One More Epicycle

Couple interesting arXiv papers today: I’ll only talk about two of them (many of the others are related to the themes of exoplanet observations from yesterday; the one by McCullough on modelling glint from alien oceans looks kind of neat.)

One of the cool papers is by J. Harrington, B. Hansen, S. Luszcz, S. Seager, D. Deming, K. Menou, J. Cho, and L. J. Richardson. I list all of their names because the next-to-last name on the paper, James Cho, is not only another Queen Mary researcher, but the two of us share a Thursday afternoon engineering math tutorial! He’s lots of fun, very friendly, and since he’s from North America — Maryland, originally, I think? — hearing him speak is comfortably familiar.

The paper’s worth a quick read — it was for Science, so it’s pretty short. It descibes how using 24-micron Spitzer observations of upsilon Andromedae b (which is the starhugger in the three-planet system) they were able to spot the periodic modulations in the system’s flux and see the day/night temperature variation of the planet. Enormously hot on one side (1600 degrees) and cold on the other (-100, say).

This work got a fair bit of notice in the popular press last week; here’s a quick writeup from the Times.

The other paper I liked relates to a long-standing quirky interest of mine: what various constraints from the solar system can tell us about fundamental physics. My favourite example comes from a cute four-page paper by Whitmire & Matese a few years ago (which can be found here for those who don’t have a subscription to Icarus) on the anomalous Pioneer acceleration, which showed by some simple calculations that if there were some external isotropic force which behaved like some of the crazier new-physics explanations for the acceleration then the resulting distribution of comets would be incompatible with observations.

I gave a talk on the acceleration back when I was a PhD student.  By the end of my readings, I had decided that since at the time the two most careful papers which listed the forty or so main possible systematic errors disagreed with each other on the size of the effects — with disagreement on the order of the acceleration detection itself — that it was almost certainly not a hint of anything new and exciting.

Today’s paper by Erickcek et al. — also a short four-pager! — entitled “Solar System tests DO rule out 1/R gravity” deals with similar subjects.

A while back, Chiba wrote a paper showing that you can’t simply add a 1/R term to the Einstein-Hilbert action to explain the acceleration of the universe (note that this is a different observation than the Pioneer acceleration, although as you might expect many people have tried to come up with unifying models to tie the two together.) Chiba demonstrated that the solar system constraints rule it out.

However, this result got some static: Erickcek and company list five papers arguing that no, in fact the 1/R theory was indistinguishable from GR on solar system scales.

Erickcek et al. patiently explain why Chiba was right in the first place, and give a nice analogy from basic second-year electrostatics which shows where the counterarguments went wrong; they then work through some straightforward computations to confirm. An enjoyable read.

My arXiv reading has been pretty spotty for the past few months (yeah, let’s go with months) during the half-mad, half-leisurely dash to finish my thesis. The arXiv is an amazing resource for preprints in a lot of mathematical sciences, and I should really be checking it more frequently.

Just the other day I found out about an important upcoming paper by Caroline Terquem and John Papaloizou on the formation of hot Neptune systems, which is one of the things that Nelson and I are planning to look at with my parallel planetary N-body code miranda. I’ve been working on initial conditions for the runs for the last week or so.

So to help discipline myself, I figure that semi-intermittently, I’ll take some time in the morning and skim through the papers looking for ones on subjects of interest to those of us in planetary formation and dynamics. And at the moment the boss is off in Cardiff refereeing an enormous grant proposal, so now’s as good a time as any!

Only two planetary papers of interest today, and neither of them are really in my area:

CoRoT and the search for exoplanets. The Italian contribution by Poretti et al.

As you might guess from the title, this one’s about, er, what the Italians are doing vis-a-vis the upcoming satellite CoRoT (COnvection, ROtation, and planetary Transits — which as astronomical acronyms goes isn’t actually that bad). They explain that the Italian Space Agency hadn’t officially supported the project, but some of the authors (e.g. Poretti) decided that it was worth continuing to work on it even without an imprimatur. There seems to be a subtle “see? we should’ve signed on!” suggestion in the background..

They’re leveraging the high-precision photometry from the astroseismology for the exoplanet search: if I’m understanding them right they’re hoping to detect not via the usual Doppler tricks but chiefly through transits. They quote probable detection numbers of ~25 1.6 Earth radii bodies at 0.3 AU; ~40 x2.0 at 0.3 AU; “a few” 2 Earth radii bodies in the habitable zone; and hundreds of hot Jupiter-to-Uranus planets. Those numbers may seem low — after all, they’re looking at 60000 targets — but they could just as easily be too optimistic.

(Personally, I think that predictions for where bodies of sizes below our current detection limits — such as Earth-mass objects — will be are little more than wild guesses. Every sane, sensible expectation would’ve ruled out almost all of the planetary systems we’ve seen so far. Given the orbits of the dominant bodies, I can probably say via integrations where terrestrial objects are unlikely to be, but that’s about it.)

They give some details of their expectations for the data and their data modelling process, and the challenges that the confusion due to eclipsing binaries will pose. And because no Canadian can talk about observational astroseismology and exoplanets without mentioning you-know-what, here’s a link to Canada’s own entry in the field, MOST.

The Response of Atmospheric Chemistry on Earthlike Planets around F, G and K Stars to Small Variations in Orbital Distance by Grenfell et al.

This one’s a little dense for a morning read, but the abstract is interesting. It asks the question “How would the atmospheric chemistry of the Earth respond if we moved it to different orbital distances or changed its host star?”

They use (what seems to me, who doesn’t do atmospherics) quite a complicated numerical model to look at the problem, with an emphasis on biomarkers (chemical signals which correlate strongly with life). They take an Earthlike planet and move it around the habitable zone (+/- 5~10%), finding that the chemistry could change significantly even with such slight changes in planet semimajor axis. For example, ozone levels increased slightly with greater orbital radius; water decreased by an order of magnitude; methane increased by a factor of 20; that sort of thing.

This isn’t necessarily evidence in favour of a Rare Earth hypothesis [1], though, as they explain that one of the major assumptions of their model is that “biogenic output” (like methane, chloromethyl, and nitrous oxide) stays at Earth’s values even while they move the planet. It’s possible that the planet’s species would have developed to provide a feedback mechanism. There’s no real way to predict this, and they say (kind of wistfully I think) that it’d be useful to couple a vegetation model to their atmospheric model as a step towards solving the whole problem.

(Right now, some grad students somewhere are working on alien vegetation models. Good luck, kids!)

One of their interesting bottom-line results is that despite the strong variation in biomarker signal they predict, they’re not sure if the Darwin/TPF projects would be able to see it anyway.

We’re still in the very early stages of this game; it’s going to be fiendishly difficult.

[1] For the record, I accept the hypothesis. I expect that complex, intelligent life is probably very rare in the universe.