Survey Finds Mysterious ‘Dark Accelerators’

The central part of the Milky Way has never been surveyed in gamma ray wavelengths with the sensitivity offered by HESS, the High Energy Stereoscopic System. And as announced in the March 25th issue of Science, the HESS team has not only found eight new very high energy (VHE) gamma ray sources in the galactic disk, thus doubling the number of known sources, but has also discovered two ‘dark accelerators,’ objects that emit energetic particles but have no known optical or x-ray counterpart.

It takes a particle accelerator of cosmic proportions to produce gamma rays, such as the explosion of a supernova. But such sources should be visible in other wavelengths. Says Dr. Paula Chadwick of the University of Durham (UK):

“Many of the new objects seem to be known categories of sources, such as supernova remnants and pulsar wind nebulae. Data on these objects will help us to understand particle acceleration in our galaxy in more detail; but finding these ‘dark accelerators’ was a surprise. With no counterpart at other wavelengths, they are, for the moment, a complete mystery.”

From a press release from the Particle Physics and Astronomy Research Council:

Another important discovery is that the new sources appear with a typical size of the order of a tenth of a degree; the H.E.S.S. instrument for the first time provides sufficient resolution and sensitivity to see such structures. Since the objects cluster within a fraction of a degree from the plane of our Galaxy, they are most likely located at a significant distance – several 1000 light years from the sun – which implies that these cosmic particle accelerators extend over a size of light years.

The HESS array is located in Namibia, a system of four 13-meter telescopes that comprises the most sensitive detector of VHE gamma rays in the world. Because gamma rays are absorbed by the atmosphere, the HESS detectors work indirectly, collecting the blue ‘Cherenkov light’ the rays emit upon being absorbed. HESS uses this information to construct images of astronomical objects as they appear at these wavelengths.

The paper on the HESS results is Aharonian et al., “A New Population of Very High Energy Gamma-Ray Sources in the Milky Way,” Science 307: 1938-1942 (2005).

Defining Habitable Zones in the Galaxy

When is a planet habitable? The assumption, in studies of the ‘circumstellar habitable zone’ (CHZ) ranging back as far as 150 years, is that a planet is habitable if liquid water can be maintained on its surface. That this is a ‘life as we know it’ scenario is obvious: it works best if you assume a planetary system not so different from our own, one with roughly the same configuration of planets (gas giants in outer orbits, rocky worlds in close). Venus and Mars have served as test cases of the boundaries of habitable zones.

But our view of habitable zones is evolving. I relied on Stephen Dole’s groundbreaking study Habitable Planets for Man (New York: Blaisdell Publishing Company 1964) in Centauri Dreams. Dole’s work was prepared for the RAND Corporation, and was released in popularized form as Planets for Man, in collaboration with Isaac Asimov (New York: Random House, 1964). Dole defined a stellar ecosphere as “. . . a region in space, in the vicinity of a star, in which suitable planets can have surface conditions compatible with the origin, evolution to complex forms, and continuous existence of land life; and, in particular, surface conditions suitable for human beings, together with the whole system of life forms on which they depend.”

The habitable zone concept has a long history. In an 1853 book called Of the Plurality of Worlds, William Whewell proposed that a ‘temperate zone’ existed where the Solar System was inhabitable. His work was extended by Alfred Russel Wallace in Man’s Place in the Universe (1903), and later Hubertus Strughold introduced the concept of the ‘ecosphere’ as it applied to conditions around the Sun (“The Ecosphere of the Sun,” Aviation Medicine 26, pp. 323-328, 1955). In 1959, Su-Shu Huang discussed the variations in the Earth’s temperature that would occur if it were moved closer or farther from the Sun (“Occurrence of Life in the Universe,” American Scientist 47, pp. 397-402).

It was Michael Hart’s 1979 study “”Habitable Zones About Main-Sequence Stars” (Icarus 37, pp. 351-357) that produced a mathematical model of the circumstellar habitable zone, with an inner boundary marked by runaway greenhouse effects and an outer one by runaway glaciation. Hart’s habitable zone was tiny: the Solar System was friendly to life between 0.958 and 1.004 AU.

Following Hart, a number of studies continued to explore the CHZ with models that took into account our deepening understanding of the greenhouse effect and changes in planetary geophysical processes. But all in all, the concept of a habitable zone between 0.95 and 1.2 AU settled in for our own Solar System, a widening of Hart’s parameters but not by much. This was more or less the conclusion that Dole reached in his 1964 study.

A thorough analysis of work on the concept since Hart now appears in Guillermo Gonzalez’ paper Habitable Zones in the Universe. Gonzales explains why he believes we should keep the habitable zone concept despite our broadening view of where life might exist, including liquid biospheres in the interior of icy moons like Europa and Callisto:

Although these places are located outside the traditional CHZ, some argue that they should also be called habitable zones. Nevertheless, there are several reasons to keep the traditional definition. First, one of the primary motivations for study of the CHZ is to learn how to detect habitable terrestrial planets in other systems. Since we know Earth has been inhabited most of its history, finding another planet like Earth is probably the surest bet to find another inhabited planet. Second, since life requires much more than just liquid water, merely locating other pockets of liquid water will not suffice. Each new potential niche will have to be evaluated on its ability to supply the necessary ingredients for life in the right forms. It has yet to be demonstrated that these other niches can harbor life.

Galactic habitable zoneIf we move beyond stellar habitable zones, we can talk about a galactic habitable zone, one that considers the effects of nearby supernovae and gamma ray bursts, and takes into account the orbits of comets as they may be subject to large-scale gravitational perturbations. Encounters with nearby stars must also be considered. The largest galactic picture studies the metallicity of star-forming nebulae and the evolution of stars in various environments. A full treatment of the concept is “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way,” by Charles Lineweaver, Yeshe Fenner and Brad Gibson, which ran in Science’s January 2, 2004 issue. The article is also available online (PDF warning).

Image: The formation and expansion of the Habitable Zone of the Milky Way Galaxy. In the early stages of galaxy formation (lower panel) there were not enough heavy elements to form terrestrial planets except in the most central regions of the Galaxy, where the danger due to nearby supernovae was very high (shown in red). As heavy elements spread through the Galaxy, terrestrial planets formed and a habitable zone emerged and broadened (shown in green in the upper panels). Credit: Swinburne University, Australia.

A galactic habitable zone also takes into account the concept of Cosmic Habitable Age, the period during which a universe can be inhabited — after all, life cannot form in a cosmos before atoms have formed, or after the stars have burned out. An intriguing 2003 study by Werner von Bloh and colleagues calculated the peak time for Earth-like planets to appear and found it to be near the time of formation of our own Earth.

The von Bloh paper is “Maximum Number of Habitable Planets at the Time of Earth’s Origin: New Hints for Panspermia?,” Orig. Life and Evol. Bios. 33, pp. 219-231 (2003). Other notes: Whewell’s Of the Plurality of Worlds: An Essay (London: John W. Parker and Son, 1853) is still worth reading, as is Alfred Russel Wallace’s Man’s Place in the Universe (New York: McClure, Phillips & Co. 1903).

The Light of Distant Worlds

As discussed in yesterday’s entry, being able to work with actual light from distant planets is a major breakthrough. It opens the possibility of studying characteristics like temperature and atmospheric composition, further fusing astronomy with the nascent science of astrobiology. And with the Spitzer Space Telescope’s proven ability to make such observations, we can expect a whirlwind of exoplanetary data ahead.

A few further details from yesterday’s announcements:

A study of the work on HD 209458b, a ‘hot Jupiter’ that orbits its parent star in 3.5 days, ran in today’s online edition of Nature. The paper is Deming, D., Seager, S. et al., “Infrared radiation from an extrasolar planet.” Dr. Sara Seager of the Carnegie Institution, a co-author of the study, provided more about HD 209458b:

“This planet was discovered indirectly in 1999 and was later found to transit its star–the star dims as the planet moves in front of it during the course of the planet’s orbit. With Spitzer, we first measured the combined light of the planet and star just before the planet went out of sight. Then when the planet was out of view, we measured how much energy the star emitted on its own. The difference between those readings told us how much the planet emitted.”

The result: HD 209458b was found to be a scorching 1,574 F (1130 K).’s news site also offers the article “Light from alien planets,” by Mark Peplow, a useful overview.

Diagram of Spitzer planet findOther facts: HD 209458b lies 153 light years from Earth in the constellation Pegasus. TrES-1 is 489 light years away in the constellation Lyra. And while yesterday’s entry said that both HD 209458b and TrES-1 orbit stars very much like our sun, this is true only of the former; TrES-1’s star is smaller and cooler.

Image: Frame from a video simulation shows in a simplified schematic how the brightness of a star/planet system varies as the planet is eclipsed by the star. The false colors represent infrared images. Credit: NASA/JPL/Caltech-R. Hurt.

The paper on TrES-1 by David Charbonneau et al. is in press at The Astrophysical Journal, with publication slated for the June 20 issue. A preprint of the paper, “Detection of Thermal Emission from an Extrasolar Planet,” is available at the ArXiv site.

Also intriguing on the exoplanet front is planet-hunter Geoff Marcy’s intention to issue a catalog that will cover all exoplanets found to date. Marcy’s own team has thus far found almost 100 planets, and he is now turning to The Planetary Society in search of donations for the catalog.

Why is a catalog necessary? Here’s what The Planetary Society has to say in a recent release:

…most of the data is not available to the scientific community, or to the public at large. Of course, preliminary information about each world was published as it was discovered. But scientists have continued to study them, adding mountains of data to those initial observations.

So, while a massive reservoir of fantastic information about these new worlds now exists… almost no one can tap into it because the new data has never been properly processed and released. There could be critical discoveries buried in those mounds of data that have never seen the light of day.

Centauri Dreams agrees that such an updatable resource would be of scientific importance as well as providing broad educational options for students and teachers.

Confirmed Detection of Extrasolar Planets’ Light

Light from two planets orbiting other stars has now been directly detected by the Spitzer Space Telescope, in findings announced today at a NASA news conference. Spitzer scrutinized both planets using the ‘transit’ method, in which a planet eclipses its star and blocks a small fraction of its light.

The space-based telescope has been able to detect not only the primary transit but the secondary eclipse, occurring when a planet comes out from behind its star on the far side of its orbit. It thus became possible for astronomers to subtract the planetary ‘signal’ from the otherwise overwhelming light of the parent star, the first confirmed detection of the light from extrasolar worlds.

Two view of Spitzer planetsBoth planets fall into the category of ‘hot Jupiters’ — massive worlds that orbit at extremely close distances from their primaries. The first (studied with Spitzer’s Infrared Array Camera) is TrES-1, orbiting its star at a distance of four million miles and boasting a temperature of 1340 degrees Fahrenheit. Spitzer was able to show that the planet’s reflectivity is 31 percent — the planet absorbs, in other words, most of the light that falls upon it.

Image: The artist’s concept above shows what a fiery hot star and its close-knit planetary companion might look like close up if viewed in visible (left) and infrared light. In visible light, a star shines brilliantly, overwhelming the little light that is reflected by its planet. In infrared, a star is less blinding, and its planet perks up with a fiery glow.

Astronomers using NASA’s Spitzer Space Telescope took advantage of this fact to directly capture the infrared light of two previously detected planets orbiting outside our solar system. Their findings revealed the temperatures and orbits of the planets. Upcoming Spitzer observations using a variety of infrared wavelengths may provide more information about the planets’ winds and atmospheric compositions. Credit: NASA/JPL-Caltech/R. Hurt (SSC).

Infrared is a huge plus for detecting extrasolar planets, since in visible light a star outshines its planet by a factor of 10,000, whereas in the infrared the star is only 400 times brighter, making its light much easier to separate from any planetary companion.

“Planets like TrES-1 are tiny and faint compared to their stars, but the one thing they can’t hide is their heat,” said David Charbonneau of the Harvard-Smithsonian Center for Astrophysics (CfA), who led the team making the discovery. “We are like detectives. Previous clues told us the planet must be there, so we put on our ‘infrared goggles’ and suddenly, it popped into view.”

A second ‘hot Jupiter’ is HD 209458b, detected using Spitzer’s Multiband Imaging Photometer for Spitzer (MIPS) by a team under Drake Deming of the Goddard Space Flight Center. Interestingly, this planet’s shape seems distended, too large for its apparent mass. A previous theory, that this ‘puffy’ effect was caused by a non-circular orbit, has now been discounted, as both planets have been shown to follow nearly circular orbits. Each of the planets discussed today had been previously detected by indirect means (i.e., without observing actual light from the planets themselves).

Intriguingly, both stars in question are Sun-like. Two earlier claims for direct detections of planetary light (neither of which have been confirmed) involved other stellar types.

As for Spitzer, the future looks bright indeed. The study of planetary characteristics such as color, reflectivity, and temperature now comes into play. “We’ve caught our first ‘firefly,'” said Charbonneau. “Now we want to study a swarm of them.”

The TrES-1 detection will be published in the June 20th issue of The Astrophysical Journal. A press release from the Spitzer site can be found here. Also check this release from the Harvard-Smithsonian Center for Astrophysics.

Extrasolar Planet Announcement Moved to Today

“Astronomers will announce major findings about planets outside our solar system, known as extrasolar planets, at a NASA Science Update at 3 p.m. EST today.” So says a media advisory just out from the agency. NASA TV plans to carry the event, which will discuss discoveries made by the Spitzer Space Telescope. The panelists:

  • Dr. Drake Deming, chief, planetary systems laboratory, NASA’s Goddard Space Flight Center, Greenbelt, Md.
  • Dr. David Charbonneau, assistant professor of astronomy,
    Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.
  • Dr. Alan Boss, staff research astronomer, Department of Terrestrial Magnetism, Carnegie Institution of Washington
  • Dr. Kim Weaver, moderator; Spitzer program scientist, NASA’s Science Mission Directorate, Washington.