts will help answer this question.