Knowing what comets are made of -- dust and ice -- only begins to answer the mystery of what is inside them. A compact object with this composition should be heavier than water, but we know that many comets have densities much lower than that of water ice. The implication is that comets are porous, but what we'd still like to know is whether this porosity is the result of empty spaces inside the comet or an overall, homogeneous low-density structure. For answers, we turn to the European Space Agency's continuing Rosetta mission. In a new paper in Nature, Martin Pätzold (Rheinische Institut für Umweltforschung an der Universität zu Köln, Germany) and team have gone to work on the porosity question by analyzing Comet 67P/Churyumov-Gerasimenko, around which Rosetta travels. It's no surprise to find that 67P/Churyumov-Gerasimenko is a low-density object, but an examination of the comet's gravitational field shows that we can now rule out a cavernous interior. Image: These images of comet...

If there's one thing Pluto turned out to have beyond all expectation, it's geological activity. New Horizons is now showing us what researchers are calling 'hills of water ice' floating in a sea of frozen nitrogen, much like icebergs moving through our own Arctic Ocean. The isolated hills are thought to be fragments of the water ice in the surrounding upland regions. Measuring several kilometers across, they are found in Sputnik Planum, a plain within Pluto's 'heart.' Image: The image shows the inset in context next to a larger view that covers most of Pluto's encounter hemisphere. The inset was obtained by New Horizons' Multispectral Visible Imaging Camera (MVIC) instrument. North is up; illumination is from the top-left of the image. The image resolution is about 320 meters per pixel. The image measures a little over 500 kilometers long and about 340 kilometers wide. It was obtained at a range of approximately 16,000 kilometers from Pluto, about 12 minutes before New Horizons'...

Given that we have four planets in our Solar System with rings, it's a natural thought that if so-called Planet Nine does exist, it might likewise show a system of rings. After all, Caltech's Konstantin Batygin and Mike Brown are talking about a planet with a mass on the order of ten times that of the Earth. Neptune is about 17 Earth masses, while Uranus is 14.5 as massive. If Planet Nine is an ejected ice giant, perhaps it joins Uranus, Neptune, Jupiter and Saturn in having a ring system of its own, along with a thick atmosphere of hydrogen and helium. Of course, we have to discover Planet Nine first, a process that may take some time if, indeed, it is successful. Meanwhile, we have interesting developments in the Solar System's most intriguing ring system. As compared with those of other planets, Saturn's rings are visually stunning. The B ring is the brightest and most opaque of the planet's rings, but now we're finding out that brightness and opacity have little correlation with...

Newly interpreted data from the New Horizons spacecraft tells us that Pluto has more water ice on its surface than we once thought. The image below tells the tale, a false-color view derived from observations by the Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument. Here we're at infrared wavelengths and can see areas showing the spectral signature of water ice. Note the sharp contrast between the left and right sides of the image below. Image: This false-color image is derived from observations in infrared light by the Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument. It is based on two LEISA scans of Pluto obtained on July 14, 2015, from a range of about 108,000 kilometers. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. The two scans, as this JHU/APL news release explains, were taken about fifteen minutes apart and merged into a 'data cube' -- a three-dimensional array covering the hemisphere New Horizons...

In the past few years, several readers have talked to me about changes to the comment format on Centauri Dreams. In particular, some way of setting up comment 'threads' seemed to make sense, and there are various plugins to make this happen. Thanks to all for their input, and in particular Michael Spencer and Daniel Suggs, the latter of whom suggested I check with Judith Curry, who runs the Climate Etc site. A few tweaks with the aid of Dr. Curry and it was done. The new format became available as of last night and I hope the 'reply' function proves useful. On to the Ninth Planet What stirred me about yesterday's story on a possible ninth planet was the involvement of Caltech's Mike Brown, whose general disbelief in any large outer system planet was known. But as Brown tweeted yesterday, he's now a believer in a nine-planet system (the reference being to Pluto, the planetary status of which was demoted not long after Brown's discovery of Eris). If Brown were involved, this promised...

A new planet ten times the mass of Earth deep in the outer system? That's the word out of Caltech, where Konstantin Batygin and Mike Brown report the evidence from computer modeling and simulations, though no planet has yet been directly observed. The planet would orbit 20 times further from the Sun than Neptune, with an orbital period between 10,000 and 20,000 years. "This would be a real ninth planet," says Brown. "There have only been two true planets discovered since ancient times, and this would be a third. It's a pretty substantial chunk of our solar system that's still out there to be found, which is pretty exciting." Image: This artistic rendering shows the distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Hypothetical lightning lights up the night side. Credit: Caltech/R. Hurt (IPAC). From what we know so far, the planet would explain features in the Kuiper Belt, including the fact that from a list of...