Brown Dwarf or Planet: The Case of XO-3b

Odd planets seem to be sprouting in our data like mushrooms. Take the case of XO-3b. It’s got the mass of thirteen Jupiters but orbits its star in less than four days, making it the largest, most massive planet ever found in such a tight orbit. But XO-3b also seizes the attention because its orbit is significantly elliptical rather than circular. Is this evidence for the gravitational effects of another object in the same system?

We should be able to learn a lot more about this and other questions because XO-3b is also a transiting world, passing between its star and the Earth. This is the third transiting planet identified by the XO Project, which uses two small telescopes at Haleakala (operated by the University of Hawaii) to identify transit candidates before passing the data on to a network of amateur astronomers for further study. After sufficient evidence is gathered, the work goes back to large telescopes at McDonald Observatory (University of Texas) for confirmation.

Announced at the American Astronomical Society meeting in Honolulu, the new planet is close enough to a brown dwarf in size to make its actual status uncertain. Sustainable hydrogen fusion seems to need about eighty Jupiter masses to occur, marking the upper limit between brown dwarf and star. But XO-3B may be right at the lower mass limit. Christopher Johns-Krull (Rice University), says this:

“The controversy lies at the lower end of the scale. Some people believe anything capable of fusing deuterium, which in theory happens around 13 Jupiter masses, is a brown dwarf. Others say it’s not the mass that matters, but whether the body forms on its own or as part of a planetary system.”

All of which may help us with a mystery. The so-called ‘brown dwarf desert’ is the term coined for the unexpected dearth of brown dwarfs in our exoplanet hunts. Radial-velocity techniques ought to be able to spot objects of XO-3b’s size and larger relatively easily given their mass, but the number of brown dwarfs so identified has remained small. Finding objects on the borderline may help us sort out some of the constraints on brown dwarf formation both in and out of planetary systems.

Let’s linger for a moment on the collaboration that’s happening here. Telescope time on the largest and best instruments is scarce, but time is precisely what planet-hunting astronomers need to firm up their findings. “…[T]hat’s where our amateur partners come in,” says XO Project director Peter McCullough (Space Telescope Science Institute), “culling our long lists of candidates down to more manageable size to observe with the big telescopes.” That kind of synergy is likely to produce a continuing harvest of transits, another confirmation of the role amateurs working with commercially available equipment can play in delivering good data.

An Asteroid Strike in North America?

Earth’s geological history could have a lot to say about our future in space. Every time we investigate a huge crater like Chicxulub, the Yucatan impact site that may have played a role in the demise of the dinosaurs, we’re reminded that the Solar System is an active and dangerous place. And the evidence multiplies. The Wilkes Land crater in east Antarctica may bear witness to the Permian-Triassic extinction that destroyed almost all life on Earth some 250 million years ago.

A defensive system in space with the capability of deflecting dangerous Earth-crossing objects is vital for species survival, whether the next strike occurs in ten or a hundred thousand years. But it’s a hard concept to sell because Earth’s major strikes, unlike those on the Moon, for example, tend to be obscured over time. Absent a visible historical context, a relatively minor strike like the 1908 Tunguska event can come to be seen as a quirky accident rather than evidence of a larger threat.

Now comes word of an explosion over or into the Laurentide ice sheet north of the Great Lakes some 13,000 years ago, causing the extinctions of big mammals like the mammoth and the mastodon by triggering a round of climatic cooling. Here again we have no obvious crater to look at, but rather layers of sediment drawn from numerous sites around the North American continent. In the sediments, a team led by James Kennett (UC Santa Barbara) found amounts of iridium and specks of nano-diamond, along with small spheres of glass and carbon. All that adds up, says the research team, to an explosion 12,900 years ago.

The BBC does a good job covering this story, with this quote from Kennett on the extinctions:

“All the elephants, including the mastodon and the mammoth, all the ground sloths, including the giant ground sloth – which, when standing on its hind legs, would have been as big as a mammoth …All the horses went out, all the North American camels went out. There were large carnivores like the sabre-toothed cat and an enormous bear called the short-faced bear.”

As to human populations, they would have lived through a 1,000 year period of cooling, an era known as the Younger Dryas whose effects were global and may have influenced the beginning of farming in the Middle East. In North America, the impact on the Clovis people, who hunted mammoths, may have been catastrophic. Such speculations enlivened the American Geophysical Union meeting in Acapulco when presented there last week. Let’s hope they also energize a new round of thinking about what a human presence in space can do to head off future catastrophes.

28 New Exoplanets Announced

What better indication of the success of our planet hunting efforts than the news out of the American Astronomical Society’s annual meeting in Honolulu. There, the California & Carnegie Planet Search team announced at least 28 new planets, with four multi-planet systems among them and two borderline cases that need further investigation. That’s a bump of 12 percent in the number of known planets over the last year. Behold:

Table of new planets

With the exoplanet count now not that far from 250, planetary discoveries are coming fast enough that a certain ennui seems to have settled in among press and public. Sure, Gliese 581 c was big news because we thought it was potentially habitable, but finding more and more gas giants probably won’t trigger the public imagination, even if GJ 436 b did cause a ripple because of the presence of water. That ripple lasted only long enough for scientists to explain what kind of water they were talking about. Here’s Geoff Marcy (UC Berkeley) on the subject:

“From the density of two grams per cubic centimeter – twice that of water – it must be 50% rock and about 50% water, with perhaps small amounts of hydrogen and helium. So this planet has the interior structure of a hybrid super-Earth/Neptune, with a rocky core surrounded by a significant amount of water compressed into solid form at high pressures and temperatures.”

Hard, hot, high-pressure water isn’t conducive to life and researchers have been quick to note that habitability is not an issue here. But it shouldn’t be neglected that a rocky place with high water content, discovered through a transit of a ‘hot Neptune,’ is an exciting reminder that the essentials for life are likely widespread. Marcy thinks about one in ten planets in our galaxy are habitable, which by his reckoning makes for around 20 billion conceivably living worlds in the Milky Way.

Interior of GJ 436 b

Image: The interior structure of the planet, Gliese 436 b. The core is probably composed of rock, including silicates, iron, and nickel. The envelope is probably composed primarily of water under high pressure (from the weight of material above it), and of smaller amounts of methane and ammonia. The density of the planet is 2.0 grams/cc, as determined from the planet’s mass (from Doppler measurements) and the diameter (from the dimming of the starlight as the planet transits in front of the star, blocking starlight). Credit: Jason Wright/California & Carnegie Planet Search.

The news release on these findings is here, containing John Asher Johnson’s interesting discoveries around A and F stars, with masses between 1.5 and 2.5 solar mass. Coupling his observations with earlier exoplanetary discoveries around older A stars, Johnson notes that planets around massive stars seem to orbit farther from their primary. “Only one of the 9 planets is within 1 AU (astronomical unit, or 93 million miles), and none of them is within 0.8 AU, of their host stars, which is very different than the distribution around sun-like stars,” Johnson said.

So it will be interesting to see what happens as this team works through the 450 older A stars it has added to its target list. Massive A stars like these normally rotate quickly and can mask the radial-velocity ‘signal’ of accompanying planets, which is why the team focuses on older A stars that have nearly completed hydrogen burning, a short period of stability that can reveal much about planetary companions. Large planets seem to show up more often around more massive stars, a trend that, if confirmed, seems to show the core accretion model of planetary formation at work, there being more disk material available early in the process.

Gliese 581: Right System, Wrong Planet?

New work on Gliese 581’s interesting planetary system may prove dismaying for those hoping for a planet in the habitable zone. With two ‘super-Earths’ and a Neptune class world, this is a system that cries out for close analysis. The Geneva team that detected the super-Earths had calculated surface temperatures on Gliese 581 c at roughly 20 degrees C. What they left out was the likely greenhouse effect of the atmosphere.

For habitability — defined here as the presence of liquid water at the surface — is not dependent on the central star alone, but also on the properties of the planets circling it. Werner von Bloh (Potsdam Institute for Climate Impact Research) and team tackle the habitability question in terms of atmosphere. From their paper:

…habitability is linked to the photosynthetic activity of the planet, which in turn depends on the planetary atmospheric CO2 concentration, and is thus strongly in?uenced by the planetary dynamics. In principle, this leads to additional spatial and temporal limitations of habitability, as the stellar HZ (de?ned for a speci?c type of planet) becomes narrower with time due to the persistent decrease of the planetary atmospheric CO2 concentration.

From this, the team studies habitable zone limitations for super-Earths, using a thermal evolution model that takes into account atmospheric carbon dioxide and varying ratios between water and land surfaces. The results: Both super-Earths in the Gliese 581 system are inside the tidal locking radius, keeping one face turned toward the star at all times. Gliese 581 c, the source of so much press speculation about terrestrial worlds, turns out to be far too hot to support life. Even scaling for stellar luminosity, it’s closer to its star than Venus is to ours.

But we’re not quite through. Interestingly, Gliese 581 d, a super-Earth of roughly eight Earth masses, could well have built up a dense atmosphere. With at least some of the climate models and carbon dioxide pressures assumed, the planet nudges inside the habitable zone and could be habitable for a period as long as 7.2 billion years. From the paper:

A planet with eight Earth masses has more volatiles than an Earth size planet to build up such a dense atmosphere. This prevents the atmosphere from freezing out due to tidal locking. In case of an eccentric orbit of Gl 581d (e = 0.2), the planet is habitable for the entire luminosity range considered in this study, even if the maximum CO2 pressure is assumed as low as 5 bar. Williams & Pollard (2002) concluded that a planet with a suffciently dense atmosphere could harbour life even if its orbit is temporarily outside the HZ. In conclusion, one might expect that life may have originated on Gl 581d.

The authors go on to add this caveat: Complex life is unlikely due to what they describe as ‘rather adverse environmental conditions.’ But of course we won’t know until we can get a look at potential biomarkers in the atmospheres of both these super-Earths. That will have to await later space missions like ESA’s Darwin and whatever mission grows out of the Terrestrial Planet Finder research. Given uncertainties of funding and ongoing technological reassessment, we may not have a definitive answer on the Gliese system for quite some time.

Thanks to Andy for the pointer to this one, and congratulations as well. Read through the comments on our first story on Gliese 581 c and you’ll see that Andy came up with remarkably similar conclusions not long after the story broke. Nice work indeed! The paper is von Bloh et al., “The habitability of super-Earths in Gliese 581,” available online.

A Nanotech Comeback for Big Ideas

There was a time when images like that of the space station under construction below were standard issue for futurists. It seemed inevitable that after our first tentative orbital flights, we would quickly graduate to building an enormous platform above the Earth, using it as a base for a Moon landing as well as a research establishment in its own right, even a vacation getaway for the well-heeled. It would eventually be part of the supply chain that would create and support a colony on Mars.

Space station vision from 1950s

The Pastelogram blog featured this futuristic vision by Frank Tinsley recently, done as one of a series of ads that ran in 1958 and 1959 for defense contractor American Bosch Arma. From the ad copy:

New vistas in astronomy will be opened up by such a space station, because of perfect conditions for photography and spectroscopy. It will also provide unique conditions for advanced research in physics, electronics, weather prediction, etc. Three such stations, properly placed, could blanket the entire world with nearly perfect TV transmission. Atomic rocket vehicles with prefabricated skin layers provide building materials for the station, then return to Earth. Similar craft will service an established station, docking by electromagnetic pull exerted from lower section of the station’s axis. Inertial navigation systems will play an increasing role in the exploration of outer space. Arma, now providing such systems for the Air Force Titan and Atlas ICBMs, will be in the vanguard of the race to outer space.

Today, with thoughts of Mir and the International Space Station in our heads, space seems infinitely more constrained. But don’t give up on a future of large space constructs just yet. Watching a Babylon 5 episode last night, I mused that its “two million, five hundred thousand tons of spinning metal, all alone in the night,” though beyond the imagination of current industrial technique, might one day be spawned by nanotechnological methods given suitable raw materials.

Indeed, just as we’ve been adjusting to the idea of nanotechnology as a way to reduce payload sizes for possible interstellar missions, we might go on to think how the same techniques could rejuvenate some older notions. Robert Forward’s lightsail concepts, relying on sails a hundred kilometers in diameter and a Fresnel lens between the orbit of Saturn and Uranus that would have to be as wide as Texas, may start to look a little less dubious. How fast can nano methods, using countless smart assemblers, actually build such a lens to specification?

I have no idea, but as it is the business of the future to surprise us, it seems unwise to rule out such scenarios. Build a Forward-style lens in the outer Solar System and a powerful laser could take a probe to a tenth of lightspeed for a Centauri flyby, or as Forward envisioned, push a manned mission to Epsilon Eridani with return capability. It all comes down to the building of large structures in space, an idea that could be making a nanotech comeback that would gladden the hearts of 1950’s dreamers.