Science News
Washington, D.C.

Ron Cowen

June 26, 1998

Astronomers find a planet orbiting one of the closest stars to Earth

Two teams of astronomers this week reported that one of the sun's nearest neighbors -- a star just 15 light-years from Earth -- possesses a planet 1.6 times as massive as Jupiter. The finding is reported in the June 27 issue of Science News, a weekly news magazine.

As with other planets recently discovered, this object was not imaged, but betrayed its presence through its gravitational tug on its parent star. A leader of one of the teams, Geoffrey W. Marcy of San Francisco State University and the University of California, Berkeley, reported the finding on June 22 at a symposium of the International Astronomical Union in Victoria, British Columbia. Marcy and his colleagues used telescopes at Lick Observatory and the Keck I telescope atop Hawaii's Mauna Kea to detect a telltale wobble in the motion of the nearby star Gliese 876. Just 2 hours after his presentation, a colleague presented him with an e-mail from a team led by Xavier Delfosse of Geneva and Grenbole observatories. The message said that the team had confirmed the finding. These astronomers used telescopes at the Haute-Provence Observatory in France and the European Southern Observatory in La Serena, Chile. Details about the planet, which has some intriguing properties, appear in the June 27 issue of Science News.

The entire Science News article is available at http://www.sciencenews.org/sn_arc98/6_27_98/fob3.htm

Technical information presented at the IAU symposium in Victoria is available at http://cannon.sfsu.edu/~gmarcy/planetsearch/gl876/gl876.html


PHYSICS NEWS UPDATE

The American Institute of Physics Bulletin of Physics News

Number 379, June 25, 1998

by Phillip F. Schewe and Ben Stein

NEAREST EXTRA-SOLAR PLANET

The existence of a planet around the star Gliese 876, only 15 light years distant from Earth, was announced this week by planeteer Geoffrey Marcy of San Francisco State University (SFSU) at a meeting in Victoria, British Columbia. The star, whose presence is inferred not from direct observation but by the wobble it imparts to the star, has a mass about 1.6 that of Jupiter. Gliese itself only has a mass of about one third that of our sun, making it the lightest known star to have a planet. The planet circles the star every 61 days at a radius of one-fifth the Earth-Sun distance. The discovery was soon confirmed by other astronomers. (Science News, 27 June 1998.)

Still other extra-solar planets (perhaps a half dozen) will be presented by several observing teams at a meeting a week from now in Santa Barbara. (Science NOW, 24 June 1998.)


SEARCHING FOR EXTRASOLAR PLANETS

USEFUL LINKS:


May 12, 1997

Astronomers Geoffrey Marcy and R. Paul Butler describe how they detected six planets orbiting other stars in a feature article for Encarta Encyclopedia's Yearbook.

April 24, 1997

A planet around pulsar Geminga?


Harvard-Smithsonian Center for Astrophysics

April 24, 1997

NEW EXTRA-SOLAR PLANET DISCOVERED; CIRCULAR ORBIT SUGGESTS IT FORMED LIKE PLANETS IN OUR SOLAR SYSTEM

CAMBRIDGE, MA -- The notion that giant, Jupiter-like bodies may be a common occurrence around stars like the Sun has been bolstered by the discovery of such an object orbiting Rho Coronae Borealis, a star in the constellation Northern Crown. The newly discovered planet offers additional evidence for how such systems form, and bolsters the idea that other worlds like our own may be widespread throughout the galaxy.

The discovery was made by a team of scientists from three institutions -- the Smithsonian Institution's Astrophysical Observatory (SAO) in Cambridge, MA, the National Center for Atmospheric Research (NCAR) in Boulder, CO, and the Pennsylvania State University in State College, PA -- based on observations made at the Smithsonian's Fred Lawrence Whipple Observatory on Mt. Hopkins, Arizona.

The scientific team includes Sylvain Korzennik, Martin Krockenberger, Peter Nisenson, and Robert Noyes of SAO; Harvard University graduate student Saurabh Jha; Timothy Brown and Edward Kennelly of NCAR; and Scott Horner of Penn State.

Using a special instrument known as the Advanced Fiber Optic Echelle (AFOE) spectrograph located at the 1.5-meter Tillinghast Reflector of the Whipple Observatory, the scientists detected extremely small variations in the recession velocity of Rho Coronae Borealis that are thought to be caused by the presence of an orbiting companion.

With the AFOE capable of measuring velocity variations smaller than 10 meters per second (about 22 miles per hour), the scientists found that the speed of Rho Coronae Borealis varied back and forth by about 67 meters per second, or 150 miles per hour, over a 40-day period. This led the team to conclude that the star has a companion in a 40-day orbit and, from the size of the velocity variation and the mass of the star (almost identical to the Sun), they calculated that the companion must be slightly more massive than the planet Jupiter.

The short orbital period means the planet must lie only about 1/4 of an Astronomical Unit from the star -- closer than Mercury orbits the Sun (an AU is the distance of the Earth from the Sun). This also implies its temperature would be about 300 degrees C, or more than 500 degrees F -- much too hot for liquid water to exist, and hence not a likely place for life to form.

According to the researchers, the circular nature of the orbit suggests that the planet was formed like the planets in our own solar system, that is, through the slow coalescence of dust and gas from the circularly rotating disk that is thought to surround all newborn stars. A more eccentric, or highly elliptical orbit, could imply that the companion object was a failed star, the unsuccessful second partner in a potential binary star system.

"This discovery helps show that giant planets like Jupiter may be reasonably common around ordinary stars," says Robert Noyes of SAO. "Moreover, they can be found at a variety of distances from their parent stars, ranging from very close in, like the companion to 51 Pegasi, to very far away, like Jupiter relative to the Sun. The planet around Rho Coronae Borealis, like several others, is in between.

"It is exciting to think that there may be many smaller planets much more like the Earth in orbit around these stars, as in our own Solar System," says Noyes.

Timothy Brown, of NCAR, carried out the design and fabrication of the AFOE spectrograph's optics. He added, "All the giant planets found so far orbit Sun-like stars. The star Rho Coronae Borealis is another one of these, but it appears to be about 10 billion years old -- twice as old as the Sun."

Scott Horner, of Penn State, designed and built the AFOE's iodine cell (a precise velocity-reference device). "It was the star's solar similarity that led us to target it for study in the first place," he agreed. "Soon after we began to look at it, we thought that its radial velocity was varying. Now, after 11 months of monitoring, we're sure."

As one of the stars forming the "crown" of the constellation, Rho Coronae Borealis is visible from February through September to naked-eye observers in the Northern Hemisphere . It is about 50 light years from Earth.

A scientific paper describing the discovery has been accepted for publication in the Astrophysical Journal Letters. A pre-publication version of the paper has also been made available, along with other details about the AFOE program, on the World-Wide Web at http://cfa-www.harvard.edu/afoe. NCAR is sponsored by the National Science Foundation.

Figure available as an anonymous ftp at:
ftp://cfa-ftp.harvard.edu/pub/afoe/np.ps

Caption:
Graphic representation of variations in the velocity of the star Rho Coronae Borealis observed at the Smithsonian Institution's Whipple Observatory in Arizona, showing a 40-day period indicative of a Jupiter-sized companion.


16 GYG B: ANOTHER EXTRASOLAR PLANET

October 23, 1996

Today, during a Meeting of the Division of Planetary Sciences in Tucson, Arziona, the finding of a new extrasolar planet was announced by Drs. William Cochran, Artie Hatzes and the team of Geoff Marcy and Paul Butler. Star: 16 Cyg B = HR 7504

Spectral Type: G2.5
Distance: 85 light years
Object Mass: 1.6 Mjupiter
Distance from primary: 0.6 to 2.7 AU
Orbital Period: 804 days (= 2.2 years)
Eccentricity: 0.65


There are now at least 8 confirmed extrasolar planets found around normal stars (a few more were found around pulsars). Next to 61 Cygni B we have 51 Pegasi, 47 Ursae Majoris, 55 Cancri, Tau Bootis, Upsilon Andromedae, 70 Virginis, HD 114762 and possibly Lalande 21185. Check out the SFSU-site on the discovery of extrasolar planets by Eric Williams.

A good overwiew on all extrasolar planets discovered so far can be found on Darwin's page by Alan J. Penny. Darwin is a space-based infrared interferometry which could detect Earth-like planets around nearby stars, and look fror ozone in their spectra. Large amounts of ozone can only be present if the atmosphere is rich in oxygen, which in turn can only be produced if there is life on the planet. The Darwin project proposal is currently being studied by the European Space Agency as one of two options for their Cornerstone 6 mission for launch in 2015.


SFSU Public Affairs Press Release

Published by the Communications / Public Affairs Office at San Francisco State University, Diag Center.

October 22, 1996

SFSU RESEARCHERS DISCOVER NEW PLANET WITH OBLONG ORBIT

"Eccentric" orbit shatters long-held theory of circular orbits
SAN FRANCISCO -- A remarkable new planet around a Solar-like star (16 Cygni B) has been discovered by Drs. Geoff Marcy and Paul Butler of SFSU, and Drs. Bill Cochran and Artie Hatzes of the University of Texas - two teams working independently. This planet orbits its star with the most extreme ``eccentricity'' (i.e., oblong shape) ever found for any planet, e = 0.6, on a scale of 0 to 1. All of the planets in our Solar System reside in nearly circular orbits, having eccentricities less than 0.2. This new planet dismantles the long-held theory that other planets in the universe would all have nearly circular orbits.

The discovery was made by measurements of the Doppler shift of the light from the Solar-type Star, 16 Cyg B, (spectral type = G2.5) which is 85 light years from Earth. The star exhibits a periodic Doppler variation, with a period of 804 days (= 2.2 years). The star changes its velocity by +- 46.5 meters/sec every 2.2 years, in a pattern that is NOT a perfect sine wave.

This wobble implies that a planet orbits the star with an orbital period of 2.2 years and has a mass of at least 1.5 Jupiter masses. The actual mass of the planet may be slightly greater than 1.5 Jupiter masses, the uncertainty being due to the unknown tilt of the orbit plane which enters into the orbital physics (as the trigonometric sine of inclination).

Of extreme importance is the unprecedented eccentricity of the orbit, unlike that for any other planet. Its orbit carries the planet from a closest distance of 0.6 Earth-Sun distances to 2.7 Earth-Sun distances at its farthest from its host star, 16 Cyg B. The planet would experience extreme variations in the heat energy it receives from its star, as it varies from Venus-like distances to Mars-like distances.

The oblong shape of the orbit is easily determined from the graph of Doppler-shift versus time. This graph is not a sine wave, which occurs for circular orbits. The departure from a sine wave is due to the speeding-up of the planet as it rounds the star at closest approach, much as the sound of a car engine changes pitch (also by the Doppler effect) as it rounds a sharp curve.

This planet adds to the mystery of a previously discovered planet around the star, 70 Virginis (discovered by the SFSU Marcy and Butler team). Its planet also has a large eccentricity of 0.4 , the previous record holder. But that non-circular orbit was so discordant with the expected circular orbits from theory, that some theorists hoped it could be dismissed as a failed star (i.e., a "brown dwarf"), thereby ignoring the problem of how a planet (or any object) might become so eccentric. Now, new theories must be found to explain these two eccentric planets. Proposed theories involve collisions of two planets that scatter them into wacky orbits (Doug Lin, UC Santa Cruz and Fred Rasio MIT) , or gravitational perturbations from the disk of gas and dust out of which the planets formed (Pat Cassen of NASA Ames Research Center, and Pawel rtymovicz of Stockholm Observatory).

This new planet was discovered completely independently by two teams: Drs. Bill Cochran and Artie Hatzes from the University of Texas and Drs. Paul Butler and Geoff Marcy of San Francisco State University and U.C. Berkeley. Each team has an ongoing, extremely sensitive technique for measuring the Doppler shifts of stars, designed explicitly to detect the perturbations imposed on the stars due to the gravitational force exerted on it by orbiting planets. This planet represents the sixth planet discovered by the team of Butler and Marcy, and brings the total of known planets outside our Solar System to eight.

TECHNICAL SOLUTION

Formally, this is the solution for 16 Cyg B from the COMBINED measurements of both teams. The San Francisco State team provides Doppler measurements that have better precision (8 m/s compared with 27 m/s). But both teams detect virtually the same orbit.

ORBITAL ELEMENTS
P=      804.4 days      s.e. =  12.4
T (JD)= 48941.508 J.D.  s.e. =  10.523
K =     46.592 m/sec    s.e. =   8.219
e =     0.666           s.e. =   0.091
omega = 86.807 degrees  s.e. =  12.908

PHYSICAL ELEMENTS
a*sin(i)   =    3.84328E+08 meters          s.e. =  6.21915E+07
f(m)       =    3.49068E-09 solar masses    s.e. =  1.69798E-09

One deduces that 16 Cyg B is about 1.0 solar mass, as it's spectrum (G2.5 V) is nearly the same as the Sun's (including age and metalicity). Indeed, it is often deemed a ``Solar Twin''.

This gives a companion mass of :

M_comp = 1.52/sin i Jupiter masses.

The semimajor axis of the planet about the star is:

a = 1.7 AU (1.7 earth-sun distances) coming directly from Kepler's 3rd Law.


SKY & TELESCOPE NEWS BULLETIN
JUNE 15, 1996

THE FAMILY OF LALANDE 21185

At last week's meeting of the American Astronomical Society, George Gatewood of Allegheny Observatory announced evidence for not one but two planets around a star in our neighborhood. Lalande 21185, 8.2 light-years away, is the sixth-nearest star to our Sun. Gatewood bases his claim on two sets of observations: a 50-year sequence of photographic plates taken by Allegheny's 30-inch refractor, and 8 years of tracking with an ultraprecise photometer. After allowing for the star's parallax and proper motion, Gatewood sees a 30-year wobble in the star's motion, implying that a Jupiter-mass planet circles Lalande 21185 at a distance of 11 astronomical units -- an orbit slightly larger than Saturn's around our Sun.

What's more, the photometer data also hint that a second body, slightly lower in mass, is about 2.2 a.u. from the star. Since the orbits appear to be titled by at most 30 degrees to our line of sight, each planet should be causing the star to move toward and away from Earth at 25 to 30 meters per second. But at least one team has watched Lalande 21185 spectroscopically for just this type of motion, without success. Lalande 21185 has a visual magnitude of 7.5 and is located in Ursa Major at right ascension 11h 03m 20s, declination +35 58.2'.


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