The Real Planet X

A day after the news about 2003 EL61, a Kuiper Belt object originally thought to be larger than Pluto, we now have another world that appears significantly larger still. 2003 UB313 was discovered with the Samuel Oschin Telescope at the Palomar Observatory by astronomers Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University). Evidently the lower limit of its size is Pluto, and it may be (and probably is) larger.

View of 2003 UB313

Image: Three views of the new planet. Credit: Mike Brown, California Institute of Technology.

Now some 97 AU from the Sun, the planet is the farthest-known object in the Solar System. A news release from Caltech quotes Brown on 2003 UB313 and its credentials as a planet:

“It’s definitely bigger than Pluto,” says Brown, who is professor of planetary astronomy. Scientists can infer the size of a solar-system object by its brightness, just as one can infer the size of a faraway light bulb if one knows its wattage. The reflectance of the planet is not yet known–in other words, it’s not yet possible to tell how much light from the sun is reflected away–but the amount of light the planet reflects puts a lower limit on its size.

“Even if it reflected 100 percent of the light reaching it, it would still be as big as Pluto,” says Brown. “I’d say it’s probably one and a half times the size of Pluto, but we’re not sure yet of the final size.

“But we are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system.”

But there is also an upper size limit, one determined by the fact that the Spitzer Space Telescope cannot detect this world. That pegs its diameter at something less than 3000 kilometers (Pluto’s is 2300 kilometers). And yes, we’ll have a better name than 2003 UB313 for this object soon once the name the astronomers have proposed to the International Astronomical Union has been accepted. More on the new planet can be found here; this page is likely to be updated significantly over the weekend.

Centauri Dreams‘ note: If Clyde Tombaugh had known to look 44 degrees off the ecliptic, he might have found 2003 UB313 instead of Pluto. No one would have expected a planet in an orbit with such a steep inclination, raising all kinds of questions about what forces drove the world to its present position. The elliptical orbit takes 560 years to complete and brings 2003 UB313 as close as 3.3 billion miles from the Sun (inside the orbit of Pluto). At 97 AU, it is currently nine billion miles out.

Meanwhile, this further information about 2003 EL61 — remember, this is a different object — also from Caltech in Brown’s Web pages. The evidence strongly suggests 2003 EL61 is actually a good deal smaller than Pluto:

“Many times when objects like this are discovered we don’t actually know how big we are, just how bright they are. They could be bright because they are large or they could be bright because they are highly reflective, like a ball of snow. In the case of 2003 EL61, however, we have gotten lucky, because we have discovered a moon orbiting it. By following the orbit of the moon over the course of 6 months we are able to precisely determine the mass of 2003 EL61 and its moon. The mass is about 32% that of Pluto, implying that it has a diameter of perhaps 70% that of Pluto or around 1500 km. We don’t know the diameter for sure, though, just the mass. It could be made of high density material like rock and be smaller or it could be made of low density material like ice and be larger and still be the same mass. If the size is indeed 1500 km 2003 EL61 is larger than any other known object in the Kuiper belt other than Pluto, with the closest runners-up being Quaoar and Pluto’s moon Charon at about 1250 km. It is also possibly larger than Sedna, a similar object well outside the Kuiper belt.”

Brown, Trujillo and Rabinowitz had been studying 2003 EL61 at the same time the Sierra Nevada team was working on it, and were delaying announcement of the object until further observations could yield more information about its size. The Spanish team thus gets credit for the discovery, while the Palomar team supplies a slew of further detail. Their observations at the Keck Observatory, for example, have shown that 2003 EL61’s moon appears to compose only about one percent of the mass of the system, making it the smallest satellite relative to its primary thus far found in the Kuiper Belt (Charon is roughly ten percent of Pluto’s mass).

A New Planet Larger Than Pluto?

A bright, slowly moving object in the outer Solar System may be a world larger than Pluto. A team of astronomers led by Jose-Luis Ortiz at the Sierra Nevada Observatory in Baja, California found the object, called 2003 EL61, using observations made in 2003. It is some 51 AU from the Sun (one AU, or Astronomical Unit, is the distance from Earth to the Sun), and evidently comes as close as 35 AU, inside Pluto’s average distance of 39 AU. An analysis of older observations shows the object in images dating back to 1995. [Note: the Sierra Nevada Observatory was mistakenly identified as being in Spain in an earlier version of this post].

Image of 2003 EL61

Is 2003 EL61 a new planet? And for that matter, how do we define what a planet is? That debate is sure to be reignited as we weigh the possibilities here, for a world larger than Pluto surely has to be considered a planet. But size measurements this far out from the Sun are tricky, and rely on an object’s albedo, a measure of how much light the object reflects. If 2003 EL61 is similar to other distant objects like Sedna and Quaoar, then the large size estimate makes sense. If, on the other hand, it is similar to Pluto in having a bright albedo, it would be smaller, though still significant at some 1500 kilometers in diameter (Pluto’s diameter is 2300 kilometers).

Image: The bright moving object now known as 2003 EL61 in a photograph made by Jose-Luis Ortiz and colleagues. Credit: Instituto de Astrofísica de Andalucia.

New Scientist has a write-up of the discovery here. The Sierra Nevada team has asked amateur astronomers to study the object, and its observations have been verified by the International Astronomical Union’s Minor Planet Center (MPC) in Cambridge, Massachusetts. Interestingly, 2003 EL61 could only be found using observations taken a full day apart, an indication of how slowly it is moving. And like Pluto, the new object is tilted substantially in relation to the orbital plane of the other planets, some 28° as compared to Pluto’s 17°.

Centauri Dreams‘ take: Given the widening hunt for planetoids in the Kuiper Belt, it comes as no surprise that a major object like this one should appear, although it does seem unusual that something this large has eluded detection until now. But before we draw too many conclusions, it will be necessary to get some hard data about the object’s true reflectivity and other characteristics that will give us some clue as to its composition.

A second thought is to note the heartening collaboration that is once again taking place between professional and amateur astronomers. With time on the big telescopes severely allocated, and with amateur equipment constantly increasing in sophistication, this is a synergy we can exploit to map the location of numerous deep space objects in the Kuiper Belt and beyond. We may indeed find that the Solar System is embedded in a veritable halo of Kuiper Belt planetoids, some of them large enough to qualify as planets in their own right.

The Ortiz team evidently intends to publish an animation of 2003 EL61 showing its movement against background stars, but the link is not yet operational; keep an eye on the site. A separate page gives background (in Spanish) on the equipment deployed at Sierra Nevada Observatory.

Tantalizing Evidence for Cosmic Strings

An object called CSL-1 may have a lot to say about the nature of the universe. The odd thing about this double source — evidently a pair of galaxies — is that both galaxies appear identical. They share a common redshift, a similar shape, and their luminosity profiles match that of two giant elliptical galaxies. Moreover, the spectra of the two components seem to be identical.

Is this a double image of the same galaxy? If so, then something tantalizing is going on. String theory, the latest and still evolving explanation for how the universe works, says that there should be gigantic counterparts to the strings that make up the fundamental particles of matter. A single-dimensional string millions of light years in length — think of it as a thread of energy — is one prediction made by string theory, and CSL-1 may indicate the presence of just such a cosmic string.

For a cosmic string would be so energetic that it would warp spacetime around it, with the effect that a string lying between Earth and a distant object would create two different routes for light to reach us. What we would see would be identical images of the same object, separated by a tiny distance. Mikhail Sazhin (Capodimonte Astronomical Observatory, Naples and the Sternberg Astronomical Institute, Moscow) and his colleagues found CSL-1 last year, and now report on their work in a new paper available on the arXiv site.

The key is to determine whether the two images represent the same object, or whether they are simply two extremely similar galaxies in close proximity to each other. Using the European Southern Observatory’s Very Large Telescope (Paranal, Chile), the team recorded detailed spectra of the objects and now present the case that they are identical. Further work will be definitive, and awaits the observing time on the Hubble Space Telescope that the team has now been granted. “The resolution of the HST will allow us to detect the specific signature produced by the cosmic string,” says Sazhin, in an article running in the July 30 issue of New Scientist. “We hope it will reduce the scepticism of other astronomers.”

Centauri Dreams‘ note: the first alternative that leaped to mind here was conventional gravitational lensing; i.e., lensing caused by a massive object between Earth and the distant galaxy. But Sazhin notes that standard lensing models rule out this configuration. “…in this second case, due to the lack of asymmetry in the two images, the lens could not be modelled with the standard lensing by a massive compact source. Actually, the usual gravitational lenses, i.e. those formed by bound clumps of matter, always produce inhomogeneous gravitational fields which distort the images of extended background sources… The detailed modelling of CSL-1 proved that the two images were virtually undistorted.”

Diagram of cosmic strings

Only gravitational lensing produced by a cosmic string seems to fit the data. The Hubble work will be vital because the cosmic string model predicts the images should have sharp edges of a precision not verifiable from Earth-based telescopes. The observations for these tests have not yet been taken, and so the coincidental two-galaxy possibility cannot yet be ruled out.

Image: A computer simulation of cosmic strings run at the University of Southampton (UK). Formed in a tangled mass, the strings would have quickly started to straighten out at speeds close to that of light, under the influence of their enormous tension. Remnants would still be around today: perhaps a few lengths stretching across the the visible Universe, strings so large that they have not had time to disappear, and a debris of smaller oscillating loops. The Southampton work on massively parallel simulations of cosmic strings can be found online.

If the CSL-1 data are confirmed as the result of a cosmic string, this finding would suggest that string theory is correct in its prediction of extra dimensions in the universe. Indeed, the theory makes the startling claim that the universe may be a three-dimensional ‘brane’ afloat in a sea of other dimensions. A collision between two such branes could have caused the Big Bang, and produced at the same time the kind of cosmic strings hypothesized here.

The paper on CSL-1 is Sazhin, M., Capaccioli, M, Longo, G. et al., “Further spectroscopic observations of the CSL-1 object,” available here.

Enceladus Flyby Reveals Bizarre Geology

No body in the solar system is as reflective as Saturn’s moon Enceladus. Its terrain also appears relatively young, with the early Cassini flybys revealing regions that are only lightly cratered. It seems that Enceladus has undergone a number of episodes of geologic convulsion, with the southernmost latitudes seeing the most recent activity, producing a tortured surface marked by crisscrossing faults, folds and ridges. All this comes from findings revealed by the July 14 Enceladus flyby and discussed recently in a news release from the Cassini Imaging Central Laboratory for Operations.

Boulders on EnceladusThe latest flyby brought Cassini within 175 kilometers (109 miles) of the moon, showing that the landscape near its south pole is studded with ice boulders the size of houses, while impact craters in the region are almost entirely absent. Some of the ice blocks are up to 100 meters (328 feet) across, and they appear in an area that lacks the fine-grained frost found elsewhere on Enceladus. All that adds up to yet another Enceladus surprise.

Image: The tortured southern polar terrain of Enceladus appears strewn with great boulders of ice in this fantastic view, one of the highest resolution images obtained so far by Cassini of any world. Credit: NASA/JPL/Space Science Institute.

“A landscape littered with building-sized blocks was not expected,” said Dr. Peter Thomas, an imaging team member from Cornell University, Ithaca, N.Y. “The minimal cover of finer material and the preservation of small, crossing fracture patterns in the surrounding areas indicate that this region is young compared to the rest of Enceladus.”

If the south pole region is indeed younger than the rest of the moon, then material from Enceladus may be implicated, as some scientists have argued, in the formation of Saturn’s extensive E ring, which coincides with the moon’s orbit. Enceladus is considered too small to generate the heat needed to make major modifications to its surface. But the latest images show a Y-shaped tectonic feature made from parallel ridges and valleys, perhaps evidence of a shift in the moon’s spin rate. Such a shift could be a source for Enceladus’ energetic geology.

“These tectonic features define a boundary that isolates the young, south polar terrains from older terrains on Enceladus,” noted Dr. Paul Helfenstein, an associate of the Imaging Team also at Cornell University. “Their placement and orientation may tell us a very interesting story about the way the rotation of Enceladus has evolved over time and what might have provided the energy to power the geologic activity that has wracked this moon.”

More on Saturn: if you want to put a chill down your spine, listen to the radio sounds of Saturn as registered by Cassini’s radio and plasma wave science instrument. They’re available here as an audio file. The radio waves seem to be generated by Saturn’s aurorae, producing a radio spectrum with rising and falling tones that sounds like something out of a 1950’s science fiction movie sound track. All similarities to Forbidden Planet are strictly coincidental!

The Hunt for ‘Hot Earths’

By now we all know what a ‘hot Jupiter’ is — a gas giant orbiting breathtakingly close to its parent star. The radial velocity searches for extrasolar planets that have found so many new worlds are particularly sensitive to high-mass planets in close orbits, so it makes sense that the early list of discoveries would be populated mostly with hot Jupiters. It’s intriguing (and typical of the entire field of extrasolar planet detection) that this is a category of planet few scientists expected to find, especially in such numbers.

But look what has happened to the planet hunt. In 2000, Geoff Marcy and Paul Butler detected the first planet with a mass below that of Saturn. It orbits the star HD 46375, some 109 light years away in the constellation Monoceros. The duo also discovered a planet 70 percent of Saturn’s mass orbiting the star 79 Ceti, 117 light years away in the constellation Cetus.

In 2004, a team led by Portuguese researcher Nuno Santos discovered a planet 14 times the size of Earth orbiting the star mu Arae, and a later detection by Barbara McArthur and colleagues (University of Texas at Austin) found what may be a rocky world 18 times larger than Earth around the star 55 Cancri. The numbers, in other words, are decreasing as we find smaller and smaller worlds. The 2005 finding of a planet a mere 7.5 times the mass of Earth around the red dwarf Gliese 876 marked the smallest extrasolar planet yet detected (this work was led by Jack Lissauer at NASA Ames and Eugenio Rivera at Lick Observatory).

Thus we have a new category of planet that some are already calling ‘hot Earths.’ The Gliese 876 planet, for example, orbits its star in just 1.94 days, and is roughly a tenth of the distance from it that Mercury is from our Sun.

A new paper by Joshua Pepper (Ohio State) and B. Scott Gaudi (Harvard-Smithsonian Center for Astrophysics) suggests we can do better still. The authors believe that intensive monitoring using current technology could uncover close-in planets down to Earth-like mass. From the paper: “…we conclude it should be possible – from the ground and with current technology – to place interesting constraints on the frequency of Hot Earths and Hot Neptunes in Galactic open clusters. This will in turn constrain the properties of the low-mass planets recently detected in RV [radial velocity] surveys, as well as theories of planet formation and migration in general.”

The Hyades ClusterAn open cluster is a small collection of stars that probably formed at the same time (think the Pleiades, for example, or the Hyades, both in the constellation Taurus). Not to be confused with globular clusters, which are found throughout the galactic halo, open clusters tend to line up with the galactic plane. Pepper and Gaudi note that despite a number of surveys, no planets have been found around stars in open clusters. But if hot Earths and hot Neptunes are more common than hot Jupiters, transit surveys of such clusters may well yield planets.

Image: The Hyades star cluster, the nearest open cluster to our Solar System at a distance of some 150 light years. The large red star, not a member of the cluster, is Aldebaran. Open clusters may prove the best places to look for Earth-mass planets in close orbits. Credit: Sobolev Astronomical Institute, St. Petersburg.

Transit surveys are tricky; they rely on a planet being lined up so that, as seen from Earth, it passes directly in front of its parent star. Only ten percent of close-in planets make such transits, and only two planets originally found through radial velocity methods have been found to make transits, although deep field surveys of galactic disk stars have found others. But transits offer priceless information about a planet’s radius and are thus a priority. Pepper and Gaudi think that transit surveys of open clusters like the Hyades would be able to detect hot Earths with radii as small as the Earth and orbital periods of between one and four days.

Centauri Dreams‘ note: While not listing them in this paper, the authors point out that surveys of open clusters offer advantages over deep field surveys. They are thus suggesting that for smaller worlds, open clusters are a much likelier hunting ground, even though the number of stars being surveyed is much smaller. The paper, which has been submitted to Astrophysical Journal Letters, is “Detecting Transiting Hot Earths and Hot Neptunes in Galactic Open Clusters, now available at the arXiv site.