Strange Object Boosts Kuiper Belt Mystery
Andrew Cooper/W. M. Keck Observatory
March 13, 2013, marks 20 years since the W. M. Keck Observatory began taking observations of the cosmos. Located in arguably one of the most extreme and beautiful places on the planet -- atop Mauna Kea, Hawai'i, 13,803 ft (4,207 m) above sea level -- the twin Keck domes have observed everything from asteroids, planets, exoplanets to dying stars, distant galaxies and nebulae. Seen in this photograph, the Keck I and Keck II telescopes dazzle the skies with their adaptive optics lasers -- a system that helps cancel out the turbulence of the Earth's atmosphere, bringing science some of the clearest views attainable by a ground-based observatory.
To celebrate the last two decades of incredible science, Discovery News has assembled some of the most impressive imagery to come from Keck.
William Merline, SWRI / W.M. Keck Observatory
Starting very close to home, the Keck II captured this infrared image of asteroid 2005 YU55 as it flew past Earth on Nov. 8, 2011.
Larry Sromovsky (University of Wisconsin)
Deeper into the solar system, the Keck NIRC2 near-infrared camera captured this beautiful observation of the oddball Uranus on July 11-12, 2004. The planet's north pole is at 4 o'clock.
W.M Keck Observatory/NASA/JPL-G.Orton
This is a mosaic false-color image of thermal heat emission from Saturn and its rings on Feb. 4, 2004, captured by the Keck I telescope at 17.65 micron wavelengths.
Antonin Bouchez (W. M. Keck Observatory)
A nice image of Saturn with Keck I telescope with the near infrared camera (NIRC) on Nov. 6, 1998. This is a composite of images taken in Z and J bands (1.05 and 1.3 microns), with the color scaling adjusted so it looks like Saturn is supposed to look to the naked eye.
Antonin Bouchez, W.M. Keck Observatory
This is Saturn's giant moon Titan -- a composite of three infrared bands captured by the Near Infrared Camera-2 on the 10-meter Keck II telescope. It was taken by astronomer Antonin Bouchez on June 7, 2011.
W. M. Keck Observatory/SRI/New Mexico State University
Another multicolored look at Titan -- a near-infrared color composite image taken with the Keck II adaptive optics system. Titan's surface appears red, while haze layers at progressively higher altitudes in the atmosphere appear green and blue.
Mike Brown, Caltech / W.M. Keck Observatory
This image of Neptune and its largest Tritan was captured by Caltech astronomer Mike Brown in September 2011. It shows the wind-whipped clouds, thought to exceed 1,200 miles per hour along the equator.
A color composite image of Jupiter in the near infrared and its moon Io. The callout at right shows a closeup of the two red spots through a filter which looks deep in the cloud layer to see thermal radiation.
Christian Marois, NRC and Bruce Macintosh, LLNL/W. M. Keck Observatory
HR 8799: Three exoplanets orbiting a young star 140 light years away are captured using Keck Observatory's near-infrared adaptive optics. This was the first direct observation by a ground-based observatory of worlds orbiting another star (2008).
Bob Goodrich, Mike Bolte, and the ESI team
Now to the extremes -- an image of Stephan's Quintet, a small compact group of galaxies.
W.M. Keck Observatory
The Egg Nebula: This Protoplanetary nebula is reflecting light from a dying star that is shedding its outer layers in the final stages of its life.
W. M. Keck Observatory
This is WR 104, a dying star. Known as a Wolf Rayet star, this massive stellar object will end its life in the most dramatic way -- possibly as a gamma-ray burst. The spiral is caused by gases blasting from the star as it orbits with another massive star.
W. M. Keck Observatory/UCLA
Narrow-field image of the center of the Milky Way. The arrow marks the location of radio source Sge A*, a supermassive black hole at the center of our galaxy.
Dr. Mark Morris (UCLA) Keck II, Mirlen instrument
A high resolution mid-infrared picture taken of the center of our Milky Way reveals details about dust swirling into the black hole that dominates the region.
Mansi Kasliwal, Caltech and Iair Arcavi, Weizmann Institute of Science/W. M. Keck Observatory
A false-color image of a spiral galaxy in the constellation Camelopardalis.
A scintillating square-shaped nebula nestled in the vast sea of stars. Combining infrared data from the Hale Telescope at Palomar Observatory and the Keck II telescope, researchers characterized the remarkably symmetrical “Red Square” nebula.
ESA, NASA, J.-P. Kneib (Caltech/Observatoire Midi-Pyrenees) and R. Ellis (Caltech)/W. M. Keck Observatory
Galaxy cluster Abell 2218 is acting as a powerful lens, magnifying all galaxies lying behind the cluster's core. The lensed galaxies are all stretched along the shear direction, and some of them are multiply imaged.
UC Berkeley/NASA/W. M. Keck Observatory
The central starburst region of the dwarf galaxy IC 10. In this composite color image, near infrared images obtained with the Keck II telescope have been combined with visible-light images taken with NASA’s Hubble Space Telescope.
There’s something odd floating around in the outer solar system. Actually, there’s lots of odd things floating around in the outer solar system, but 2002 UX25 is one of the most baffling.
The mid-sized Kuiper belt object (KBO) measures 650 kilometers (400 miles) across, and yet it has a density less than water (less than 1 gram per cubic centimeter). Yes, if you put it in a huge bathtub, 2002 UX25 would float.
As we probably all know by now, the Kuiper belt — a populated region of the solar system found just beyond the orbit of Neptune — is a strange place. Once thought to have a population of just one, astronomers have identified thousands of other minor planetary bodies. In fact, it was the accelerated discoveries in the Kuiper belt that ultimately led to the reclassification (or demotion, depending on which way you look at it) of Pluto from “planet” to lowly “dwarf planet.”
Now, in a paper accepted for publication in the The Astrophysical Journal Letters, planetary scientist Mike Brown, of the California Institute of Technology (Caltech) in Pasadena, has taken a measure of 2002 UX25′s density and discovered that it is “the largest solid known object in the solar system with a measured density below that of pure water ice.” Measuring the density of these distant objects are very difficult and require a small moonlet in orbit around the KBO so its orbital characteristics can be accurately measured and KBO density probed. The KBOs satellite was discovered by Hubble in 2005 and follow-up observations by the Keck Observatory in Hawaii refined its orbit.
This finding adds an extra twist to a strange dichotomy of KBOs. Objects with diameters less than 350 kilometers (218 miles) generally have densities less than that of water; objects over 800 kilometers (500 miles) have densities greater than water. One point in the gray area — between the diameter range of 350-800 kilometers — has just been added by 2002 UX25. But it is very large to have a density 18 percent less than water ice, a fact that surprised the veteran KBO hunter.
Densities of objects in and from the Kuiper belt.From “The density of mid-sized Kuiper belt object 2002 UX25 and the formation of the dwarf planets,” Michael Brown, 2013. arXiv:1311.0553 [astro-ph.EP]
“The inferred low rock fraction of the 2002 UX25 system makes the formation of rock rich larger objects difficult to explain in any standard coagulation scenario,” Brown writes.
It is thought that KBOs formed in a similar way to asteroids and planets. Over the evolution of our solar system, small bits of rocky and icy debris coalesced, eventually forming planetesimals that then gathered more and more debris as their gravitational oomph grew. In this scenario, one would expect the density of minor planetary bodies to increase with increasing mass; the gravitational pressure of progressively larger bodies would cause more compression, thus increasing the density.
However, the very low densities of smaller KBOs are hard to explain without assuming that the bodies have a high degree of porosity. Porosity is a known factor in the formation of asteroids throughout the solar system — gaps throughout the structure of rocky bodies less than 350 kilometers in diameter are thought to lower the overall density. Asteroids over 350 kilometers become so massive that porosity decreases; the gravitational compression pulls the material closer together, reducing porosity and increasing density.
According to Brown, this porosity transition should occur in KBOs larger than 350 kilometers wide. But as 2002 UX25 shows, this transition hasn’t happened up to a size of 650 kilometers. This factor creates a problem. If larger KBOs over 1,000 kilometers (620 miles) formed through the coalescence of smaller KBOs (like 2002 UX25), it isn’t possible that large rock-rich KBOs could have such high densities.
In the case of an object the size of Eris, for example, with a measured density of 2.5 g/cm3, even with the gravitational compression exerted by the 2,326 kilometer-wide dwarf planet, the low density, high porosity material from an objects like 2002 UX25 cannot be compressed to such a high degree. Such an object “would still have a density close to 1 g/cm3 rather than the 2.5 g/cm3 density of Eris,” writes Brown. On this evidence alone, large KBOs cannot form through agglomeration of many small KBOs like 2002 UX25.
So what’s going on in the Kuiper belt? Brown offers a few explanations.
Perhaps there is some observational bias in the measurements of KBO density, or perhaps 2002 UX25′s density is not representative of mid-sized KBOs — it could be the ‘black sheep’ of the Kuiper flock. Could it be that the highest density, large KBOs formed through conventional agglomeration processes, only to have their densities beefed-up by energetic collisions early in the solar system’s history? Dwarf planet Haumea shows evidence for a massive collision in its past, which smashed the majority of its icy mantle away, leaving a rocky core behind — this had the effect of increasing the overall density of the object.
“None of these alternatives appears likely,” concludes Brown. “We are left in the uncomfortable state of having no satisfying mechanism to explain the formation of the icy dwarf planets. While objects up to the size of 2002 UX25 can easily be formed through standard coagulation scenarios, the rock rich larger bodies may require a formation mechanism separate from the rest of the Kuiper belt.”
Brown often refers to the Kuiper belt as a “war zone” or the “blood spatter” of the solar system; the outermost region of shattered rocky and icy bodies preserved for aeons, unchanging evidence of the violent formative years of our star. Is 2002 UX25 just a forensic oddity? Or does it challenge planetary formation theories, proving the Kuiper belt is even stranger than we imagined?