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The CfA Almanac Vol. XIII No. 1, March 2000
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For the first time, astronomers--including the CfA's David Latham and graduate student David Charbonneau--have directly detected an extrasolar planet by watching its shadow cross the disk of a Sun-like star. By observing this "transit" of a planet in front of the star HD209458, scientists have taken a giant step toward learning about the nature of planets outside our Solar System.

Previously, astronomers had found many "extrasolar planets," bodies that orbit stars other than our Sun, indirectly. By observing the subtle gravitational effects that an orbiting body has on its parent star, astronomers have been able to detect large planets around more than a score of planets. (In fact, this technique was pioneered by Latham. In addition, Charbonneau's CfA advisor, Robert Noyes, and his colleagues, earlier used so-called radial velocity measurements made at the Whipple Observatory to find the first trio of planets orbiting a Sun-like star.)

By measuring the slight dimming of the star's light as the planet passes in front, astronomers can deduce, in addition to mass and radius, other critically interesting properties, such as the surface gravity (coincidentally, HD209458 has roughly the same value as the Earth!) and average density of the transiting planet. Gleaning this data from their observations, the team suspects that the planet orbiting HD 209458 is a giant gas planet with a density of approximately 0.38 g/cm3, much less than Saturn, the least dense of our Solar System's own gas giants.

light curve of planet transiting HD 209458

The observed light curve of the gas giant planet transiting HD 209458 by Charbonneau et.al.

Transit observations of the same star, but covering less than one full transit, were made later by a second team. The work by the CfA team, on the other hand, covered two full transits, thus providing stronger and more accurate evidence for the discovery.

Other members of the discovery team include Timothy Brown (High Altitude Observatory, National Center for Atmospheric Research) and Michel Mayor (Observatoire de Geneve). Brown, who is Charbonneau's advisor during his current stint at NCAR, is the leader of the STARE project, a photometric survey to search for "hot Jupiters" around tens of thousands of Sun-like stars. Brown's telescope for this project was used to detect the transits.


The development of low-field magnetic resonance imaging (MRI) by SAO scientists has been cited as one of the outstanding developments in current physics. A typical MRI device uses a huge, high-field magnet to polarize hydrogen nuclei inside water molecules in the human body. The spinning molecules produce radio signals that can be used to image most organs in great detail to detect tumors, for example.

SAO researchers, led by Ron Walsworth, use lasers to increase the nuclear spin-polarization of inert gases like helium, enabling MRI of the inhaled gas in the lung, the sinuses, and other body cavities where MRI has been ineffective. This new biomedical imaging technique, a spin-off of research in atomic physics, is only about five years old. Already, doctors are using laser-polarized gas MRI to diagnose and plan treatment for people with lung diseases, such as emphysema and asthma.

The SAO innovation, developed in cooperation with the Massachusetts General Hospital, uses small, low-field magnets for MRI of laser-polarized gas. It promises much simpler, less intimidating, and lower-cost MRI units in hospital settings, as well as portable instruments for use in remote, cramped environments, such as space vehicles. Recently, the SAO group has begun to apply laser-polarized gas MRI in other fields, such as probing the porous structure of rocks that can hold oil, natural gas, and subterranean water. These innovations demonstrate the vital synergy between basic science and practical applications and the role SAO plays in making these connections.


For the first time, the Smithsonian Astrophysical Observatory (SAO) Bibliography is available online. The comprehensive list of SAO publications appearing between October 1, 1997, through September 30, 1998, that will later appear in Smithsonian Year 1998, Annals of the Smithsonian Institution, can be found at http://cfa-www.harvard.edu/cfa/ep/98SAOBIB1.html.

Also, the abstracts of the CfA Preprint Series are now available either via http://cfa-www.harvard.edu/cfa/ep/abstracts.html, or, through an email listserver. To request a free subscription to the CfA Preprint listserver, send a message to majordomo@cfa.harvard.edu. In the body of the message, enter "subscribe pre-abs" on the top message line. The subject line is not relevant. (Similarly, use the "unsubscribe pre-abs" command to remove yourself from the list.)

In addition, full papers in the CfA Preprint Series (starting with No. 4777) are now available through the NASA Astrophysics Data System (ADS) and linked to the pre-abs mailing list: http://adswww.harvard.edu/cfa/.


The popular image of nascent planetary systems as thin, spinning pancakes of cosmic dust and debris may be changed by a new computer model that shows how that disk of debris is transformed into a very distinct ring once Pluto-like bodies start to form.

By analyzing Hubble Space Telescope images of a suspected young planetary system recently discovered around the star HR 4796A, Scott Kenyon and Kenny Wood of the CfA and Barbara Whitney and Michael Wolff of the Space Science Institute have produced a computer model that suggests the ring around that object probably is a common feature of all planetary systems. Indeed, the well-known Kuiper Belt of asteroids in our own Solar System may even be the residual remains of such a ring.


A space experiment with major contributions from the CfA has been selected for NASA's Medium-Class Explorer, or MIDEX, program, and scheduled to be launched in 2004.

The Full-Sky Astrometric Mapping Explorer (FAME) is an Earth-orbiting optical telescope that will gather information on roughly 40 million stars in the Milky Way galaxy with unprecedented measurement accuracy. For bright stars, positions will be determined to the equivalent of the width of a footprint on the Moon as seen from Earth (50 millionths of a second of arc). This exacting precision is central to the study of key issues of scientific and general interest including the existence of other "solar systems," the size and age of the Universe, and an investigation of the mysterious "dark matter" in our portion of the Galaxy.

Artist's conception of FAME satellite. (Courtesy of US Naval Observatory)

"FAME will use a solar sail instead of thrusters to provide the propulsive force needed to reorient itself to scan the entire sky," according to NASA's Ed Weiler. "And, this invention by Project Scientist, Robert Reasenberg of the CfA, will increase the accuracy of the astrometric determinations and open the way for a highly extended mission by obviating a major use of consumables."

"FAME will increase by more than 1000-fold the volume of space in which we can determine the distances to stars. By using the parallax method, we will directly determine the lower rungs of the 'cosmic distance ladder,'" says Reasenberg. "In addition, the star coordinates determined by FAME will be more than 20 times more accurate than any available today, opening the way for a rich scientific yield from the mission and producing a resource for future researchers."

"By measuring the wobbling of star positions, FAME will discover companions, including 'brown dwarfs' and giant planets," says James Phillips of the CfA, who serves as Deputy Project Scientist. "Because of the large number of stars FAME will observe, it will provide the first statistically useful survey of such companions and elucidate the transition region between brown dwarfs and giant planets."

In addition to determining the positions, motions, and distances of the stars, this satellite will measure the brightness of stars in each of several color bands, repeatedly during the mission, to achieve millimagnitude accuracy for bright stars. When combined with the distance measurements, this photometric information will permit a determination of stellar type and intrinsic brightness, and will contribute to an understanding of the evolution of stars.

Four other CfA scientists are participating in the FAME mission. John Geary is a member of the project team, to which he lends his expertise on detectors. John Huchra, David Latham, and Irwin Shapiro are members of the science team and will conduct scientific investigations with the FAME data. The FAME mission also has an educational component. The major portion of that will be conducted by the CfA Science Education Dept. under the leadership of Philip Sadler and Bruce Ward. FAME is a collaborative effort of the U.S. Naval Observatory, the Naval Research Laboratory, Lockheed Martin Missiles and Space, and SAO.


Until just a few years ago, many astronomers believed the planet Uranus was a bit strange. That's because, unlike the other giant members of the Solar System, Uranus did not appear to have any so-called irregular satellites, or, distant moons with unusual orbits. However, recent observations have found what appear to be three new irregular moons around Uranus, thus suggesting that the seventh planet from the Sun is just one of the gang after all.

An international team of astronomers, including Matt Holman of the CfA's Planetary Sciences Division, used the Canada-France-Hawaii Telescope (CFHT) to find these extremely faint objects. If confirmed, and tallied with two other irregular satellites discovered in 1997, Uranus would have 16 regular and five irregular moons, making it the most populated planetary satellite system known.

Irregular satellites do not follow the normal, near-circular orbits of most satellites, such as the Earth's Moon. Instead, these irregular objects either travel in highly elliptical orbits, or follow paths that are severely tipped to the plane of the planet's equator.

"The discovery of these irregular satellites is very important because it means that Uranus is not some oddball, but rather is just like Neptune, Saturn, and Jupiter," says Holman. "It might help us better understand how the irregular satellites of the giant gas planets originated and how they've evolved."

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