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 theClimate 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 to be pretty solid evidence, even if we didn’t yet have a planet to look at through our telescopes.

OK, OK, I am now willing to admit: I DO believe that the solar system has nine planets.

— Mike Brown (@plutokiller) January 20, 2016

The image below, taken from the new Search for Planet Nine site, helps make sense of the evidence that leads us to a putative new planet. Back in early 2015, we looked at a paper by Chadwick Trujillo and Scott Sheppard (Carnegie Institution for Science, Washington) that made the case that Sedna and other ‘extreme trans-Neptunian objects’ (ETNOs) could signal the presence of not only a large number of similar objects, but a much larger planet. See A Dwarf Planet Beyond Sedna (and Its Implications).

Mike Brown and Konstantin Batygin used the Trujillo/Sheppard paper, and the discovery of 2012 VP113, also on a Sedna-like orbit, as an inducement to push further into the idea of an outer system planet. This is what Batygin says in an entry on the site:

Prompted by their discovery of 2012 VP113, a second object residing on a Sedna-type orbit, Trujillo and Sheppard pointed out that all Kuiper belt objects with orbits that do not veer into inter-planetary space and spend longer than approximately 2000 years to complete a single revolution around the Sun, tend to cluster in the argument of perihelion. As it turns out, this clustering represents only a part of the full picture. A closer look at the data shows that six objects that occupy the most expansive orbits in the Kuiper belt (including Sedna and 2012 VP113) trace out elliptical paths that point into approximately the same direction in physical space, and lie in approximately the same plane.

Image credit: Mike Brown/Konstantin Batygin/Caltech.

What’s immediately striking here is that Batygin and Brown could use perturbation theory to see what should happen given the gravitational influence of Jupiter, Saturn, Uranus and Neptune. The orbits in question should become randomly oriented over a timeframe much shorter than the age of the Solar System, which means that we can’t harken back to something that happened billions of years ago. Something must be holding these orbits together now.

Looking back at data from Trujillo and Sheppard, Batygin and Brown could, as Batygin says, see that the long axes of the orbits traced out by these distant objects tended to point in the same direction, providing further evidence of something larger influencing these objects. But add a large planet into this scenario and we should see a set of objects whose orbits are sharply tilted when compared to the plane of planetary orbits. And in fact we do know of six objects that behave exactly like this.

What we need to do now is to find the possible planet and take its picture, because all we have is the inference of a planet based upon orbital anomalies in a small number of outer system objects. But the model that the two researchers have developed combines a number of interesting points about the Kuiper Belt as we know it. Batygin adds:

In the end, our model ties together three elusive aspects of the Kuiper belt (namely, physical alignment of the distant orbits, generation of detached objects such as Sedna and the existence of a population tracing our perpendicular orbital trajectories) into a single, unifying picture. As a dynamical model, this appears compelling. But it is simultaneously important to keep in mind that until Planet Nine is caught on camera, it remains a theoretical prediction.

Until we actually see a new planet, both Batygin and Brown will probably continue to experience what the former calls “an uneasy exhilaration.” Meanwhile, the idea that there may be a planetary discovery of the kind Percival Lowell was looking for — a large world deep in the outer system — awakens an almost atavistic enthusiasm. I’ve always been open to the idea that there must be undiscovered planets in our system, but something ten times the mass of the Earth seemed out of the question. Now we have to find out if, as in the case of Neptune, precise mathematical calculations can indeed lead us to an object we can see.

The Brown and Batygin paper is “Evidence for a Distant Giant Planet in the Solar System,” Astronomical Journal, published online 20 January 2016 (full text). Trujillo & Sheppard’s paper is “A Sedna-like body with a perihelion of 80 astronomical units,” Nature 507 (27 March 2014), pp. 471–474 (abstract).

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 thirteen of the most distant objects in the Belt, six of them follow elliptical orbits that point in the same direction in physical space, as this Caltech news release explains. Says Brown:

“It’s almost like having six hands on a clock all moving at different rates, and when you happen to look up, they’re all in exactly the same place,” says Brown. The odds of having that happen are something like 1 in 100, he says. But on top of that, the orbits of the six objects are also all tilted in the same way—pointing about 30 degrees downward in the same direction relative to the plane of the eight known planets. The probability of that happening is about 0.007 percent. “Basically it shouldn’t happen randomly,” Brown says. “So we thought something else must be shaping these orbits.”

A Kuiper Belt with 100 times the mass it has today could explain the phenomenon, but that’s obviously out. Simulations involving a massive planet in an anti-aligned orbit seemed to work, however. By anti-alignment, the researchers mean an orbit in which the planet’s perihelion is 180 degrees across from the perihelion of all other objects and known planets. Mean-motion resonance could keep Kuiper Belt objects from colliding with the planet and maintain the necessary alignment, with the new planet nudging KBOs to maintain the configuration. Says Batygin: “I had never seen anything like this in celestial mechanics.”

Image: A predicted consequence of Planet Nine is that a second set of confined objects should also exist. These objects are forced into positions at right angles to Planet Nine and into orbits that are perpendicular to the plane of the solar system. Five known objects (blue) fit this prediction precisely. Credit: Caltech/R. Hurt (IPAC) [Diagram was created using WorldWide Telescope.

Brown and Batygin are continuing to refine their simulations to learn more about the planet’s orbit and gravitational effects, while at the same time searching the sky for it. Remember that the orbit is only approximately known. It may well show up in images taken through previous surveys, though if in the most distant part of its orbit, large telescopes like Keck or the Subaru instrument on Mauna Kea may be needed to see it. “I would love to find it,” says Brown. “But I’d also be perfectly happy if someone else found it. That is why we’re publishing this paper. We hope that other people are going to get inspired and start searching.”

The paper, titled “Evidence for a Distant Giant Planet in the Solar System,” appears in the Astronomical Journal, published online 20 January 2016 (full text). Needless to say, more on this tomorrow.

Knowing that the data from New Horizons continues to arrive gives me a warm feeling about the months ahead. Below we have the highest resolution color image of one of the two potential cryovolcanoes found on the surface during the Pluto flyby last summer. This is Wright Mons, some 150 kilometers across and 4 kilometers high. If this is indeed a volcano, none has been discovered in the outer system that can compare with it in size.

Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

The image is a composite drawn from New Horizons’ Long Range Reconnaissance Imager (LORRI) on July 14, 2015. The range is approximately 48,000 kilometers, giving us features down to 450 meters across. JHU/APL has also incorporated color data from the Ralph/Multispectral Visible Imaging Camera (MVIC) taken about 20 minutes after the LORRI images were taken, from a range of 34,000 kilometers, and with a resolution of 650 meters per pixel. The scene on the right is 230 kilometers across.

The question most directly raised by the image is the nature of the sparse red material — why is it where it is and why is it not more widespread? You can also see that this must be a relatively young surface, to judge from the fact that there is only one clear impact crater on Wright Mons. This JHU/APL news releasespeculates that the young surface is an indication that Wright Mons was active relatively late in Pluto’s history.

So hard to believe it’s been ten years since launch…

10 years ago @NewHorizons2015 had raced past the moon’s orbit on its way to Pluto. https://t.co/PgnWuLQfkx#OTDpic.twitter.com/ii7BPxOqtO

— Corey S. Powell (@coreyspowell) January 20, 2016

In 2006, just before the launch, David Grinspoon wrote this in the Los Angeles Times:

Theory alone cannot teach us what there is to know. The universe is stranger and more varied than we can predict or calculate, and that is why we explore. We can’t know exactly what New Horizons will find. But we can safely predict that when we get to Pluto — and the whole new class of worlds it represents — what we discover will baffle, surprise, delight and enlighten.

Pluto As We Used to See It

If you think back to what we knew about Pluto when we could only see it from Earth orbit, it’s heartening to see how much we surmised. As Amanda Zangari (SwRI, and a member of the New Horizons’ Geology, Geophysics and Imaging Team) mentions in a recent blog post, we knew that the surface was covered with nitrogen and methane, and also that tholins were common (Zangari calls them the ‘brown gunk made when UV light hits nitrogen and methane’). And the carbon monoxide patch we saw where Pluto was brightest turned out to be Sputnik Planum, the area forming the left side of Pluto’s now famous ‘heart.’

The New Horizons mission has been an outstanding success, but there is a slight frustration in Zangari’s post having to do with the fact that it was a flyby:

I like to think of the whole experience as getting to look at the answer in the back of the book. To extend the back-of -the-book analogy, like many textbooks, we are only getting half the answer. With Pluto, that missing element isn’t the even problems, but time. Our greatest images are from the July 14 closest encounter, and thus we only have one glimpse of one side of Pluto on one day. Yet these images show Pluto as an active world, undergoing volatile-ice transport and change.

True enough — who wouldn’t want to see an orbiter in this fascinating system? — but have a look at the progress we’ve made. Here I’m directly poaching from Zangari’s post to show the best maps we had before the flyby, made from Hubble Space Telescope images in 2002 and 2003.

Image credit: NASA/JHUAPL/SwRI/Marc Buie.

And here we are from New Horizons:

Image credit: NASA/JHUAPL/SwRI.

Zangari tells us that her next project involves comparing measurements of Pluto’s brightness as seen by New Horizons from 2013 to 2015 and comparing these to Hubble Space Telescope observations from 2002 to 2003. The goal: To see how Pluto’s surface has changed over time, a good thing to consider given how surprisingly active the Pluto/Charon system has turned out to be. Sad to think that watching Pluto’s continuing evolution now involves Earth-based telescopes alone. Someday there will be a Pluto orbiter, but none of us can know when.