HR 8799b: Low Temperatures, Surprising Spectrum

Photos of the Earth from a significant distance are always fascinating, dating back to the spectacular shot of the rising Earth over lunar mountains taken by Apollo 8 in December of 1968. The image below, showing Earth and its Moon, comes from the Messenger spacecraft, taken at a distance of some 183 million kilometers. I see things like this and think about our future imaging of exoplanets, and the possibilities of space-based missions that can study their atmospheres. Learning how we look helps us understand what to look for around other stars, and also offers a bit of the ‘wow’ factor.

We’re nowhere near this kind of imaging with exoplanets but we’re getting better all the time, and that’s providing some curious results. HR 8799 is the interesting young system some 130 light years from Earth (in Pegasus) that has yielded direct images of three planets. Some eighteen months after the discovery of the system here, we’ve now managed to get a spectrum of HR 8799b, useful for what it can tell us about temperatures, clouds, and chemical composition.

Image: The Earth/Moon system as seen from the Messenger spacecraft. Credit: NASA/JHU-APL.

HR 8799b shows little or no methane in its atmosphere, a fact that used in conjunction with models of low-temperature atmospheres yields an estimate of 1200 K (926 degrees Celsius or 1700 Fahrenheit) as the coolest possible temperature for this young object. Oddly, the planet ought to be some 400 Kelvin cooler than what the new measurements show, extrapolating from the amount of energy the planet emits and its assumed age.

The culprit? Scientists at the University of Hawaii, who made these measurements at the Keck Observatory, think dust in the planet’s atmosphere must be to blame. If you change the computer models of gas giant planets to incorporate thick dust clouds, you wind up with essentially the same result. Thus direct spectroscopy of exoplanets may be telling us that young gas giants are cloudier than we had realized. The results are a reminder, as if we needed one, that exoplanets will continue to surprise us, especially given the fact that direct imaging of these worlds has just begun. Only six planets — three in this system — have been directly imaged. Says Michael Liu, co-author of the paper on this work:

“Direct studies of extrasolar planets are just in their infancy. But even at this early stage, we are learning they are a different beast than objects we have known about previously.”

True enough, and we should consider the advantages that direct imaging puts on the table. For one thing, it gives us the opportunity to study planets in wide orbits — the planets around HR 8799, discovered by Christian Marois (Herzberg Institute of Astrophysics) and team in 2008, orbit the star with semimajor axes of 24 AU, 38 AU and 68 AU respectively, with the interesting possibility of a 1:2:4 orbital resonance (not yet confirmed). At 68 AU, HR 8799b is too far from its star for radial velocity detection, but direct imaging gives us both planet and spectrum.

Image: Keck II image of the young extrasolar planet HR 8799b, seen as the point source in center of image. The bright light from the parent star HR 8799 is seen in the background in yellow/red and has been removed in an annular region centered on the planet. Credit: Brendan Bowler and Michael Liu, IfA/Hawaii.

But are the HR 8799 objects planets or brown dwarfs? At least one recent paper has suggested the latter, but this work advocates a younger age for the system and sees the objects as hot, young planets. Even so, explicit comparisons with brown dwarfs are useful, and the authors note that these objects are massive analogs of giant planets, objects that offer up similar physics, radii, effective temperatures and cooling histories. The comparison is intriguing, and allows the researchers to peg HR 8799b as a unique object:

Altogether, our spectral and photometric comparisons to field brown dwarfs suggest a spectral type between L5 and T2 for HR 8799 b. Although peculiar compared to most L and T dwarfs in the field, the planet’s photometry is consistent with the reddest field L dwarfs. These results imply that HR 8799 b is the lowest-mass L/T transition object currently known.

And later, the paper refers to the planet this way:

These observations, combined with spectroscopy of HR 8799 c and d, will elucidate the physical properties of this emerging class of low-mass objects which are characterized by low surface gravities, low luminosities, and exceptionally cloudy atmospheres.

‘An emerging class of low-mass objects’ is a phrase that reminds us how much we’ve learned in the past decade about small, cool stars and planets that skirt the boundary between planet and star. The paper is Bowler et al., “Near-Infrared Spectroscopy of the Extrasolar Planet HR 8799b,” accepted by The Astrophysical Journal (preprint).


Poul Anderson’s Answer to Fermi

Enrico Fermi’s paradox has occupied us more than occasionally in these pages, and for good reason. ‘Where are they,’ asked Fermi, acknowledging an obvious fact: Even if it takes one or two million years for a civilization to develop and use interstellar travel, that is but a blip in terms of the 13.7 billion year age of the universe. Von Neumann probes designed to study other stellar systems and reproduce, moving outward in an ever expanding wave of exploration, could easily have spread across the galaxy long before our ancestors thought of building the pyramids.

Where are they indeed. Kelvin Long, one of Project Icarus’ most energetic proponents, recently sent me Poul Anderson’s thoughts on the subject. I probably don’t need to tell this audience that Anderson was a science fiction author extraordinaire. His books and short stories occupied vast stretches of my youth, and I still maintain that if you want to get not so much the tech and science but the sheer wonder of the interstellar idea, you can tap it in its pure form in his writing. Poul was also the author of Tau Zero, the novel which gave our Tau Zero Foundation its name, and we’re delighted to have Karen Anderson, Poul’s wife, as a valuable part of the organization.

In a letter to the Journal of the British Interplanetary Society in 1986, Anderson sketched the reasons why Fermi was asking his question, citing the von Neumann probes mentioned above, and noting that while interstellar travel was likely hard enough that civilizations practicing it might be rare, all it takes is one to eventually fill the galaxy with its artifacts. He found the notion that Fermi could be answered by saying we are the only high-technology civilization unlikely, but his reason for writing was to offer an entirely different suggestion based on practicality.

Let’s assume a stable civilization arises that achieves extremely long lifespans, if not physical immortality — this may be too big an assumption, but there are those arguing that our successors may be a form of artificial intelligence for whom this could apply. Such a civilization naturally would explore its neighborhood, moving out to local star systems and gradually spreading beyond. Anderson saw this as a problem: The farther from home you go, the longer it takes you to return information. The galaxy itself is 100,000 light years wide, he noted, and that means most information would be utterly outdated by the time it spread throughout the disk.

And what of this self-replication idea? Anderson saw problems there too:

…self-replication would probably already have broken down. Quantum mechanics alone guarantees gradual degradation of the programmes, an accumulation of ‘mutations’ generation by generation — without any natural selections to winnow out the unfavourable majority — until ultimately every machine is useless and every line of its descent extinct.

Can we conjecture a kind of self-healing technology that extends to fixing these errors to maintain the integrity of the expansion? Perhaps, but the data flaw remains paramount:

…long before this has happened, the sphere of exploration will include so many stars that the data flow from them saturates the processing capacity of the present civilisation. After all, with some 1012 stars in the Galaxy, a small fraction amounts to a huge number. Moreover, while they may fall into categories with predictable properties, we are learning in our own back yard that every planet any of them may have is a world, replete with mysteries and surprises. Every life-bearing planet offers endless matter for research, especially since the life will always be changing, evolving.

In short, Fermi’s ‘they’ are not here because they are kept too busy within a few score light-years of their various homes.

If Anderson is right, then we can imagine a galaxy in which technological civilizations arise here and there, each of them gradually filling a sphere of exploration and colonization until a kind of equilibrium is reached and there is no practical advantage to pushing further. Earth, then, could be seen as being in the spaces between such civilizations, not yet aware of their existence, preparing over the next few centuries to begin its own expansion to nearby stars.

Is the galactic population sufficiently dense that such ‘bubbles’ of expansion ever meet? Or is SETI our only chance to confirm the idea that the galaxy has brought forth other technological civilizations? If the latter, we may know them only by the whisperings of their local traffic, exchanging information and perhaps speculating as we do about still more distant suns.

Anderson’s letter appeared in JBIS Vol 39 No (7 July 1986), p. 327.


SETI and the ‘Long Stare’

It’s been a week with an exoplanet focus, what with the interesting Kepler results yesterday and the five, or perhaps seven, planets found around the same star by the HARPS instrument. But I can’t close the week without recourse to Seth Shostak’s recent comments on biological versus machine intelligence. Paul Davies took much the same tack in his recent book The Eerie Silence (Houghton Mifflin Harcourt, 2010), arguing that any civilization we encounter will likely be composed of intelligent machines. Shostak thinks SETI should take that seriously.

Searching for Doppelgängers

Right now we’re searching for what Shostak calls ‘doppelgängers of humans’ — i.e., SETI has focused on places that could support life forms that do more or less what we do, which includes not only using radio to communicate, but much broader traits like living for finite lifetimes, following basic biochemical dictates and being subject to evolution. That biases the search toward places that could sustain life as we know it, a reasonable bias in my opinion, but one that may not take our own development into consideration. After all, we may be living in the short window between developing radio and building true artificial intelligence.

Suppose, Shostak asks in this BBC story (with accompanying audio interview), we develop true AI by the end of this century. What would happen next? This is where things get interesting. Shostak:

At some point they may just pick up and leave, at least some of them, maybe most of them… If you’re a machine, you’re interested in only two things I can think of. And that is matter and energy, because those facilitate whatever it is you’re doing. And matter and energy are not in particularly great supply here.

The result: AI lifeforms go to places more suited for their kind of existence, which could include the galactic core or, perhaps, the neighborhood of a hot, young star. Shostak is arguing that we should allocate a small percentage of our observing time — perhaps up to ten percent — for searching in places AI is more likely to call home. Thus far we’ve searched fewer than a thousand star systems intensively, and our all-sky search is of necessity unable to linger on a target. We’re new at the game, in other words, but let’s tune up our target list.

Is Biology the Issue?

The problem with SETI is that we’re forced to make assumptions about how aliens would operate, an issue that continues to bedevil the field today. Recently we’ve looked at the Benford brothers’ call for a different kind of search, one homing in on the kind of interstellar beacons an alien culture would be likely to create. The discipline is rife with new ideas as we try to figure out the basic parameters that any intelligent species would have to possess in our galaxy. But getting into an alien mind, much less an artificial one, is tricky business. The best we can do is build on our knowledge of physics and extrapolate a line of rational behavior.

The Benfords extrapolate from both physics and economics to argue that an interstellar beacon will likely use short, powerful bursts rather than continuous broadcasts. But SETI has lacked the ‘long stare’ needed to find such a signal. To me, the issue is less AI vs. biology than it is continuous survey vs. pinpoint search. The SETI League’s Project Argus, discussed in these pages recently, is an attempt to set up 5000 amateur receiving stations to implement the ‘long stare.’ It would be a low sensitivity survey, but as the cost of equipment drops and its power increases, it should become possible to implement at the amateur level, and it could be a powerful adjunct to more sophisticated (and focused) searches.

Methods like these could detect an alien beacon, whether built by machine or biological beings, out to several hundred light years, with the sphere of detection growing as we replace older stations with newer technology. They’re a great complement to higher-powered instruments. If we’re looking for beacons, a continuous, high-sensitivity stare along the galactic plane is a sensible way to proceed. But there’s a place for minimal assumptions and broad coverage too, and the advantage of an all-sky survey is that it takes what it finds, which might involve the kind of surprises SETI is made for.


New Kepler Planets in Resonance

Somewhere around 2000 light years away in the direction of the constellation Lyra is a Sun-like star orbited by at least two Saturn-class planets. What’s interesting about this news, as just discussed in the Kepler press conference I’ve been listening to this afternoon, is that for the first time we’ve detected and confirmed more than one planet around a single star using the transit method. But much more important, transit timing variations — the leads and lags of the two planets as they transit the star as seen from Kepler — can be used to tease out new and significant information.

Kepler-9b and 9c mark the first clear detection of transit timing variations by Kepler, allowing us to study the gravitational interactions between the planets involved. And that’s useful stuff: We see two planets in a 2:1 orbital resonance, one with a 19.2-day orbit, the other with a 38.9-day orbit. As the inner planet completes two orbits, the outer planet completes one. The variations in transit time (TTV) help us establish the mass of the two planets, showing that both have a mass and radius slightly smaller than Saturn. Peg size and mass and you can derive planetary density.

Image: Lightcurves of the Kepler 9 system. Credit: Matthew Holman (Harvard-Smithsonian Center for Astrophysics).

We’ve known for some time that transit timing variations should be a useful part of the exoplanet toolbox, but seeing them working in practice is a powerful proof of concept. The hope, of course, is that similar TTV methods can be put to work on smaller worlds down to terrestrial mass planets in the habitable zones of their stars. In fact, in this case, the Kepler 9 system sports a candidate that is 1.5 times the radius of Earth, a possible super-Earth whose barely detectable signature is not yet sufficient for us to declare the planet confirmed.

The other aspect of the Kepler 9 system that received discussion at the news conference was what it can tell us about planetary formation theories. These planets orbit well within the orbit of Mercury in our own system, and the assumption is that they could not have formed there. Planetary migration resulting in a 2:1 orbital resonance is something that points back to an earlier set of conditions whose nature will take many more precision measurements — in this and many other systems — to understand. But ultimately, the way a planetary system looks today can reveal much about its history.

Is there good news in this for terrestrial worlds? Not yet, but there is at least a hint. Alycia Weinberger (Carnegie Institution of Washington) made this case at the conference:

Ultimately Kepler will find many multiple planet systems. We will know how many systems show these resonances, how often and when different kinds of migration occurred while planets were forming. Our 1.5-Earth radius candidate, if real, survived whatever migration it and the other planets went through, a fact that bodes well for systems with substantial migration.

In other words, the Kepler 9 system may eventually tell us that the movement of gas giants into the inner system does not necessarily spell the doom of smaller worlds there, if and when we confirm the existence of the super-Earth. Weinberger goes on:

It would be interesting to know that planetary systems with different histories can produce low mass planets or planets more Earth-like in size. Resonances like those Kepler-9b and 9c demonstrate can ensure stability and produce planetary systems that last billions of years. Frequent resonances, in other words, are good news for low mass planets, giving them stable orbits. And transit timing variations can help us deduce the masses of the planets involved.

Not a bad haul for this interesting system, about which we’ll learn more as the Kepler mission progresses. As I said above, produce figures on the size and mass of a planet and you can derive its density, helping astronomers understand its composition, from gaseous to rocky or water planets. We’ll see what transpires with that candidate super-Earth — if it’s there, it’s in a scorching 1.6-day orbit, another hellish world singed by its star. But perhaps Kepler 9 helps point the way to a future news conference when an Earth-like planet in the habitable zone will be announced.


HD 10180: A Planetary Harvest

In a sense the planets discovered around the Sun-like star HD 10180 are no surprise. We’ve long assumed that planetary systems with numerous planets were common. We lacked the evidence, it’s true, but that could be put down to the limitations of the commonly used radial velocity method, which favors massive worlds close to their stars. But we’re getting much better at radial velocity work and, using instruments like the HARPS spectrograph at the European Southern Observatory’s La Silla (Chile) telescope, we’re teasing out ever more exquisite signals from distant systems. More and more multiple-planet scenarios are in our future.

Noting that high-precision radial velocity surveys are now able to detect planets down to roughly 1.9 Earth masses, the paper on the HD 10180 work frames the situation this way:

Preliminary results from the HARPS survey are hinting at a large population of Neptune-like objects and super-Earths within ?0.5 AU of solar-type stars (Lovis et al. 2009). Moreover, hundreds of small radius candidate planets have been announced by the Kepler Team (Borucki & the Kepler Team 2010). Clearly, the exploration of the low-mass planet population has now fully started, and will become the main focus of the field in the coming years.

But five planets at one go is still an eye-opener, especially when you consider that two others are also possible here. It took six years of study of this star, some 127 light years away in the constellation Hydrus, to bag the five leading signals, representing planets like Neptune in being between 13 and 25 Earth masses. These worlds circle their star in orbits that range from six to 600 days at distances between 0.06 and 1.4 AU. Some accounts are citing similarities with our Solar System because of the number of worlds, but we might just as well note the differences, including the crowding of the inner system and the presence of massive planets there.

Image: This wide-field image shows the sky around the star HD 10180, which appears as a fairly bright star just below the centre. The picture was created from photographs taken through red and blue filters and forming part of the Digitized Sky Survey 2. The field of view is approximately three degrees across. The coloured halos around the stars are artifacts of the photographic process and are not real. The remarkable planetary system around this star is far too faint and close in to be visible in this image. Credit: ESO, Digitized Sky Survey 2. Acknowledgement: Davide De Martin.

The paper notes that systems like this open up new realms of study:

It is expected that the characterization of planetary system architectures, taking into account all objects from gas giants to Earth-like planets, will greatly improve our understanding of their formation and evolution. It will also allow us to eventually put our Solar System into a broader context and determine how typical it is in the vastly diverse world of planetary systems. The characterization of a significant sample of low-mass objects, through their mean density and some basic atmospheric properties, is also at hand and will bring much desired insights into their composition and the physical processes at play during planet formation.

Those two additional worlds, whose existence Christophe Lovis (Observatoire de Genève), lead author of the cited paper, says is supported by solid evidence, include a 65 Earth-mass gas giant in a 2200-day orbit and a world that, if confirmed, would be the least massive exoplanet yet discovered, with a mass of about 1.4 times that of the Earth. This one is not exactly a candidate for astrobiology, though, orbiting the host star at a distance so close (0.02 AU) that a planetary year lasts a mere 1.18 Earth days. This ESO news release likens the possible world to the rocky inferno CoRoT-7b.

If there were a gas giant like Jupiter in this system, we should have evidence of it. And note that the orbits of all these planets seem to be almost circular. Says Lovis:

“Systems of low-mass planets like the one around HD 10180 appear to be quite common, but their formation history remains a puzzle.”

Although HD 10180 presents us with one of fifteen planetary systems known to have at least three worlds, that number will grow quickly. The new planets were found in a radial velocity survey of about 400 bright FGK stars in the solar neighborhood using HARPS, and the paper notes that ‘many new systems are about to be published.’ We’re homing in on the ability to derive statistical properties of the low-mass planet population, a new phase in the exoplanet hunt, one that focuses on complex planetary systems rather than individual planets.

From the paper:

The HD 10180 system shows the ability of the RV technique to study complex multi-planet systems around nearby solar-type stars, with detection limits reaching rocky/icy objects within habitable zones. Future instruments like VLT-ESPRESSO will build on the successful HARPS experience and carry out a complete census of these low-mass systems in the solar neighborhood, pushing towards planets of a few Earth masses at 1 AU.

The paper, submitted to Astronomy & Astrophysics, is Lovis et al., “The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems” (full text).