Space and astronomy news
Host: Fraser Cain (@fcain)
Special Guest: This week we welcome Paul Sutter, the CCAPP Visiting Fellow who works on the cosmic microwave background and large-scale structure.
Guests:
Jolene Creighton (@jolene723 / fromquarkstoquasars.com)
Brian Koberlein (@briankoberlein / briankoberlein.com)
Morgan Rehnberg (cosmicchatter.org / @MorganRehnberg )
Alessondra Springmann (@sondy)
Continue reading “Weekly Space Hangout – June 26, 2015: Paul Sutter, CCAPP Visiting Fellow”
Could there be another Pluto-like object out in the far reaches of the Solar System? How about two or more?
Earlier this week, we discussed a recent paper from planet-hunter Mike Brown, who said that while there aren’t likely to be any bright, easy-to-find objects, there could be dark ones “lurking far away.” Now, a group of astronomers from the UK and Spain maintain at least two planets must exist beyond Neptune and Pluto in order to explain the orbital behavior of objects that are even farther out, called extreme trans-Neptunian objects (ETNO).
We do know that Pluto shares its region Solar System with more than 1500 other tiny, icy worlds along with likely countless smaller and darker ones that have not yet been detected.
In two new paper published this week, scientists at the Complutense University of Madrid and the University of Cambridge noted that the most accepted theory of trans-Neptunian objects is that they should orbit at a distance of about 150 AU, be in an orbital plane – or inclination – similar to the planets in our Solar System, and they should be randomly distributed.
But that differs from what is actually observed. What astronomers see are groupings of objects with widely disperse distances (between 150 AU and 525 AU) and orbital inclinations that vary between 0 to 20 degrees.
“This excess of objects with unexpected orbital parameters makes us believe that some invisible forces are altering the distribution of the orbital elements of the ETNO,” said Carlos de la Fuente Marcos, scientist at UCM and co-author of the study, “ and we consider that the most probable explanation is that other unknown planets exist beyond Neptune and Pluto.”
He added that the exact number is uncertain, but given the limited data that is available, their calculations suggest “there are at least two planets, and probably more, within the confines of our solar system.”
In their studies, the team analyzed the effects of what is called the ‘Kozai mechanism,’ which is related to the gravitational perturbation that a large body exerts on the orbit of another much smaller and further away object. They looked at how the highly eccentric comet 96P/Machholz1 is influenced by Jupiter (it will come near the orbit of Mercury in 2017, but it travels as much as 6 AU at aphelion) and it may “provide the key to explain the puzzling clustering of orbits around argument of perihelion close to 0° recently found for the population of ETNOs,” the team wrote in one of their papers.
They also looked at the dwarf planet discovered last year called 2012 VP113 in the Oort cloud (its closest approach to the Sun is about 80 astronomical units) and how some researchers say it appears its orbit might be influenced by the possible presence of a dark and icy super-Earth, up to ten times larger than our planet.
“This Sedna-like object has the most distant perihelion of any known minor planet and the value of its argument of perihelion is close to 0°,” the team writes in their second paper. “This property appears to be shared by almost all known asteroids with semimajor axis greater than 150 au and perihelion greater than 30 au (the extreme trans-Neptunian objects or ETNOs), and this fact has been interpreted as evidence for the existence of a super-Earth at 250 au. In this scenario, a population of stable asteroids may be shepherded by a distant, undiscovered planet larger than the Earth that keeps the value of their argument of perihelion librating around 0° as a result of the Kozai mechanism.”
Of course, the theory put forth in two papers published by the team goes against the predictions of current models on the formation of the Solar System, which state that there are no other planets moving in circular orbits beyond Neptune.
But the team pointed to the recent discovery of a planet-forming disk around the star HL Tauri that lies more than 100 astronomical units from the star. HL Tauri is more massive and younger than our Sun and the discovery suggests that planets can form several hundred astronomical units away from the center of the system.
The team based their analysis by studying 13 different objects, so what is needed is more observations of the outer regions of our Solar System to determine what might be hiding out there.
Further reading:
Carlos de la Fuente Marcos, Raúl de la Fuente Marcos, Sverre J. Aarseth. “Flipping minor bodies: what comet 96P/Machholz 1 can tell us about the orbital evolution of extreme trans-Neptunian objects and the production of near-Earth objects on retrograde orbits”. Monthly Notices of the Royal Astronomical Society 446(2):1867-1873, 2015.
C. de la Fuente Marcos, R. de la Fuente Marcos. “Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets? Monthly Notices of the Royal Astronomical Society Letters 443(1): L59-L63, 2014.
What is a dwarf planet? Some astronomers have been asking that question after Pluto was demoted from planethood almost a decade ago, partly due to discoveries of other worlds of similar proportions.
Today, astronomers announced the discovery of 2012 VP113, a world that, assuming its reflectivity is moderate, is 280 miles (450 kilometers) in size and orbiting even further away from the sun than Pluto or even the more distant Sedna (announced in 2004). If 2012 VP113 is made up mostly of ice, this would make it large (and round) enough to be a dwarf planet, the astronomers said.
Peering further into 2012 VP113’s discovery, however, brings up several questions. What are the boundaries of the Oort Cloud, the region of icy bodies where the co-discoverers say it resides? Was it placed there due to a sort of Planet X? And what is the definition of a dwarf planet anyway?
First, a bit about 2012 VP113. Its closest approach to the Sun is about 80 astronomical units, making it 80 times further from the Sun than Earth is. This puts the object in a region of space previously known only to contain Sedna (76 AU away). It’s also far away from the Kuiper Belt, a region of rocky and icy bodies between 30 and 50 AU that includes Pluto.
“The detection of 2012 VP113 confirms that Sedna is not an isolated object; instead, both bodies may be members of the inner Oort Cloud, whose objects could outnumber all other dynamically stable populations in the Solar System,” the authors wrote in their discovery paper, published today in Nature.
The Oort cloud (named after the Dutch astronomer Jan Oort, who first proposed it) is thought to contain a vast number of smallish, icy bodies. This NASA web page defines its boundaries as between 5,000 and 100,000 AUs, so 2012 VP113 obviously falls short of this measure.
The astronomers hypothesize that 2012 VP113 is part of a collection of “inner Oort cloud objects” that make their closest approach at a distance of more than 50 AU, a boundary that is thought to avoid any “significant” interference from Neptune. Orbits of these objects would range no further than 1,500 AU, a location hypothesized as part of the “outer Oort cloud” — the spot where “galactic tides start to become important in the formation process,” the team wrote.
“Some of these inner Oort cloud objects could rival the size of Mars or even Earth. This is because many of the inner Oort cloud objects are so distant that even very large ones would be too faint to detect with current technology,” stated Scott Sheppard, co-author of the paper and a solar system researcher at the Carnegie Institution for Science. (The lead author is the Gemini Observatory’s Chadwick Trujillo, who co-discovered several dwarf planets with the California Institute of Technology’s Mike Brown.)
One large question is how 2012 VP113 and Sedna came to be. And of course, with only two objects, it’s hard to draw any definitive conclusions. Theory 1 supposes that the gas giant planets beyond Earth ejected a “rogue” planet (or planets) that in turn threw objects from the Kuiper Belt to the more distant inner Oort Cloud. “These planet-sized objects could either remain (unseen) in the Solar System or have been ejected from the Solar System during the creation of the inner Oort Cloud,” the researchers wrote.
(Planet X hopers: Note that NASA just released results from its Wide-Field Infrared Survey Explorer that found nothing Saturn’s size (or bigger) as far as 10,000 AU, and nothing bigger than Jupiter at 26,000 AU.)
Theory 2 postulates that a passing star moved objects closer to the Sun into the inner Oort cloud. The last, “less-explored” theory is that these objects are “extrasolar planetesimals” — small worlds from other stars — that happened to be close to the Sun when it was born in a field of stars.
However these objects came to be, the astronomers estimate there are 900 objects with orbits similar to Sedna and 2012 VP113 that have diameters larger than 620 miles (1,000 kilometers). How do we know which are dwarf planets, however, given their distance and small size?
The International Astronomical Union’s definition of a dwarf planet doesn’t mention how big an object has to be to qualify as a dwarf planet. It reads: “A dwarf planet is an object in orbit around the Sun that is large enough (massive enough) to have its own gravity pull itself into a round (or nearly round) shape. Generally, a dwarf planet is smaller than Mercury. A dwarf planet may also orbit in a zone that has many other objects in it. For example, an orbit within the asteroid belt is in a zone with lots of other objects.”
That same page mentions there are only five recognized dwarf planets: Ceres, Pluto, Eris, Makemake and Haumea. Brown led the discovery of the last three dwarf planets in this list, and calls himself “the man who killed Pluto” because his finds helped demote Pluto from planethood to dwarf planet status.
It’s hard for official bodies to keep up with the pace of discovery, however. Brown’s webpage lists 46 “likely” dwarf planets, which under this definition would give him 15 discoveries.
“Reality … does not pay much attention to official lists kept by the IAU or by anyone else,” he wrote on that page. “A more interesting question to ask is: how many round objects are there in the solar system that are not planets? These are, by the definition, dwarf planets, whether or not they ever make it to any offiicially sanctioned list. If the category of dwarf planet is important, then it is the reality that is important, not the official list.”
His analysis (which focuses on Kuiper Belt objects) notes that most objects are too faint for us to notice if they are round or not, but you can get a sense of how round an object is by its size and composition. The asteroid belt’s Ceres (at 560 miles or 900 km) is the only known round, rocky object.
For icier objects, he suggested looking to icy moons to understand how small an object can be and still be round. Saturn’s moon Mimas is round at 250 miles (400 km), which he classifies as a “reasonable lower limit” (since observed satellites of 125 miles/200 km are not round).
Discovery of 2012 VP113 came courtesy of the new Dark Energy Camera (DECam) at the National Optical Astronomy Observatory’s 4-meter telescope in Chile. The orbit was determined with the Magellan 6.5-meter telescope at Carnegie’s Las Campanas Observatory, also in Chile.
The paper, called “A Sedna-like body with a perihelion of 80 astronomical units”, will soon be available on Nature’s website.