Terrestrial Worlds: The Devil in the Details

In a world dominated by short-term thinking, we tend to be driven by media cycles. That makes the coverage of science, among other subjects, problematic. Science operates through the analysis of detail as various minds subject a problem to hypotheses that can be tested experimentally. In other words, good science often takes time, which is why situations like the Gliese 581c story can be so frustrating.

Announce a habitable planet around another star and the media love you. Spend months and (if needed) years subjecting the habitability question to analysis and you’re not on the public radar. Many scientists have come to question whether Gliese 581c is remotely habitable; some even argue for habitability for the next planet out, Gliese 581d. We’re still trying to weigh the data, and such deliberate processes aren’t the sort of thing to replace the latest Hollywood starlet scandals on CNN.

The good scientist ignores media vagaries and proceeds with the painstaking details. The hunt for better tools for analysis is unceasing. Take the so-called Rossiter effect. It shows up in radial velocity data during a planetary transit, causing a distortion that seems to indicate a radial velocity shift. But there is no change in the velocity of the star being studied — the Rossiter effect only mimics such a change. The transiting planet has, in other words, meddled with the light being studied.

William F. Welsh and Jerome Orosz (San Diego State University) want to know whether this radial velocity ‘anomaly’ can be used to detect an Earth-like planet. After all, they point out, in the case of a terrestrial body orbiting beyond 0.5 AU, the amplitude of the Rossiter effect can be larger than the radial velocity of the host star’s orbit. The two astronomers simulated Rossiter effect signals and threw in observational noise to study the question. Could a useful signal be extracted from the noise of periodic star pulsations and other stellar variables?

The results aren’t encouraging for planets of Earth’s radius. But it does appear that the Rossiter effect could snare a slightly larger planet, especially as we move up to two Earth radii. Here’s the gist:

While using the Rossiter e?ect for discovering an Earth-like planet is certainly not competitive with the photometric technique (due to the vast multiplexing advantage wide-?eld photometry has), the Rossiter e?ect can used to provide an important con?rmation of potential warm Earth-like planets. Since many of the most interesting transits that will be seen by the Kepler Mission will also be the most challenging, an independent con?rmation of the transit via spectroscopy of the Rossiter effect may prove to be very helpful.

So what we’re dealing with here is a technique that can help us confirm the detection of a planet found by other methods. As Gliese 581c demonstrates, the more tools at our disposal to make such confirmations, the sooner we’ll be able to reach definitive conclusions. Such painstaking science rarely makes the headlines, but papers like this wind up leading to tools for scientists working at levels of detail that were once unimaginable. Further work will demonstrate the Rossiter effect’s viability, but looking for ways to extend our planet-hunting reach will doubtless yield the methods we’ll need to better understand what Kepler gives us.

The paper is Welsh and Orosz, “On Using the Rossiter Effect to Detect Terrestrial Planets,” Transiting Extrasolar Planets Workshop, ASP Conf. Ser. Vol 366, 2007; Eds. A. Afonso, D. Weldrake and Th. Henning, p. 176 (abstract).

Galactic Drift and Mass Extinction

Theories that explain Earthly cataclysms through astronomy are always fascinating. The notion that a dwarf star dubbed ‘Nemesis’ orbits the Sun and occasionally stirs up cometary debris in the Oort Cloud emerged in the 1980s, published by two independent teams, one of which included Richard Muller. A UC-Berkeley physicist, Muller has since given up on Nemesis, but he’s still looking for the cause of what he sees as a 62-million year cycle (plus or minus 3 million years) in mass extinction events.

Berkeley’s James Kirchner, quoted in this 2005 story on Muller’s work, thinks the evidence Muller and graduate student Robert Rohde have assembled on such extinction cycles “simply jumps out of the data.” Says Kirchner:

“Their discovery is exciting, it’s unexpected and it’s unexplained. Everyone and his brother will be proposing an explanation — and eventually, at least one or two will turn out to be right while all the others will be wrong.”

Muller and Rohde used a huge fossil database of marine organisms developed by the late John Sepkoski Jr. (University of Chicago), one whose data extend back to the time of the ‘Cambrian Explosion,’ the period when so many forms of multicellular life emerged. But while Muller and Rohde pondered alternative explanations for the cycle, two University of Kansas professors have come up with a theory involving the Sun’s position in the Milky Way, one that has gone on to win Muller’s approval.

Solar System Movement in the Galaxy

The Solar System moves up and down as it orbits the galactic core (see image at left). Mikhail Medvedev and Adrian Melott, taking that motion into account, factor in the motion of the Milky Way itself, hypothesizing that its leading, north side generates a shock wave that exposes the Earth to high-energy radiation every 64 million years or so. Here’s Melott on the matter:

“I did notice that not only did these time scales appear to be almost the same, but the drops in biodiversity coincide with the times when the sun is on the north side of the galactic disc. I already knew the north side of the galactic disc was the direction toward which the galaxy is falling.”

Image: Our solar system, represented by the white dot, moves around the center of the galaxy (like planets around the sun). It also moves up and down around the mid-plane of the galaxy. The mid-plane is shown by the dashed white line. The solid green line represents the up-and-down motion of the solar system as it circumnavigates the galaxy.

With a 3-million year uncertainty in the calculations, that 64 million year cycle matches well enough with the 62 million year cycle of extinctions. The match resonates with Richard Muller, who says of the KU team: “They succeeded where I failed in coming up with a possible explanation for the effect that we observed.” And if they’re right, we have time to prepare for the next major event, since the Solar System has just passed the mid-plane of the galaxy. The next peak occurs in ten to twelve million years, assuming the KU theorists are onto something.

The extinction event that cries out for explanation here is the most recent, the Cretaceous/Tertiary dinosaur extinction that dates back some 65 million years. It’s exceptional in this context because it occurred within two million years of the Solar System’s mid-plane galactic crossing. Here’s how the authors deal with that issue in their paper:

[Rohde and Muller] noted that the 62 My signal in the fossil record emerges from integration over almost 9 periods, and while highly signi?cant does not coincide exactly with the onset of major extinction events, dated to within uncertainties in geological dating methods. These may be caused by a combination of stresses including for example CR ?ux variation, bolide impacts, volcanism, methane release, anoxia in the oceans, ionizing radiation bursts from other sources, etc. (It is an interesting aspect of this that the onset of the K/T (“dinosaur”) extinction, generally thought to be due to a bolide impact, coincides within 2 My of mid-plane crossing. Nevertheless, the 62 My cycle is strong and robust against alternate methods of Fourier decomposition and alternate approaches to computing its statistical signi?cance.

The Berkeley work on the 62-million year cycle appears in Rohde and Muller, “Cycles in fossil diversity,” Nature 434 (10 March 2005), pp. 208-210 (abstract). The KU work is found in Medvedev and Melott, “Do extragalactic cosmic rays induce cycles in fossil diversity?” accepted by the Astrophysical Journal and available online.

Scaled Composites Support Fund

The recent deaths of three Scaled Composites employees — Charles Glen May, 45; Eric Blackwell, 38; and Todd Ivens, 33 — have brought sorrow to the young commercial spaceflight industry. Those wishing to support the families of the deceased as well as the employees injured in the explosion can do so through the Scaled Composites Family Support Fund.

According to a statement from the National Space Society, contributions can be sent to:

Scaled Family Support Fund
c/o Scaled Composites
1624 Flight Line
Mojave, CA. 93501

Acct # 04157-66832

Wire transfer ABA Routing #1220-0066-1

Please make checks payable to the account number or to the name of the fund.

The first deaths in the civilian space sector remind us how many died during the development of aviation. Doubtless there will be more, but the forces driving our push to open up space to companies with good ideas are unlikely to be slowed. What is happening in the Mojave and elsewhere is igniting the dreams of an entirely new generation. We are in debt to those who gave their lives helping that dream take form.

Asteroid Impacts and the Press

In a world where climate change is everywhere under discussion, its causes pondered and its effects debated as political fodder, I suppose it makes sense that The Economist would look at the danger posed by Earth-crossing asteroids in the same context. Thus the sub-title of its recent story on the subject: “The ultimate environmental catastrophe.” Which, of course, an asteroid impact could well be, particularly if large enough or placed in a highly populated area.

I’ve subscribed to The Economist off and on for decades, always admiring its clarity and style. The magazine handles this subject with skill, noting how quickly the living ecology of Earth scrubs away the tell-tale signs of impact craters, citing the Moon as a counter-example, and going on to note that the Earth Impact Database in Canada can nonetheless identify more than 170 such craters. And it reminds us of NASA’s scientist David Morrison’s statement that a large meteorite strike is the only known natural disaster that could put civilization itself at risk.

How the press presents complicated stories to the public has consequences on the terms of subsequent debate. The current article runs through the options for changing the orbit of dangerous objects, ranging from nuclear weapons to nearby spacecraft using their tiny gravitational effects to move the asteroid gradually off course. Worthy of greater emphasis, in my opinion, is the amount of time the latter (more practical) method would need to accomplish its work. Whatever the method, we have to pick the danger up early, allowing ourselves an adequate window to muster the resources needed.

We also have to have the infrastructure in space to do what needs to be done, whatever course we choose. Instead, we are closing down options here on Earth. The Arecibo radar installation is under immediate threat of de-funding, a potentially devastating loss to our ability to find and catalog Earth-crossing objects. Meanwhile, other than the all too occasional story, the media focus instead on useful scandals that draw viewers and readers. Lisa Nowak and her tortured love affairs. Drunken astronauts being rocketed into space. All are certain to drive ratings up.

As to the latter, I’ll just say this. If I were about to board a craft that would muscle me into orbit using two solid-state boosters, neither of which could be throttled back or shut-down once lit, I would demand more than a single drink before I walked to the launch gantry. And as for the press, let’s hope for more articles that remind the public of the stakes involved in asteroid collision. We may or may not have time to avoid another Tunguska, but the need for a flexible space-based response to the danger should be kept front and center. We don’t need alarmism, but we do need a thoughtful, prepared public that understands the issues.

Black Hole Feeding Frenzy

A research team using data from the Chandra x-ray observatory has examined supermassive black hole activity in galaxy clusters of different ages. Also known as active galactic nuclei (AGN), the black holes are the result of rapid growth in gas-rich environments in the early universe, explaining why they are more common in young clusters than in older ones. Comparing the fraction of AGN in clusters at large distance (when the universe was 58 percent of its current age) to relatively nearby clusters, the team found 20 times more AGN in the more distant sample.

Paul Martini (Ohio State University) sees this as confirmation of earlier theory:

“It’s been predicted that there would be fast-track black holes in clusters, but we never had good evidence until now. This can help solve a couple of mysteries about galaxy clusters.”

Mysteries such as why the number of blue, star-forming galaxies seems to diminish as we move to nearer, older galactic clusters. The process would seem to involve supermassive black holes expelling gas from their host galaxy through powerful eruptions, stifling the star formation process so that only older, redder stars are left. That sequence is thought to consume about a billion years. Says Jason Eastman, also of OSU:

“In a few nearby clusters we’ve seen evidence for huge eruptions generated by supermassive black holes. But this is sedate compared to what might be going on in younger clusters.”

Active galactic nuclei

Keep going back in time and it appears that AGN may be still more common around ten billion years ago, when most of these clusters were forming. As we learn more about the enigmatic black holes at the center of some galaxies, we’re beginning to understand how influential they must be in shaping the development of the galaxy clusters in which they reside. Supermassive black holes can be found outside clusters as well (more commonly in the early universe), but they do not appear nearly as frequently as in the cluster environment.

Image: An artist’s conception of a rapidly growing black hole, otherwise known as an active galactic nucleus (AGN), in the center of a galaxy. A disk of hot gas is flowing into a central black hole, and is surrounded by a large doughnut or torus of cooler gas and dust. Earlier in the history of the universe, galaxies in clusters of galaxies are thought to have contained a lot more gas than galaxies in clusters do today. This abundance of fuel should mean that the piranha-like black holes were able to thrive in young, distant clusters by growing much faster than their counterparts in nearby clusters. Chandra observations have confirmed this by showing, for the first time, that there are more AGN in younger, more distant galaxy clusters. This illustration also shows jets of high energy particles (white) that are propelled away from the vicinity of the black hole by intense electric and magnetic fields. These jets can heat the gas in galaxy clusters and significantly affect their structure. Credit: NASA/CXC/M.Weiss.

The paper is Eastman et al., “First Measurement of a Rapid Increase in the AGN Fraction in High-Redshift Clusters of Galaxies,” Astrophysical Journal Vol. 664 (July 20, 2007), pp. L9-L12 (abstract).