A Mini-Pluto at the Edge of Our Solar System

Astronomers have better measurements now of what is turning out to be a smaller version of Pluto located so far away that it may take 1,100 years to orbit the Sun. Nicknamed D.D. (or DeeDee) for Distant Dwarf, this remarkable small world appears to be about 400 miles in diameter -- only about 1/4 the size of Pluto, but possibly still big enough to be round like a planet.

D.D. was found in the fall of 2016 by a team of astronomers led by David Gerdes of the University of Michigan. As so often happens in astronomy, they were looking for something completely different, but found D.D. as part of their work. Now they have used an array of radio telescopes, called ALMA, to measure D.D.'s size and also to estimate its temperature. Ninety-two times as far from the Sun as the Earth is, D.D. is thought to be at minus 405 degrees Fahrenheit (minus 243 degrees Celsius; or only 30 degrees above nature's limit, absolute zero.)

D.D. is part of a whole zone of icy objects out beyond Neptune (astronomers call them Trans-Neptunian Objects or TNO's,) About 2,000 TNOs are now known, but most of them are too small and irregular (knotty potato-shaped) to be called dwarf planets. Pluto, the first TNO dwarf planet, was discovered in 1930, but in the last decade of the 20th century and the first decade of the 21st century, others were found, including Eris, Makemake, and Haumea. (If the names sound unusual, it's because we are trying now to name these outer worlds after mythological beings from many cultures, not just the ancient Greeks or Romans!)

We need more observations of D.D. before we are ready to call it a dwarf planet. If you thought that Pluto was being insulted when we called it a dwarf planet, perhaps it will help you to know that Pluto is the "first" of a whole new category of worlds. Five dwarf planets are now officially recognized by the International Astronomical Union, and at least six other worlds out beyond Neptune may eventually be included in the group. It may be years before we know if D.D. will join them.

But it's wonderful to see the Sun's family growing in this way, with our new instruments and surveys able to find worlds that literally are "far out!"

There is a Possible Super Earth in the Outer Solar System

Out there, way beyond the Sun's family, astronomers have discovered thousands of planets orbiting other stars. Among these alien planets, we have discovered a significant number of "Super Earths" -- planets more massive than our Earth but less massive than the smallest giants in our solar system, Uranus and Neptune. Our solar system has no such Super Earths, but many other systems do; they may be quite common.

Now, Michael Brown and Konstantin Batygin at Caltech, propose that we might just have a Super Earth in our solar system, but so far from the Sun, it takes between 10,000 and 20,000 years to make one orbit! (Pluto, for comparison, takes about 250 years.)

Brown and Batygin have been examining the orbits of icy chunks way beyond Pluto, in the region we call the Kuiper Belt. There are several chunks out there, including one Brown discovered in 2003, called Sedna, that move in an oddly aligned way. After testing many computer models to explain their odd orbit, their best model indicates there could be a planet 10 times the mass of our own Earth, whose stronger gravity is affecting the motions of many objects out where it orbits. Just a few of the affected chunks have been discovered so far and the planet itself has NOT be seen.

So this is a somewhat daring hypothesis, which the two astronomers explain in this brief video: https://www.youtube.com/watch?v=6poHQ2h00ZA

Please note that our image is a painting that Caltech commissioned. No one knows what this Planet Nine looks like.

Now here is the human side of the story. Michael Brown led the team that discovered Eris, the dwarf planet that is the same size as Pluto, in 2005. When he discovered it, he told his wife that he had just discovered a tenth planet and she had made a good decision in marrying him. Alas, instead of being acknowledged as the 10th planet, Eris caused astronomers to rethink the status of Pluto, and remove it from being the 9th planet. Brown was gracious about it, but you can imagine how disappointing it all was for him.

So now Brown may have "discovered" (or at least predicted) a real new planet. He is already nicknaming it "Planet Nine" and it's so big, no one will be able to call it a dwarf! If it is confirmed one day, he can go back to his wife and tell her, "Well, maybe I only discovered a dwarf planet before, but now I have discovered a real planet at last." Not a bad thing to discuss over dinner!

You can see a video where Brown discusses his role in the Pluto and Eris story at: https://www.youtube.com/watch?v=7pbj_llmiMg

Brown and Batygin

The Best Picture from Pluto So Far

The New Horizons spacecraft, which flew by Pluto in July, is slowly continuing to send back the images and data it took. The latest batch contains some of the most detailed close-up pictures of Pluto we have ever seen and they are fascinating!
Let’s take a look at the most intriguing new picture for a minute. We are seeing the greatest detail the spacecraft cameras were capable of. In a scene about 50 miles wide, we can make out details as small as half a city block. We see the shoreline of the Sputnik plains (part of the giant heart-shaped feature that caught everyone’s attention on the early pictures.)

The “rocks” that make up the mountains in the upper left are made of water – which is harder than rock at Pluto’s freezing temperatures. Some of these mountains are more than a mile and half high, with some of their sides bright with ice and others coated in a darker material that we are still learning about. This darker material may fall out of the sky, when ultraviolet light from the distant Sun causes chemical changes in Pluto’s thin atmosphere.

Notice how abruptly and cleanly the mountain end and give way to the softer, nitrogen-rich ice that makes up the Sputnik plains. In that ice, you can see huge but subtle cell-like structures. What makes up and drives this cell-like structure is still being debated by astronomers. Material in and around these cells may be moving up or down, like the cells you see when you boil miso soup. (If you've never boiled miso soup, ask a Japanese friend to tell you about it.) Cold nitrogen and methane ice might behave similarly when it is heated by the slightly warmer insides of Pluto and the faint heat of the Sun.
In an earlier photo, you can see some dark hills poking up at the boundary between cells, so this is very complicated terrain we are looking at. See the picture below. 
But just enjoy looking at the alien vista New Horizon’s cameras revealed. Pluto is not simple or boring!

Water Found on Largest Asteroid

Astronomers working with the Herschel space observatory have discovered water vapor coming from the largest asteroid in the asteroid belt, the one called Ceres (pronounced like "series"). By strict definition, Ceres is so big and round that it is no longer considered an asteroid, but is now designated dwarf planet number one (having been discovered on Jan. 1, 1801, long before Pluto).

Ceres is about 600 miles across and takes 4.6 years to orbit the Sun (Mars takes 1.9 years, while Jupiter takes 12 years.) The Hubble Space Telescope images of it (see one attached here) show lighter and darker areas. The water vapor is not evenly distributed around Ceres and there is more of it when Ceres is closer to the Sun, so there may be some ice that is sublimating (going from frozen form to vapor) in parts of Ceres.

At Ceres' distance from the Sun, ice on the surface would have all sublimated long ago, so this must be vapor coming from a deeper (frozen) layer, which was a surprise. Some astronomers think there may be enough water ice under the surface of Ceres to make an ocean. This is the first time water vapor has been detected in the asteroid belt, although we have seen it coming from a moon of Jupiter's and a moon of Saturn's. There may be plumes or "geysers" of water vapor coming from parts of Ceres, perhaps like the ones on Saturn's satellite Enceladus.

We will know a lot more about conditions on Ceres next March and April, when the Dawn spacecraft arrives for a rendezvous with Ceres and provides images with unprecedented detail of this intriguing member of our solar system. (Dawn gave us lots of great information about Vesta, the second largest asteroid, before it departed for Ceres in 2012.) Isn't it great how nature continues to surprise and delight us as we explore our cosmic neighborhood?

Two Major Discoveries about Our Solar System

Two major finds were announced yesterday: the first rings found around an asteroid, and the most distant object -- a dwarf planet -- ever seen in our solar system. Let me fill you in on the details.

The unexpected rings were found around the asteroid Chariklo, a 150-mile wide chunk that orbits between Saturn and Uranus. A group of South American astronomers observed Chariklo moving in front of a star, and glimpsed two thin rings around it. The rings are roughly 4 and 2 miles wide and are separated by a gap of some 5 1/2 miles. In the past, when we have found distinct thin rings like this, they have been kept thin by the presence of small "shepherding moons." The analogy is the shepherds keep a flock of sheep in a thin line, and these moons keep the particles of the rings from drifting away from their formation. So we expect that one or more such moons will eventually be found around Chariklo.

How did an asteroid get itself a set of rings -- which we have previously seen only around giant planets? Our best guess is that Chariklo was involved in some sort of cosmic accident and the rings are left-over debris from that event.

By the way, Harry Potter fans and mythology buffs will appreciate the name of the asteroid. In classical mythology, Chariklo was the wife of Chiron, the best known of the Centaurs -- creatures that were half human and half horse. In the same way, the Centaurs in our solar system are also half-breeds -- they have some characteristics of asteroids and some of comets.

The second discovery was of a dwarf planet orbiting so far away from the Earth and the Sun that it is the most distant world we have seen in our solar system. The small, frozen world, estimated to be about 250 miles wide, is called 2012 VP113, a boring provisional name that includes the date of its first sighting and letters and numbers that give a code for what its order is in the discovery of small objects that year. But because the letter code happens to be VP, the discoverers have nicknamed it Biden (but that will not be its final official name.)

I can't resist noting, in case you were not aware of this, that each vice president of the U.S. already has a direct connection to the world of astronomy, since the vice-president's residence is on the grounds of the U.S. Naval Observatory. Many vice-presidents brings guests to the observatory and even have parties there.

Little "Biden" (the world) is really, really out there, beyond what we think are the borders of the Kuiper Belt -- the zone of Pluto, other dwarf planets, and small icy chunks that make a belt outside the orbit of Neptune. The conventional wisdom has been that this belt should end at 50 times the distance between the Earth and the Sun (which we call 50 astronomical units), or about 4 1/2 billion miles from the Sun.

Biden, on the other hand, makes a looping orbit further out, and never gets closer to the Sun than 80 astronomical units or 7 1/2 billion miles. And most of the time is is even further out. (Another object found earlier, now called Sedna, is also out there.) The presence of such "far-out" members of our solar system, while not impossible, is surprising enough that astronomers are now set to pondering what got them out there.

Our image shows an artist's conception (no one has a photo of it) of what Chariklo and its two rings might look like out there, provided by the European Southern Observatory.