A Bit More Detail

We live in an age of miracles and wonders, when our species’ astronomers really can detect relatively fine details of extrasolar planets from the comfort of its home planetary system. Two Universe Today stories today highlighted the scope of our accomplishements, the first being Nancy Atkinson’s post reporting the detection of light by the Spitzer telescope from the superterrestrial planet 55 Cancri e.

The first planet around 55 Cancri was reported in 1997 and 55 Cancri e – the innermost planet in the system — was discovered via radial velocity measurements in 2004. This planet has been studied as much as possible, and astronomers were able to determine its mass and radius.

But now, Spitzer has measured how much infrared light comes from the planet itself. The results reveal the planet is likely dark, and its sun-facing side is more than 2,000 Kelvin (1,726 degrees Celsius, 3,140 degrees Fahrenheit), hot enough to melt metal.

In 2005, Spitzer became the first telescope to detect light from a planet beyond our solar system, when it saw the infrared light of a “hot Jupiter,” a gaseous planet much larger than 55 Cancri e. Since then, other telescopes, including NASA’s Hubble and Kepler space telescopes, have performed similar feats with gas giants using the same method.

In this method, a telescope gazes at a star as a planet circles behind it. When the planet disappears from view, the light from the star system dips ever so slightly, but enough that astronomers can determine how much light came from the planet itself. This information reveals the temperature of a planet, and, in some cases, its atmospheric components. Most other current planet-hunting methods obtain indirect measurements of a planet by observing its effects on the star.

The new information about 55 Cancri e, along with knowing it is about 8.57 Earth masses, the radius is 1.63 times that of Earth, and the density is 10.9 ± 3.1 g cm-3 (the average density of Earth is 5.515 g cm-3), places the planet firmly into the categories of a rocky super-Earth. But it could be surrounded by a layer of water in a “supercritical” state where it is both liquid and gas, and topped by a blanket of steam.

“It could be very similar to Neptune, if you pulled Neptune in toward our sun and watched its atmosphere boil away,” said Michaël Gillon of Université de Liège in Belgium, principal investigator of the research, which appears in the Astrophysical Journal. The lead author is Brice-Olivier Demory of the Massachusetts Institute of Technology in Cambridge.

[. . .]

“When we conceived of Spitzer more than 40 years ago, exoplanets hadn’t even been discovered,” said Michael Werner, Spitzer project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Because Spitzer was built very well, it’s been able to adapt to this new field and make historic advances such as this.”

During Spitzer’s ongoing extended mission, steps were taken to enhance its unique ability to see exoplanets, including 55 Cancri e. Those steps, which included changing the cycling of a heater and using an instrument in a new way, led to improvements in how precisely the telescope points at targets.

Next, Jon Voisey described how one proposed method for detecting the existence of water oceans on distant worlds really isn’t all that. (One method, mind, for now.)

One of the most promising methods was proposed in 2008 and considered the reflective properties of water oceans. In particular when the angle between a light source (a parent star) and an observer is small, the light is not reflected well and ends up being scattered into the ocean. However, if the angle is large, the light is reflected. This effect can be easily seen during sunset over the ocean when the angle is nearly 180° and the ocean waves are tipped with bright reflections and is known as specular reflection.

Translating this to exoplanets, this would imply that planets with oceans should reflect more light during their crescent phases than their gibbous phase. Thus, they proposed, we might detect oceans on extrasolar planets by the “glint” on their oceans. Even better, light reflecting off a smoother surface like water tends to be more polarized than it might be otherwise.

The first criticisms of this hypothesis came in 2010 when other astronomers pointed out that similar effects may be produced on planets with a thick cloud layer could mimic this glinting effect. Thus, the method would likely be invalid unless astronomers were able to accurately model the atmosphere to take its contribution into consideration.

The new paper brings additional challenges by further considering the way material would likely be distributed. Specifically, it is quite likely that planets in the habitable zones without oceans may have polar ice caps (like Mars) which are more reflective all around. Since the polar regions make up a larger percentage of the illuminated body in the crescent phase than during the gibbous, this would naturally lead to a relative diminishing in overall reflectivity and could give false positives for a glint.

This would be especially true for planets that are more oblique (are “tilted”). In this case, the poles receive more sunlight which makes the reflections from any ice caps even more pronounced and mask the effect further. The authors of the new study conclude that this as well as the other difficulties “severely limits the utility of specular reflection for detecting oceans on exoplanets.”

Universe Today’s Jason Major wrote about a new theory for the formation of brown dwarfs, presented in the Astrophysical Journal paper “A Hybrid Scenario for the Formation of Brown Dwarfs and Very Low Mass Stars” by Basu and Vorobyov.

According to research by Shantanu Basu of the University of Western Ontario and Eduard I. Vorobyov from the University of Vienna in Austria and Russia’s Southern Federal University, brown dwarfs may have been flung out of other protostellar disks as they were forming, taking clumps of material with them to complete their development.

Basu and Vorobyov modeled the dynamics of protostellar disks, the clouds of gas and dust that form “real” stars. (Our own solar system formed from one such disk nearly five billion years ago.) What they found was that given enough angular momentum — that is, spin — the disk could easily eject larger clumps of material while still having enough left over to eventually form a star.

Model of how a clump of low-mass material gets ejected from a disk (S. Basu/E. Vorobyev)

The ejected clumps would then continue condensing into a massive object, but never quite enough to begin hydrogen fusion. Rather than stars, they become brown dwarfs — still radiating heat but nothing like a true star. (And they’re not really brown, by the way… they’re probably more of a dull red.)

In fact a single protostellar disk could eject more than one clump during its development, Basu and Vorobyov found, leading to the creation of multiple brown dwarfs.

If this scenario is indeed the way brown dwarfs form, it stands to reason that the Universe may be full of them. Since they are not very luminous and difficult to detect at long distances, the researchers suggest that brown dwarfs may be part of the answer to the dark matter mystery.

“There could be significant mass in the universe that is locked up in brown dwarfs and contribute at least part of the budget for the universe’s missing dark matter,” Basu said. “And the common idea that the first stars in the early universe were only of very high mass may also need revision.”

Based on this hypothesis, with the potential number of brown dwarfs that could be in our galaxy alone we may find that these “failed stars” are actually quite successful after all.

Centauri Dreams’ Paul Gilster reports on the search for Earth-size–and potentially Earth-like–planets orbiting one of the stars of Alpha Centauri, nearest star system to our own. Alpha Centauri A and B are both broadly Sun-like stars, A brighter than B, are roughly as old as our sun, and computer models suggest that rocky planets could form around the two stars and enjoy stable orbits. According to Gilster, the discovery is just a matter of time and money.

A warm and cozy planet around the K-class Centauri B would be just the ticket, and the planet hunt continues. One thing we’ve learned in the past decade is that neither Centauri A or B is orbited by a gas giant — planets of this size should have shown up in the data by now. We’ve also learned that stable orbits reach out maybe 2 AU from either star. Remember that while Centauri A and B are separated by almost 40 AU at their widest point, they close to within 11 AU, thus disrupting outer orbits, as demonstrated by computer simulations. We should expect planets, if they exist, to be no further out than the main asteroid belt in our own system.

Debra Fischer (Yale University) has been working on the Alpha Centauri problem at Cerro Tololo (Chile) in addition to her efforts at improving instrument sensitivity for planet hunting at the Keck and Lick observatories. The goal is to reach the precision needed to turn up planets the size of the Earth with radial velocity methods. If we’re going to get a Centauri detection, odds are it favors Centauri B because A does not seem to be as stable as B, and the latter is more likely to be the first to yield what Fischer calls the ‘tiny whisper’ that would flag an Earth-like world. Usefully, the 79 degree orbital plane of these stars means that planets in this system, assuming they share this tilt, should be generating a reflex velocity close to the line of sight from the Earth.

Radial velocity methods, in other words, should work here if we can attain sufficient sensitivity. The detection effort calls for telescope time at the Cerro Tololo Inter-American Observatory this spring and summer, and The Planetary Society is campaigning to raise money to support the effort. What Fischer needs is 20 nights of observing time, but the team’s NASA and NSF grants cannot be used to pay for telescope time, which at Cerro Tololo runs to $1650 per night. A total of $33,000 will do it, then, money the community should be able to raise. Have a look at the Planetary Society’s donation page and let’s see if we can’t make this happen.

Anyone involved with The Planetary Society is probably already aware of Fischer’s work with astronomer and Tau Zero practitioner Geoff Marcy (UC-Berkeley) on FINDS Exo-Earths (Fiber-optic Improved Next generation Doppler Search for Exo-Earths). The collaboration has resulted in a high-end optical system installed on the 3-meter Lick Observatory telescope and is now feeding the FINDS 2 effort to provide advanced optics for the Keck Observatory in Hawaii. Marcy and Fischer are working with a fiber optics array that adjusts light entering the telescope’s spectrometer and an adaptive optics system that offers the best signal to noise ratio.

FINDS worked out well at the Lick Observatory, improving the ability to detect Doppler velocities from the pre-existing 5 meters per second down to the 1 meter per second range, allowing us to detect smaller planets. Fischer and Marcy are hopeful of attaining precisions down to 0.5 meters per second with their work at Keck, which should get us into the range of Earth-sized planets. FINDS 2 will then be used with Keck to provide follow-up data about planets found by the Kepler mission, ruling out false positives in the ongoing hunt for planets like our own. The work on FINDS has led directly into the commissioning of a new spectrometer at Cerro-Tololo.

The latest news item announcing that the experiments used by the Vikings Mars landers of the 1970s to determine if there might be Mars actually did detect life, just life that we did know how to identify back in the 1970s before we learned of the extremophiles of Earth, something that Livejournaler absinthe-dot-ca linked to, just as james-nicoll did. The former linked to the paper, “Complexity Analysis of the Viking Labeled Release Experiments”.

The only extraterrestrial life detection experiments ever conducted were the three which were components of the 1976 Viking Mission to Mars. Of these, only the Labeled Release experiment obtained a clearly positive response. In this experiment 14C radiolabeled nutrient was added to the Mars soil samples. Active soils exhibited rapid, substantial gas release. The gas was probably CO2 and, possibly, other radiocarbon-containing gases. We have applied complexity analysis to the Viking LR data. Measures of mathematical complexity permit deep analysis of data structure along continua including signal vs. noise, entropy vs.negentropy, periodicity vs. aperiodicity, order vs. disorder etc. We have employed seven complexity variables, all derived from LR data, to show that Viking LR active responses can be distinguished from controls via cluster analysis and other multivariate techniques. Furthermore, Martian LR active response data cluster with known biological time series while the control data cluster with purely physical measures. We conclude that the complexity pattern seen in active experiments strongly suggests biology while the different pattern in the control responses is more likely to be non-biological. Control responses that exhibit relatively low initial order rapidly devolve into near-random noise, while the active experiments exhibit higher initial order which decays only slowly. This suggests a robust biological response. These analyses support the interpretation that the Viking LR experiment did detect extant microbial life on Mars.

At best, this is provocative stuff, and makes the case for a followup mission to Mars. Comments in Jason Major’s Universe Today item make the point that retroactive analyses of the data are great at picking up patterns, just patterns that are imposed by the researchers combing through the data again as much as patterns that actually exist. One of the authors of the paper, Gilbert Levin, designed some of the Viking probes’ life-detection experimental kit and has since argued at length that NASA scientists almost went of their way to interpret the evidence as proof of an absence of life.

Not two days after my post describing speculation that the gas giant believed to have been imaged orbiting the nearby star of Fomalhaut didn’t exist, via io9 I learned that the European Southern Observatory in Chile has found different, smaller planets using the Atacama Large Millimeter/submillimeter Array.

The ALMA images show that both the inner and outer edges of the thin, dusty disc have very sharp edges. That fact, combined with computer simulations, led the scientists to conclude that the dust particles in the disc are kept within the disc by the gravitational effect of two planets — one closer to the star than the disc and one more distant.

Their calculations also indicated the probable size of the planets — larger than Mars but no larger than a few times the size of the Earth. This is much smaller than astronomers had previously thought. In 2008, a NASA/ESA Hubble Space Telescope image had revealed the inner planet, then thought to be larger than Saturn, the second largest planet in our Solar System. However, later observations with infrared telescopes failed to detect the planet.

That failure led some astronomers to doubt the existence of the planet in the Hubble image. Also, the Hubble visible-light image detected very small dust grains that are pushed outward by the star’s radiation, thus blurring the structure of the dusty disc. The ALMA observations, at wavelengths longer than those of visible light, traced larger dust grains — about 1 millimetre in diameter — that are not moved by the star’s radiation. They clearly reveal the disc’s sharp edges and ringlike structure, which indicate the gravitational effect of two planets.

“Combining ALMA observations of the ring’s shape with computer models, we can place very tight limits on the mass and orbit of any planet near the ring,” said Aaron Boley (a Sagan Fellow at the University of Florida, USA) who was leader of the study. “The masses of these planets must be small; otherwise the planets would destroy the ring,” he added. The small sizes of the planets explain why the earlier infrared observations failed to detect them, the scientists said.

The ALMA research shows that the ring’s width is about 16 times the distance from the Sun to the Earth, and is only one-seventh as thick as it is wide. “The ring is even more narrow and thinner than previously thought,” said Matthew Payne, also of the University of Florida.

The ring is about 140 times the Sun-Earth distance from the star. In our own Solar System, Pluto is about 40 times more distant from the Sun than the Earth. “Because of the small size of the planets near this ring and their large distance from their host star, they are among the coldest planets yet found orbiting a normal star,” added Aaron Boley.

• Anders Sandberg at Andart approves of entrepreneur Elon Musk’s desire to use space travel to create offworld backups for our biosphere.
• Burgh Diaspora’s Jim Russell believes that the close links between Brazil and Boston–driven by migration, at first strictly economic but then driven by interest in Massachusetts’ education institutes–could serve Boston quite well.
• Two links from Centauri Dreams today, one describing the planetary system of HD 10180, a Sun-like star that supports nine planets to our eight, and the other describing hypothetical laser-based defenses for starships against interstellar dust.
• At Extraordinary Observations, Rob Pitingolo describes the difficulties tourism planners in destination cities have with getting people to visit sites that aren’t the most heavily trafficked.
• Geocurrents’ Asya Pereltsvaig deflates the myth that Chinese men (lacking spouses owing to a male-biased sex ratio at birth) will flood into Russia (especially Siberia) looking for Russian women (lacking spouses owing to a high male death rate). Among other things, there actually isn’t much of a shortage of theoretically marriageable men in Siberia.
• The Global Sociology Blog discusses what happens when celebrity culture and social networking sites like Twitter insect. The answer? It’s easier to get social capital than ever before.
• At GNXP, Razib Khan notes that Argentina–unlike English-speaking countries also products of mass European immigration–still evidences the genetic trace of indigenous populations.
• Open the Future’s Jamais Cascio points out that, at long last, global climate change is kicking off (as expected as early as 1981).
• Registan features a guest post from Uzbekistan commentator Azamat Seitov, who discusses the possibility that the Eurasian Economic Community–a Russia-centered bloc also including Belarus, Kazakhstan, Kyrgyzstan, and Tajikistan–will take off. He’s skeptical.

I think that io9 may have overreached in titling a post. The post “Why we won’t find Earth-2 around a red dwarf star” links to a very interesting paper regarding unconsidered problems facing potentially Earth-like planets around red dwarf stars, “Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating” by Barnes, Mullins, et al., but the paper consider a specific known exoplanet orbiting a red dwarf–Gliese 667C c, covered by me back in February here–and concludes that it could be habitable after all.

What’s going on? It all has to do with the habitable zones around stars, the set of orbits in which a planet could plausibly support an Earth-like climate friendly to liquid water. Traditional calculations of a habitable zone have considered the radiant energy produced by a star. For red dwarfs–dim, low mass stars–a planet in the habitable zone would be closely bound by gravitation to its star, quite possibly with one side forever facing its sun in much the same way that one side of the Moon forever faces the Earth and the other forever faces away. This degree of tidal locking wouldn’t prevent such a planet from being habitable, as atmospheric models suggest that an atmosphere only slightly denser than Mars would be capable of transporting enough heat to prevent the planet’s atmosphere from freezing on the dark side. Other constraints, however, might exist. The authors identify the heat produced by the gravitational tides exerted by a star on such a close planet as a major source of heat.

As a planet moves from periastron, its closest approach to the star, to apoastron, the furthest point, and back again, the gravitational force changes, being inversely proportional to distance squared. This difference creates an oscillating strain on the planet that causes it to undergo periodic deformation. The rigidity of the planet resists the deformation, and friction generates heat. This energy production is called tidal heating.

Tidal heating is responsible for the volcanism on Io (Strom et al. 1979; Laver et al. 2007), which was predicted, using tidal theory, by Peale et al. (1979). Io is a small body orbiting Jupiter with an eccentricity of 0.0041, which is maintained by the gravitational perturbations of its fellow Galilean moons, that shows global volcanism which resurfaces the planet on a timescale of 100 – 105 years (Johnson et al. 4 1979; Blaney et al. 1995; McEwen et al. 2004). The masses of Jupiter and Io are orders of magnitude smaller than a star and terrestrial exoplanet, and thus the latter have a much larger reservoir of orbital and rotational energy available for tidal heating. Moreover, some exoplanets have been found with orbital eccentricities larger than 0.9 (Naef et al. 2001; Jones et al. 2006; Tamuz et al. 2008). Thus, the tidal heating of terrestrial exoplanets may be much more effective than on Io (Jackson et al. 2008c,a; Barnes et al. 2009a, 2010; Heller et al. 2011). This expectation led to the proposition that terrestrial exoplanets with surface heat fluxes as large or larger than Io’s should be classified as “Super-Ios”, rather than “Super-Earths” (Barnes et al. 2009b).

The authors go on to calculate that it’s quite possible for some planets closely orbiting red dwarf stars, especially worlds orbiting low-mass red dwarf stars (less than 20% the mass of our sun, perhaps) and worlds with very eccentric orbits, to be located within the “classical” habitable zone of their star but nonetheless be so heated by the tidal forces exerted by their star as to become “Tidal Venuses”, becoming superheated worlds which lose their water to evaporation in space in just hundreds of millions of years. The aforementioned Gliese 667C c is not likely to be such a planet, according to the team’s calculations, as its orbit is too distant. Other worlds, as yet undiscovered, may not be so lucky.

HIP 11952 has made the news recently for its planets. This star 375 light-years away is noteworthy since, although it’s broadly similar to the Sol mass-wise–it’s only one-sixth less massive than our Sol, perhaps comparing to nearby famous Tau Ceti–it is a remnant of the very early universe. Our sun is 4.57 billion years old, our universe 13.75 billion (plus or minus 110 million years), but HIP 11952 formed in the youth of the universe, and is 12.8 billion years old. Because of its extreme age, when HIP 11952 was formed in the infant Milky Way it did so in an environment where there just hadn’t been enough nuclear fusion in stars to make the elements heavier and helium that dominate our terrestrial worlds; spectrograms indicate that this Population II star has 1% of the “metals” of our own sun. There was a consensus in astronomy that stars with so few metals shouldn’t have been able to support the formation of planets.

But HIP 11952 has two Jovian-mass planets regardless.

It is widely accepted that planets are formed in disks of gas and dust that swirl around young stars. But look into the details, and many open questions remain – including the question of what it actually takes to make a planet. With a sample of, by now, more than 750 confirmed planets orbiting stars other than the Sun, astronomers have some idea of the diversity among planetary systems. But also, certain trends have emerged: Statistically, a star that contains more “metals” – in astronomical parlance, the term includes all chemical elements other than hydrogen and helium – is more likely to have planets.

This suggests a key question: Originally, the universe contained almost no chemical elements other than hydrogen and helium. Almost all heavier elements have been produced, over time inside stars, and then flung into space as massive stars end their lives in giant explosions (supernovae). So what about planet formation under conditions like those of the very early universe, say: 13 billion years ago? If metal-rich stars are more likely to form planets, are there, conversely, stars with a metal content so low that they cannot form planets at all? And if the answer is yes, then when, throughout cosmic history, should we expect the very first planets to form?

Now a group of astronomers, including researchers from the Max-Planck-Institute for Astronomy in Heidelberg, Germany, has discovered a planetary system that could help provide answers to those questions. As part of a survey targeting especially metal-poor stars, they identified two giant planets around a star known by its catalogue number as HIP 11952, a star in the constellation Cetus (“the whale” or “the sea monster”) at a distance of about 375 light-years from Earth. By themselves, these planets, HIP 11952b and HIP 11952c, are not unusual. What is unusual is the fact that they orbit such an extremely metal-poor and, in particular, such a very old star!

For classical models of planet formation, which favor metal-rich stars when it comes to forming planets, planets around such a star should be extremely rare. Veronica Roccatagliata (University Observatory Munich), the principal investigator of the planet survey around metal-poor stars that led to the discovery, explains: “In 2010 we found the first example of such a metal-poor system, HIP 13044. Back then, we thought it might be a unique case; now, it seems as if there might be more planets around metal-poor stars than expected.”

What does this mean for planetary formation? What are the minimum requirements for planetary formation? When did the first planets in our galaxy form?

Centauri Dreams’ Paul Gilster notes that this has implications from the perspectives of galactic history and the possibilities of extraterrestrial life. If planets formed so early in the universe’s history, well, where is everybody?

The idea of ‘deep time’ exerts an abiding fascination. H.G. Wells took us forward to a remote futurity when his time traveler looked out on a beach dominated by a red and swollen Sun. But of course deep time goes in the other direction as well. I can remember wanting to become a paleontologist when I discovered books about the world of the dinosaurs, my mind reeling from the idea that the world these creatures lived in was as remote as any distant star. Paleontology was a grade-school ambition I never followed up on, but the Triassic and Jurassic eras still have a hold on my imagination.

In a SETI context, deep time presents challenges galore. Charles Lineweaver’s work offers up the prospect that the average Earth-like planet in our galactic neighborhood may well be far older than our own — Lineweaver calculates something like an average of 1.8 billion years older. Would a civilization around such a star, if one could survive without destroying itself for so long, have anything it wanted to say to us? Would it have evolved to a level where it had merged so completely with its environment that we might not be able to recognize its artifacts even if we saw them?

[. . .]

Are there, then, more planets around metal-poor stars than we have previously thought? We need to find more planetary systems in this age bracket to learn more, but Anna Pasquali (Heidelberg University), a co-author of the paper on this work, says “The discovery of the planets of HIP 11952 shows that planets have been forming throughout the life of our Universe,” a thought that reminds us to be careful about drawing hasty conclusions about planet formation. Don’t be surprised, either, if the idea doesn’t once again bring Dr. Fermi knocking at the door.

A commenter at Centauri Dreams suggests that gas giants, which are made substantially of the non-metals that were in exclusive abundance early in the universe’s history, would likely form more easily than rocky planets like the Earth. Against this, a commenter at the first news link wonders–jokingly–if HIP 11952 is the home system of the Vorlons.

Neil deGrasse Tyson, the man who has the best claim to being the living heir to Carl Sagan as an astronomer and a popularizer of science, has an opinion piece up on Wired Science, “We Can Survive Killer Asteroids — But It Won’t Be Easy”.

I’m with him on everything, until the last paragraph, reproduced below.

Every few decades, on average, house-sized impactors collide with Earth. Typically they explode in the atmosphere, leaving no trace of a crater. Once in about a hundred million years, though, Earth is visited by an impactor capable of annihilating all life-forms bigger than a carry-on suitcase.

One killer asteroid we’ve been monitoring is Apophis, which is large enough to fill the Rose Bowl. On Friday the 13th, April 2029, it will dip below the altitude of our communication satellites. If its trajectory on that day passes within a narrow range of altitudes called the “keyhole,” then the influence of Earth’s gravity on its orbit will guarantee that seven years later, in 2036, on its next trip around the Sun, the asteroid will hit Earth directly, likely slamming into the Pacific Ocean. The tsunami it creates will devastate all the coastlines of the Pacific Rim. If Apophis misses the keyhole in 2029, we’ll have nothing to worry about in 2036.

A more recent discovery, half the size of Apophis, is expected to pass Earth at a distance of a million miles in 2023 and ten million miles in 2028, has been stirring up the scaremongers but rates only a 1 on the 1–10 scale of impact hazards. Unscarily named 2011 AG5, it will become much more visible and trackable during 2013. Earth’s gravity could conceivably convince it to collide with us in 2040, but NASA deems that a remote chance.

Some people would like to blow potentially hazardous rocks out of the sky with a nuclear bomb. Others would deploy a radiation-intensive neutron bomb (the Cold War–era bomb that kills people but leaves buildings intact) to induce a recoil and alter the asteroid’s orbit. A kindler, gentler approach would be to nudge it into a different orbit with slow but steady rockets that have somehow been attached to one side — or with a solar sail, which harnesses the pressure of sunlight for its propulsion.

The odds-on favorite solution, however, is the gravitational tractor. This involves parking a probe in space near the killer asteroid. As their mutual gravity draws the probe to the asteroid, an array of retro rockets fires, instead causing the asteroid to draw toward the probe and off its collision course with Earth.

Saving the planet requires commitment. First we have to catalogue every object whose orbit intersects Earth’s, then task our computers with carrying out the calculations necessary to predict a catastrophic collision hundreds or thousands of orbits into the future. Meanwhile, space missions would have to determine in great detail the structure and chemical composition of killer comets and asteroids.

If humans one day become extinct from a catastrophic collision, we would be the laughing stock of aliens in the galaxy, for having a large brain and a space program, yet we met the same fate as that pea-brained, space program-less dinosaurs that came before us.

Does the last paragraph hint at Tyson’s boosterism for manned space travel as a way to jump-start growth of all kinds on Earth, irrespective of the evidence suggesting that it’s inordinately expensive and dangerous and generally a waste of resources? Or am I reading too much into it?

The BBC’s Paul Rincon writes about the ways in which the world Vesta–traditionally classified as an asteroid–is built along the lines of rocky worlds like the Earth and Mars. As I’ve blogged in the past, by virtue of its structure and–perhaps–its size–Vesta should be counted as a dwarf planet, like Pluto and like fellow asteroid-belt resident Ceres.

Vesta has been viewed as a massive asteroid, but after studying the surface in detail, scientists are describing it as “transitional”.

The Dawn spacecraft has been orbiting Vesta – one of the Solar System’s most primitive objects – since July 2011.

They have documented many unexpected features on its battered surface.

Mission scientists presented their latest results at the Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas.

Dawn’s principal investigator, Christopher T Russell, told the meeting that the science team found it hard not to refer to the object as a planet.

He said the rounded asteroid showed evidence of geological processes that characterise rocky worlds like Earth and the Moon.

Vesta is the second most massive of the asteroids, measuring some 530km (330mi) in diameter. It is dominated by a huge crater called Rheasilvia and bears many other scars left by the hammering it has received at the hands of other asteroid belt denizens.

One important transitional feature of Vesta can be found in its topography, or elevation. Vertical elevation on the Moon or Mars might reach tens of kilometres, but these objects are also very large.

“This means the topography is about 1% of the radius,” Dr Ralf Jaumann, from the German Aerospace Center (DLR), told BBC News, “If you go to Vesta, it is 15%, and if you go to the largest outer asteroid – Lutetia – it is 40%.”

In short, this mathematical relationship between topography and radius (half an object’s diameter), puts Vesta in an intermediate position between small asteroids and rocky planets.

Another aspect concerns the way its surface has been modified, or “processed”, by the many collisions. This is evident in dark material that can be seen in images of its terrain.

The dark material seems to be related to impacts and their aftermath. Scientists think carbon-rich asteroids could have hit Vesta at speeds low enough to produce some of the smaller deposits without blasting away the surface.

Higher-speed asteroids could also have collided with Vesta’s surface and melted the volcanic basaltic crust, darkening existing surface material.

Scientists are confident there has been volcanism on the asteroid during its history. This is because there are hundreds of pieces of Vesta sitting in museums around the world.

They form a particular class of meteorite called the HEDs; more of these objects have fallen to Earth than all the meteorites from the Moon and Mars put together. Studies of HED meteorites have revealed telling chemical signatures of volcanic activity.

Dave Williams, from Arizona State University, told BBC News: “We know [from the HED meteorites] there were lava flows at some point in history, so I expected there to be at least a few lava flows, maybe a few channels, shields or cones. Looking at all the images in places that have been illuminated thus far, we don’t see any evidence of that.

“That’s because of all the impact processing over Solar System history. It has destroyed all the evidence.”