Tag Archives: Pluto

Pluto Has Tail, X-Rays

Did you ever notice that Pluto doesn’t have much of a tail? No, not that Pluto! This Pluto:


This has been known for a while. NASA noted this last year:

New Horizons has discovered a region of cold, dense ionized gas tens of thousands of miles beyond Pluto — the planet’s atmosphere being stripped away by the solar wind and lost to space. Beginning an hour and half after closest approach, the Solar Wind Around Pluto (SWAP) instrument observed a cavity in the solar wind — the outflow of electrically charged particles from the Sun — between 48,000 miles (77,000 km) and 68,000 miles (109,000 km) downstream of Pluto. SWAP data revealed this cavity to be populated with nitrogen ions forming a “plasma tail” of undetermined structure and length extending behind the planet.

Not long ago it was not known that Pluto had an atmosphere. But it does, and it is probably made from solid ice that makes up a good portion of the planet. When Pluto is nearer the Sun, this atmosphere burns off and forms an unimpressive tail. (Existentially impressive, but not fireworks impressive.) If Pluto were to come really close to the sun, like a typical comet, it would … well, it would essentially be a a comet. A pretty big one, at first. But then after several passes…

Anyway, more recently, it has been discovered that Pluto also puts out X-rays, and if confirmed, this is interesting. The total number of X-rays that have been detected is very small. The existence of these X-rays is likely linked to the atmosphere. From NASA:

Scientists using NASA’s Chandra X-ray Observatory have made the first detections of X-rays from Pluto. These observations offer new insight into the space environment surrounding the largest and best-known object in the solar system’s outermost regions.

While NASA’s New Horizons spacecraft was speeding toward and beyond Pluto, Chandra was aimed several times on the dwarf planet and its moons, gathering data on Pluto that the missions could compare after the flyby. Each time Chandra pointed at Pluto – four times in all, from February 2014 through August 2015 – it detected low-energy X-rays from the small planet.

Pluto is the largest object in the Kuiper Belt, a ring or belt containing a vast population of small bodies orbiting the Sun beyond Neptune. The Kuiper belt extends from the orbit of Neptune, at 30 times the distance of Earth from the Sun, to about 50 times the Earth-Sun distance. Pluto’s orbit ranges over the same span as the overall Kupier Belt.

“We’ve just detected, for the first time, X-rays coming from an object in our Kuiper Belt, and learned that Pluto is interacting with the solar wind in an unexpected and energetic fashion,” said Carey Lisse, an astrophysicist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, who led the Chandra observation team with APL colleague and New Horizons Co-Investigator Ralph McNutt. “We can expect other large Kuiper Belt objects to be doing the same.”

The team recently published its findings online in the journal Icarus. The report details what Lisse says was a somewhat surprising detection given that Pluto – being cold, rocky and without a magnetic field – has no natural mechanism for emitting X-rays. But Lisse, having also led the team that made the first X-ray detections from a comet two decades ago, knew the interaction between the gases surrounding such planetary bodies and the solar wind – the constant streams of charged particles from the sun that speed throughout the solar system ­– can create X-rays.

New Horizons scientists were particularly interested in learning more about the interaction between the gases in Pluto’s atmosphere and the solar wind. The spacecraft itself carries an instrument designed to measure that activity up-close – the aptly named Solar Wind Around Pluto (SWAP) – and scientists are using that data to craft a picture of Pluto that contains a very mild, close-in bowshock, where the solar wind first “meets” Pluto (similar to a shock wave that forms ahead of a supersonic aircraft) and a small wake or tail behind the planet.

The immediate mystery is that Chandra’s readings on the brightness of the X-rays are much higher than expected from the solar wind interacting with Pluto’s atmosphere.

“Before our observations, scientists thought it was highly unlikely that we’d detect X-rays from Pluto, causing a strong debate as to whether Chandra should observe it at all,” said co-author Scott Wolk, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “Prior to Pluto, the most distant solar system body with detected X-ray emission was Saturn’s rings and disk.”

The Chandra detection is especially surprising since New Horizons discovered Pluto’s atmosphere was much more stable than the rapidly escaping, “comet-like” atmosphere that many scientists expected before the spacecraft flew past in July 2015. In fact, New Horizons found that Pluto’s interaction with the solar wind is much more like the interaction of the solar wind with Mars, than with a comet. However, although Pluto is releasing enough gas from its atmosphere to make the observed X-rays, in simple models for the intensity of the solar wind at the distance of Pluto, there isn’t enough solar wind flowing directly at Pluto to make them.

Lisse and his colleagues – who also include SWAP co-investigators David McComas from Princeton University and Heather Elliott from Southwest Research Institute – suggest several possibilities for the enhanced X-ray emission from Pluto. These include a much wider and longer tail of gases trailing Pluto than New Horizons detected using its SWAP instrument. Other possibilities are that interplanetary magnetic fields are focusing more particles than expected from the solar wind into the region around Pluto, or the low density of the solar wind in the outer solar system at the distance of Pluto could allow for the formation of a doughnut, or torus, of neutral gas centered around Pluto’s orbit.

That the Chandra measurements don’t quite match up with New Horizons up-close observations is the benefit – and beauty – of an opportunity like the New Horizons flyby. “When you have a chance at a once in a lifetime flyby like New Horizons at Pluto, you want to point every piece of glass – every telescope on and around Earth – at the target,” McNutt says. “The measurements come together and give you a much more complete picture you couldn’t get at any other time, from anywhere else.”

New Horizons has an opportunity to test these findings and shed even more light on this distant region – billions of miles from Earth – as part of its recently approved extended mission to survey the Kuiper Belt and encounter another smaller Kuiper Belt object, 2014 MU69, on Jan. 1, 2019. It is unlikely to be feasible to detect X-rays from MU69, but Chandra might detect X-rays from other larger and closer objects that New Horizons will observe as it flies through the Kuiper Belt towards MU69.

The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, designed, built, and operates the New Horizons spacecraft and manages the mission for NASA’s Science Mission Directorate. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA’s Science Mission Directorate. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra’s science and flight operations.


Why is Pluto not a planet?

Short answer: Pluto has only two of the three necessary characteristics to be called a planet. Pluto has not cleared its neighborhood, or orbit. But, of course, there are additional details.

The simplest reason that Pluto is not a planet is that planet experts say so, and this is their job. But you may be looking for a more detailed explanation.

Let’s look at what defines a planet. This could be a very long and tedious discussion, because “planet” is an ancient concept used long before scientists knew very much about them. Also, frankly, in many areas of science the definition of a thing, perhaps counter-intuitively to non-scientists, is often pretty irrelevant to its study. Definitions that change over time that are never quite in line with the phenomenon being observed, etc. may seem like an impediment to science, but they often are not. The definition of a “gene” has changed dramatically as we’ve learned more about them, but this shifting description has not hampered genetic research. To some extent this may be the same with planets. A “planetologist” who studied Pluto back when it was still counted as a planet would not have to find a new job when our solar system went from 9 to 8.

The International Astronomical Union has settled on a set of definitions of solar system bodies, which includes planets, dwarf planets (which are mostly minor planets), small solar system bodies, trans-Neptunian objects which also might be called plutoids (those are also minor planets) and some small solar system bodies (including some comets) and satellites, and satellites are, of course, things that go around things that are not the Sun. Confused? Probably, but that is not a big problem because while these various identified flying objects have complex overlapping categorical status, one type of object does not. Planets are planets and they are not anything other than planets.

To be a planet, you have to be in orbit around the Sun. This would rule out the Moon, which, if it was in orbit around the Sun instead of the Earth, could well be a planet.

To be a planet the object has to have sufficient mass to have been shaped by gravity to be (mostly) a globe. This depends on various things so at the low end of the mass spectrum there will be different masses and different sizes of things that don’t quite make it to globular status.

To be a planet the object has to have cleared its orbit. In other words, as an object orbits around the sun, it is likely to bump into other objects. Over a period of time, the object has finished bumping into everything it is likely to bump into, and thereafter has only a low probability of bumping into something. That does not rule out something bumping into the object, of course.

A globe shaped object that goes around the sun but that has not cleared its orbit is classified as a “dwarf planet.” This is of course historically contingent. In the early days of a solar system, perhaps there would be large star-circleing round things that have not yet cleared their orbit. This speaks to the strangeness of definitions alluded to above. The definition we use today to classify our solar system’s objects applies to a solar system developed to the extent ours is developed. The IAU nomenclature would probably need revisions if applied to all planetary-star systems in the Universe.

This scheme is not without its critics and there is indeed debate. Some of that debate is a bit nitpicky but still interesting. For example, Alan Stern, with NASA, notes that many planets have not really cleared their orbit, noting in relation to the Pluto controversy, “If Neptune had cleared its zone, Pluto wouldn’t be there.” Yes, apparently heavenly bodies have irony.

Anyway, as implied, Pluto is not classed as a planet because it has not cleared its orbit. Therefore it is a Dwarf Planet. Since it is far away (farther than Neptune) it also gets classed as a Trans-Neptunian Object. Furthermore, it is a Plutoid. That is simply a newer term applied to Trans-Neptuina dwarf planets.

The term Plutoid, then, refers to a dwarf planet, which for various reasons is apparently always specifically an ice dwarf, which is a trans-Neptunian body (orbiting most of the time beyond Neptune) that is sufficiently massive to be shaped like a globe. This term, plutoid, is officially adopted A plutoid or ice dwarf is a trans-Neptunian dwarf planet, i.e. a body orbiting beyond Neptune that is large enough to be rounded in shape. The term plutoid was adopted by the International Astronomical Union’s Committee on Small Bodies Nomenclature, but not by the working group on Planetary System Nomenclature. So you can use Plutoid or Dwarf Planet, or Ice Dwarf, depending on whom you wish to annoy.

Pluto, Eris, Haumea, and Makemake are the only known Plutoids. They are small enough and far enough away that more could be discovered.