Centauri Dreams
Imagining and Planning Interstellar Exploration
Planet Nine: “An Uneasy Exhilaration”
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 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).
Evidence for 9th Planet Unveiled
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.
Viewing Pluto Over Time
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 release speculates 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 #OTD pic.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.
Is Proxima Centauri a Bound Star?
About 1.4 million years from now, the K-class star Gliese 710, now 64 light years distant in the constellation Serpens, will brush past our Solar System. Moving to within 50,000 AU, the star could be expected to have an unsettling effect on cometary orbits in the Oort Cloud, perhaps dislodging some of these comets to cause them to move into our inner planetary system. An interesting scenario, particularly remembering speculation that comets were a source of water for the early Earth, and may perform a similar function in other young systems.
So just how common are such celestial encounters? We may have one at our very doorstep in the form of Proxima Centauri. The Pale Red Dot campaign that began yesterday is focusing on a red dwarf that is roughly 15,000 AU from the close binary stars Centauri A and B. If you think about what our system would look like with a red dwarf at the inner edge of the Oort Cloud, you can see that Proxima may play a large role in the evolution of the triple-star system.
Image: Proxima Centauri, indicated by the arrow, in an image from the first night’s run at the Pale Red Dot observing campaign at La Silla. Credit: ESO/Guillem Anglada-Escudé.
Assuming, that is, that this is a bound system. It was back in 1993 that Robert Matthews and Gerard Gilmore (Cambridge University) studied kinematic data on Proxima and concluded that the star was not necessarily gravitationally bound. In fact, it was on the borderline, and could well be a star that was simply moving past Centauri A and B, having been formed elsewhere. The researchers cited the need for further kinematic data to resolve the matter.
The following year, J. Anosova (Physical Research Laboratory, Ahmedabad, India) proposed that the three Centauri stars were part of a stellar moving group, with Proxima drifting close but not necessarily bound. But the problem would not go away. Proxima’s small relative velocity in relation to Centauri A and B (0.53 ± 0.14 km s-1) makes the likelihood that it is unbound quite small, which is why the assumption of a bound triple system has generally been the rule since the star’s discovery in 1915.
Jeremy Wertheimer and Gregory Laughlin (UC-Santa Cruz) have had the most recent word, finding in 2006 that Proxima is indeed bound to Centauri A and B:
The availability of Hipparcos data has provided us with the ability to implement a significant improvement over previous studies of the α Cen system. Our results indicate that it is quite likely that Proxima Cen is gravitationally bound to the α Cen A-B pair, thus suggesting that they formed together within the same birth aggregate and that the three stars have the same ages and metallicities. As future observations bring increased accuracy to the kinematic measurements, it will likely become more obvious that Proxima Cen is bound to the α Cen A-B binary and that Proxima Cen is currently near the apastron of an eccentric orbit…
The researchers are also able to make a prediction: When we can get a more refined look at Proxima Centauri’s absolute radial velocity, we should see a value of -22.3 km s-1 < vr < -22.0 km s-1. Hence the importance of our continuing investigation of this small star. As we proceed with projects like Pale Red Dot and beyond, we should be able to eventually declare the question of Proxima’s bound status closed.
Ramifications of a Bound Proxima
This is of more than passing interest, because Proxima Centauri could well be influencing the planetary systems (if any) of the two primary Centauri stars. As I mentioned above, a star like Gl 710 passing by our system could cause disruptions in the Oort Cloud, and a passing Proxima Centauri could certainly serve the same function in that system. But a bound Proxima — one that is, as Wertheimer and Laughlin believe, currently located near the apastron position of its orbit, could have useful effects on an infant system. As the paper notes:
If Proxima were bound to the system during its formation stages, then it may have gravitationally stirred the circumbinary planetesimal disk of the α Cen system, thereby increasing the delivery of volatile-rich material to the dry inner regions.
A significant factor indeed as we ponder the question of astrobiology around these stars. Laughlin has written elsewhere about the fact that Alpha Centauri planets will likely be dry because of the close orbit of Centauri A and B — the period of the AB binary pair is 79 years, and the stars make a close approach of 11.2 AU. Silicates and metals condense out of the protoplanetary disks around Centauri A and B, but to get water, we need to go further out, into the circumbinary disk surrounding both stars. A bound Proxima Centauri, then, becomes the mechanism for driving water-laden materials inward to any dry, terrestrial-class planets.
And there is another factor that makes the question of Proxima’s relation to Centauri A and B significant. If Proxima is bound, the implication is that all three stars were formed out of the same molecular cloud, and that would imply not just the same age but the same metallicity (metals being elements higher than hydrogen and helium). We know that both Centauri A and B are richer in metals than our own Sun, and if Proxima is likewise metal-rich, it may have an elevated chance of having planets. The link between gas-giant formation and metals has been well explored, though such a link for rocky worlds is not established.
What all this comes down to is that a gravitationally bound Proxima Centauri would exist as the third member of a system that should be rich in the materials needed to form terrestrial planets. The problem now is to find them using searches like the ongoing effort at La Silla, where the Pale Red Dot campaign will be in progress until April. As mentioned yesterday, you can follow the effort on Twitter: @Pale_red_dot, and use the hashtag #PaleRedDot. Conditions were problematic last night at the observatory but it looks like the first observations were made.
The paper by Matthews and Gilmore is “Is Proxima really in orbit about Alpha CEN A/B?,” Monthly Notices of the Royal Astronomical Society Vol. 261, No. 2 (1993), p. L5-L7 (abstract). The Anosova paper is “Dynamics of nearby multiple stars. The α Centauri system,” Astronomy & Astrophysics 292 (1994), 115-118 (full text). The Wertheimer and Laughlin paper is “Are Proxima and Alpha Centauri Gravitationally Bound?” The Astronomical Journal 132:1995-1997 (2006), available online.
Pale Red Dot: Proxima Centauri Campaign Begins
A new observational campaign for Proxima Centauri, coordinated by Guillem Anglada-Escudé (Queen Mary University, London), is about to begin, an effort operating under the name Pale Red Dot. You’ll recall Dr. Anglada-Escudé’s name from his essay Doppler Worlds and M-Dwarf Planets, which ran here in the spring of last year, as well as from Centauri Dreams reports on his work on Gliese 667C, among other exoplanet projects. Pale Red Dot is a unique undertaking that brings the public into an ongoing campaign from the outset, one whose observations at the European Southern Observatory’s La Silla Observatory begin today.
The closest star to our Sun, Proxima Centauri was discovered just over 100 years ago by the Scottish astronomer Robert Innes. A search through the archives here will reveal numerous articles about the red dwarf and the previous attempts to find planets orbiting it. I’ll point you to a round-up of exoplanet work on Proxima thus far next week, when my essay ‘Intensifying the Proxima Centauri Planet Hunt’ will run as part of Pale Red Dot’s outreach campaign. For now, the short summary is that we can rule out various planet scenarios around Proxima, but the possibility of a rocky world in the habitable zone is definitely still in the mix.
Image: First images of Proxima from the Las Cumbres Observatory Global Telescope Network at Cerro Tololo Inter-American Observatory, Chile.
Pale Red Dot today begins a two and a half month campaign that will run through April using the HARPS spectrograph at ESO’s 3.6-meter telescope at La Silla (Chile). The method here is radial velocity, looking for those infinitesimal Doppler signals showing the star’s motion as affected by planetary companions. These RV studies thus add to the earlier work done by Michael Endl (UT-Austin) and Martin Kürster (Max-Planck-Institut für Astronomie), who studied Proxima using the UVES spectrograph at the Very Large Telescope in Paranal.
But Pale Red Dot’s HARPS data will be complemented by robotic telescopes around the globe, including the Burst Optical Observer and Transient Exploring System (BOOTES) and the Las Cumbres Observatory Global Telescope Network (LCOGT). These automated installations will measure the brightness of Proxima each night during the observing campaign, helping to clarify whether any RV ‘wobbles’ of the star are caused by a planet or by events on the stellar surface. After thorough data analysis, the results will be submitted to a peer-reviewed journal.
The public outreach aspect of Pale Red Dot is compelling. Anglada-Escudé explains:
“We are taking a risk to involve the public before we even know what the observations will be telling us — we cannot analyse the data and draw conclusions in real time. Once we publish the paper summarising the findings it’s entirely possible that we will have to say that we have not been able to find evidence for the presence of an Earth-like exoplanet around Proxima Centauri. But the fact that we can search for such small objects with such extreme precision is simply mind-boggling.
“We want to share the excitement of the search with people and show them how science works behind the scenes, the trial and error process and the continued efforts that are necessary for the discoveries that people normally hear about in the news. By doing so, we hope to encourage more people towards STEM subjects and science in general.”
To communicate the process, Pale Red Dot will use blog posts and social media, with essays from astronomers, scientists, engineers and science writers on the site’s blog. To keep up with the social media updates, use @Pale_red_dot on Twitter for the project account, and you’ll probably want to check the hashtag #PaleRedDot as well. I also want to mention (and this will be in my article next week) that David Kipping’s transit studies of Proxima using the Canadian MOST (Microvariability & Oscillations of STars) space telescope are continuing, and beyond this we have another gravitational microlensing possibility as the star occults a 19.5-magnitude background star this February. Proxima Centauri, 2016 looks to be your year, and perhaps it’s also the year we find out for sure whether there is a planetary system so close to our own.
KIC 8462852: A Century Long Fade?
I hadn’t expected a new paper on KIC 8462852 quite this fast, but hard on the heels of yesterday’s article on the star comes “KIC 8462852 Faded at an Average Rate of 0.165±0.013 Magnitudes Per Century From 1890 To 1989,” from Bradley Schaefer (Louisiana State University). Schaefer takes a hard look at this F3 main sequence star in the original Kepler field not only via the Kepler data but by using a collection of roughly 500,000 sky photographs in the archives of Harvard College Observatory, covering the period from 1890 to 1989.
The Harvard collection is vast, but Schaefer could take advantage of a program called Digital Access to a Sky Century@Harvard (DASCH), which has currently digitized about 15 percent of the archives. Fortunately for us, this 15 percent covers all the plates containing the Cygnus/Lyra starfield, which is what the Kepler instrument focused on. Some 1581 of these plates cover the region of sky where KIC 8462852 is found. What Schaefer discovers is a secular dimming at an average rate of 0.165±0.013 magnitudes per century. For the period in question, ending in the late 1980s, KIC 8462852 has faded by 0.193±0.030 mag. From the paper:
The KIC 8462852 light curve from 1890 to 1989 shows a highly significant secular trend in fading over 100 years, with this being completely unprecedented for any F-type main sequence star. Such stars should be very stable in brightness, with evolution making for changes only on time scales of many millions of years. So the Harvard data alone prove that KIC 8462852 has unique and large-amplitude photometric variations.
That’s useful information, especially given the possible objection to the Kepler findings that they might be traceable to a problem with the Kepler spacecraft itself. Evidently not:
Previously, the only evidence that KIC 8462852 was unusual in any way was a few dips in magnitude as observed by one satellite, so inevitably we have to wonder whether the whole story is just some problem with Kepler. Boyajian et al. (2015) had already made a convincing case that the dips were not caused by any data or analysis artifacts, and their case is strong. Nevertheless, it is comforting to know from two independent sources that KIC 8462852 is displaying unique and inexplicable photometric variations.
As Schaefer notes, KIC 8462852 can now be seen to show two unique episodes involving dimming — the dips described here yesterday for the Kepler spacecraft, and the fading in the Harvard data. The assumption that both come from the same cause is reasonable, as it would be hard to see how the same star could experience two distinct mechanisms that make its starlight dim by amounts like these. The timescales of the dimming obviously vary, and the assumption would be that if the day-long dips are caused by circumstellar dust, then the much longer fading that Schaefer has detected would be caused by the same mechanism.
Image: KIC 8462852 as photographed from Aguadilla, Puerto Rico by Efraín Morales, of the Astronomical Society of the Caribbean (SAC).
Thus we come to the comet hypothesis as a way of explaining the KIC 8462852 light curves. Incorporating the fading Schaefer has discovered, a cometary solution would require some mind-boggling numbers, as derived in the paper. From the summary:
With 36 giant-comets required to make the one 20% Kepler dip, and all of these along one orbit, we would need 648,000 giant-comets to create the century-long fading. For these 200 km diameter giant-comets having a density of 1 gm cm?3, each will have a mass of 4 × 1021 gm, and the total will have a mass of 0.4 M?. This can be compared to the largest known comet in our own Solar System (Comet Hale-Bopp) with a diameter of 60 km. This can also be compared to the entire mass of the Kuiper Belt at around 0.1 M? (Gladman et al. 2001). I do not see how it is possible for something like 648,000 giant-comets to exist around one star, nor to have their orbits orchestrated so as to all pass in front of the star within the last century. So I take this century-long dimming as a strong argument against the comet-family hypothesis to explain the Kepler dips.
If Schaefer’s work holds up, the cometary hypothesis to explain KIC 8462852 is deeply compromised. We seem to be looking at the author calls “an ongoing process with continuous effects” around the star. Moreover, it is a process that requires 104 to 107 times as much dust as would be required for the deepest of the Kepler light dips. And you can see in the quotation above Schaefer’s estimate for the number of giant comets this would require, all of them having to pass in front of the star in the last century.
The paper is Schaefer, “KIC 8462852 Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 To 1989,” submitted to Astrophysical Journal Letters (abstract).
Follow with RSS or E-Mail
