Celestial Spectacle: Planets in Tight Orbits

by Paul Gilster on June 22, 2012

I’ve always had an interest in old travel books. A great part of the pleasure of these journals of exploration lies in their illustrations, sketches or photographs of landscapes well out of the reader’s experience, like Victoria Falls or Ayers Rock or the upper reaches of the Amazon. Maybe someday we’ll have a travel literature for exoplanets, but until that seemingly remote future, we’ll have to use our imagination to supply the visuals, because these are places that in most cases we cannot see and in the few cases when we can, we see them only as faint dots.

None of that slows me down because imagined landscapes can also be awe-inspiring. This morning I’m thinking about what it must be like on the molten surface of the newly discovered world Kepler-36b, a rocky planet 1.5 times the size of Earth and almost 5 times as massive. This is not a place to look for life — certainly not life as we know it — for it orbits its primary every 14 days at a scant 17.5 million kilometers. But if we could see the view from its surface, we would see another planet, a gaseous world, sometimes appearing three times larger than the Moon from the Earth.

That may sound like a view from the satellite of the larger planet, but in this case we have two planets orbiting closer to each other than any planets we’ve found elsewhere. The larger planet, Kepler-36c, is a Neptune-class world about 3.7 times the size of the Earth and 8 times as massive. While the inner world orbits 17.5 million kilometers from its star, Kepler-36c takes up a position a little over 19 million kilometers out, making for a close orbital pass indeed. Conjunctions occur every 97 days on average, at which point no more separates the two worlds than about 5 Earth-Moon distances. Now that would make for quite an image, but rather than just sketching a huge planet in the sky, the University of Washington’s Eric Agol, one of the researchers on this work, decided to show the unknown in terms of the familiar, as below:

Image: Sleepless in Seattle? This view might keep you up for a while. Adapted by Eric Agol of the UW, it depicts the view one might have of a rising Kepler-36c (represented by a NASA image of Neptune) if Seattle (shown in a skyline photograph by Frank Melchior, frankacaba.com) were placed on the surface of Kepler-36b. Credit: Eric Agol/UW.

We can only imagine the kind of gravitational effects the two planets are having on each other. It’s interesting to see as well the powerful uses the researchers, who report their work in Science Express this week, have made of asteroseismology, as noted in this news release from the Harvard-Smithsonian Center for Astrophysics. The sound waves trapped inside Sun-like stars set up oscillations that an instrument like Kepler or CoRoT can measure. Bill Chaplin (University of Birmingham, UK), a co-author of the paper on this work, says this:

“Kepler-36 shows beautiful oscillations. By measuring the oscillations we were able to measure the size, mass and age of the star to exquisite precision. Without asteroseismology, it would not have been possible to place such tight constraints on the properties of the planets.”

The more we know about the parent star, in other words, the better we can interpret the lightcurves we are gathering as we observe planetary transits. The pattern we’re familiar with in our own Solar System — rocky planets closer to the Sun, gas giants in the outer system — breaks dramatically here with two worlds of different compositions and densities in remarkably tight orbits. At Iowa State University, researcher Steve Kawaler was on the team that worked on these data. Kawaler describes the situation in a news release from the university:

“Small, rocky planets should form in the hot part of the solar system, close to their host star – like Mercury, Venus and Earth in our Solar System. Bigger, less dense planets – Jupiter, Uranus – can only form farther away from their host, where it is cool enough for volatile material like water ice, and methane ice to collect. In some cases, these large planets can migrate close in after they form, during the last stages of planet formation, but in so doing they should eject or destroy the low-mass inner planets.

“Here, we have a pair of planets in nearby orbits but with very different densities. How they both got there and survived is a mystery.”

Indeed, we have two planets whose densities differ by a factor of eight in orbits that differ by a mere 10 percent, offering a real challenge to current theories of planet formation and migration. The star, Kepler-36a, is about as massive as the Sun but only about 25 percent as dense, and it has somewhat lower metallicity. Researchers believe it is several billion years older than our star and has entered a sub-giant phase to attain a radius about 60 percent greater than the Sun’s.

We can hope that extreme systems like this can help us refine our thinking on planetary migration and its effects. The planets in this system, some 1200 light years from Earth in the constellation Cygnus, are puzzling but doubtless not alone in representing unusual configurations of the kind we’ll see more of as we continue to sift the Kepler results. The paper is Carter et al., “Kepler-36: A Pair of Planets with Neighboring Orbits and Dissimilar Densities,” published online in Science Express June 21, 2012 (abstract). See also this news release from the University of Washington.

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{ 16 comments }

Ronald June 22, 2012 at 11:36

The univers and above all planets keep amazing us.
The tidal effects on passing each other must be tremendous, many hundreds of times our spring tide.
Is the bigger planet really a Neptune class? I mean at that modest mass and that close orbit? If so, then I would expect the volatiles and liquids to boil off very quickly. Maybe that is the very reason for its modest mass: a remnant planet. Remarkable for such an old star (and hence old planets). Is it possible that these planets migrated inward at a later stage?

Istvan June 22, 2012 at 11:54

Despite the implication in the university press release cited, I’m forced to wonder if the orbits of Kepler-36b and 36c have indeed been observed long enough to ascertain if they are even really stable. I’ve been reading a great deal in recent years about the inward migration of jovian-type worlds, including with regard to models of our own system’s evolution, but also to explain the many “hot-Jupiter” exoplanets observed. Why would the migration of 36c be considered to have halted in this intriguing close proximity to the orbit of 36b?

It seems possible without further information that the Kepler-36 system could be an example in progress of this phenomenon: Kepler-36c is migrating inward and about to gravitationally disrupt the orbit of 36b. We’d probably need to observe Kepler-36 with a device like Kepler or superior to it for thousands or tens of thousands of years further to catch the entire act, unfortunately.

Another alternative – are Kepler-36b and c instead passing close enough to “trade orbits” as do some of the tiny ring shepherds orbiting Saturn, approaching, but not passing? Could our orbital data from transits even reveal that, or would it have to be deduced based on masses, orbital planes, and proximity at closest mutual approach?

A fascinating situation, no doubt at all!

Erik Anderson June 22, 2012 at 12:13

Paul wrote: “Maybe someday we’ll have a travel literature for exoplanets…” It’s coming sooner than you think. – E

Interstellar Bill June 22, 2012 at 14:33

Earth would be tidally locked to something as big and close as the picture shows, but residual tides would still be enormous, with constant large earthquakes and tsunamis, as well as supervolcanoes. That city would be impossible on such a planet, and probably humanity as well.

FrankH June 22, 2012 at 15:18

Very impressive work. Here’s the full article from Science:
“Kepler-36: A Pair of Planets with Neighboring Orbits and Dissimilar Densities”
https://www.sciencemag.org/content/early/2012/06/20/science.1223269.full

Perry old system, too:

“The frequencies of the oscillations indicate that the star has a density (25 ± 2)% that of the Sun (15, 16). Analysis of high-resolution spectra of this star, subject to this density constraint, yields precise values for the stellar effective temperature and metallicity. The star is slightly hotter and less metal-rich than the Sun. This information combined with additional asteroseismic constraints (16) gives precise measures of the stellar mass and radius (Table 1). Based on these parameters (16), Kepler-36 is a “subgiant” star, 2-3 billion years older than the Sun. ”

The planetary masses and radii are very well constrained as well.

ljk June 22, 2012 at 16:49

The problem with having a giant planet over Seattle is not the potential negative effects on Earth from such a nearby mass, but the fact that one would hardly ever get to see such a sight from that city due to the weather. :^)

And speaking of the Space Needle, that is what all the buildings in all the cities were supposed to look like by now, sigh. At least they still kept the monorail that was supposed to wisk us through the city and across the nation.

http://www.squidoo.com/seattle-worlds-fair

andy June 22, 2012 at 17:11

Regarding the question of stability of the system, from the discovery paper:

We investigated whether these close encounters are consistent with dynamical stability by scrutinizing a random sample of allowed model parameters (16). Direct numerical integration (16) showed that more than 91% of this sample avoided disruptive encounters over ∼0.7 million years. From this surviving population, 100 parameter sets were drawn randomly and numerically integrated for 140 million years; none experienced disruptive encounters.

See also this paper. Long-lived configurations do exist but the system is chaotic due to interaction of the 6:7 resonance with the 29:34 resonance. The Lyapunov timescale is around 10 years, as opposed to our solar system where it is about 5 million years.

Figuring out how this system formed and avoided being captured into other resonances (especially the 2:1 resonance which seems to be a particularly easy one for planets to get caught into) is going to be a challenge!

torque_xtr June 22, 2012 at 17:35

It looks like there is a common mechanism for entering and retaining such blade-running configurations. But this one looks especially incredible, in addition to mutual gravitational perturbations there should be great tidal dissipation of orbital kinetic energy, which would alter orbital parameters, and there is the increasing mass loss of the primary, which is thought to be able to destabilise our own Solar System at the Sun’s red giant stage. And still Kepler-36 system is there, billions of years old!

Rob Henry June 22, 2012 at 21:22

Paul said “We can only imagine the kind of gravitational effects the two planets are having on each other.”, but, fear not! would be writers of SF, I shall come to the rescue.

Tidal forces scale by cube of mass and inverse cube of distance. Visual diameters scale to these factors in exactly the same fashion, so the tidal effect ends up being proportional to the cube of visual diameter and proportional to density. Thus if Kepler-36c has 3x our moons visual diameter and one quarter its density at apogee to their home world of Kepler-36b, then it will have seven times the maximum tidal effect with which we all familiar.

Paul, I also accuse you of being a dreadful tease, by giving us a wonderful image of a possible vista from Kepler 36b, then telling us that it is impossible also. I fell obliged to remedy that also.

Given all this talk of migration, we might start off by speculating that Kepler-36b might have begun Earth-like then migrated and become tidal locked to its sun in a way that (somehow) its dark side retained some life sustaining characteristics. On Earth, plate tectonics caused vast (and often nearly encircled) patches of surface dominated by granite or basalt. The large density difference between these is responsible for two thirds the Earths surface being several kilometres below the rest. If half Kepler-36b become baked to dehydration – tectonic activity would to confined to the night side, and it is just possible that the effect will become more extreme, and an encircled sea bed will form that is tens of km deep.

Now Kepler-36b should have a surface gravity of 22m/s/s, and could be colder than 100K on its night side, but here we want just enough light to penetrate around the fringes of its nightside’s former seabed to have a reasonable portion of it above freezing, so lets make it 270K. This puts the scale height of an Earth like atmosphere at under 4km. So 99% of the atmosphere would be retained in a basin 20km deep.

Could this be enough if the nightside ocean was slowly evaporating water, then this was rapidly hydrolysed to oxygen that was then slowly ablated from the day side?

If this is not sufficient, what if 36-b was a diamond planet. Could it then be possible for the tensile strength of the crust to be such that a large volatiles filled basin could be retained with at least 99.99% of the atmosphere. It would just need to be in the order of 50km deep.

Rob Henry June 22, 2012 at 23:47

Further b.o.t.e calculations, put the dayside temperature of Kepler at less than 1000K for all but the lowest albedo’s and at that temperature oxygen and nitrogen may be retained permanently against thermal loss. We would need to know the exact atmospheric structure around the exobase (and put it way above 1000K), or have a more detailed knowledge of non-thermal loss mechanisms, if we were to rule out an atmosphere in this deep gravitational well. These are poorly understood even for the known planets of Sol.

I believe this is already enough to allow a depression holding an Earth-like night side environment around Kepler-36b. I think that this world is back in business as a Hard Science Fiction setting.

Daniel Suggs June 24, 2012 at 14:19

Paul, shouldn’t there be a ‘million’ somewhere in this sentence?
“While the inner world orbits 17.5 kilometers from its star”
Just wondering.

Paul Gilster June 24, 2012 at 17:00

Wow, I sure missed that. Thanks, Daniel. I fixed it in the post.

Ronald June 25, 2012 at 7:10

Interstellar Bill: “Earth would be tidally locked to something as big and close as the picture shows”.
Well, no, because, as I understood it, this proximity is not the continuous situation, but the closest approach of the two planets while passing each other (conjunction). But you are right that, each time this happens (97 days), tidal forces must be enormous.

lorq June 25, 2012 at 8:04

Seems to me that by the current definition, neither of these is a planet, since neither has cleared its immediate orbital neighborhood of other significant objects.

magne June 25, 2012 at 19:59

@lorq An response to poor Pluto’s fate http://sortingoutscience.net/wp-content/uploads/2007/06/poor-pluto.jpg :o)

Don’t think it apply to this setting, actually two of Saturn’s moons have an even weirder orbit http://en.wikipedia.org/wiki/Epimetheus_(moon) they change orbits like skaters in competitions.

andy June 26, 2012 at 12:45

The pedantic semantics complaint about the current IAU definition of “planet” is as tedious now as it was when Pluto was reclassified in 2006.

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