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Jupiter-sized Planet Discovered Orbiting Epsilon Eridani

Keith Cowing 
Thursday, August 03, 2000

Editor's note: A total of ten new extrasolar planets were announced this morning at a meeting of the International Astronomical Union (IAU) in Manchester, UK. The total number of extrasolar planets now known is 50. Details on all of these discoveries can be found in an article on SpaceRef.

A Jupiter-sized planet has been discovered orbiting the nearby star Epsilon Eridani. The announcement was made by Dr. William Cochran of the McDonald Observatory in conjunction with a presentation at the IAU Symposium on "Planetary Systems in the Universe" being held in Manchester, England.

Epsilon Eridani is a sun-like (K2V orange-red dwarf) star located in the constellation Eridanus 10.5 light years from Earth. Epsilon Eridani is much younger than our sun (a G2 star) with an age estimated at less than one billion years. It is also smaller and cooler with 79% the mass, 80% the diameter, and 34% luminosity of our sun. The metallicity of Epsilon Eridani is also lower than our sun (i.e it is less enriched in elements heavier than hydrogen) with only 58% of our sun's iron. A variety of names and catalog nomenclature have been assigned to this star: 18 Eridani, HR 1084, Gl 144, SAO 130564, BD-09 697, FK5 127, Hip 16537, HD 22049, and LHS 1557.

The newly-discovered planet is in an elliptical and somewhat eccentric orbit with a semi-major axis of 297 million miles - a distance roughly equivalent to the distance that the asteroid belt is from our own sun. The planet is suspected of being a gas giant ranging from 80% to 160% the size of Jupiter and takes approximately 7 years to complete one orbit.

The world wide team

The discovery was made by Dr. William Cochran and Dr. Artie Hatzes of the McDonald Observatory at the University of Texas at Austin. According to an IAU press release "the research was based on a combination of six independent data sets taken with four different telescopes and with three different measurement techniques. Observations were made with the 2.7-meter (107-inch) Harlan Smith Telescope at McDonald Observatory, a UT Austin research facility located 16 miles north of Fort Davis."

This was truly a global, community-wide collaboration with collaborators spread out around the world:

According to a University of Texas press release, "to arrive at its discovery, the team studied nearly 20 years of high-precision radial velocity (RV) measurements of Epsilon Eridani. The team noted that the star's high level of chromospheric activity is consistent with its relatively young age, less than a billion years old. "We looked very hard at several years worth of spectrophotometric data for Epsilon Eridani, to make sure that the star's low RV -- 19 meters per second -- was not due to periodic stellar cycles," stated Dr. Artie Hatzes of the McDonald Observatory. "Especially helpful were the Ca II H and K S-index measurements that Dr. Sallie Baliunas, our collaborator at the Harvard-Smithsonian Center for Astrophysics, had made and analyzed of 100 lower main sequence stars, Epsilon Eridani among them."

First hints of planets found in 1998, 1999, and 2000

This discovery did not pop up out of nowhere. Instead, it was the latest in a methodical sequence of events which gained new momentum in recent years. In July 1998 by the Joint Astronomy Centre (JAC), the University of California at Los Angeles, and the Royal Observatory in Edinburgh, announced the discovery Images published in 1998 (right) included a large prominent bright spot within the dust ring that was thought to be indicative of a possible large planet. Closer to the star, the dust has been removed by some process. According to the JAC "a region near the star that is partially evacuated indicates that planets may have formed ... the presence of planets is the most likely explanation for the absence of dust in this region because planets absorb the dust when they form."

Subsequent comparisons between the observations made of this dust cloud and what has been observed in our own solar system led astronomers to further suspect that there was some sort of planetary system around Epsilon Eridani.

A paper presented at the 30th Annual Lunar and Planetary Science Conference in Marc 1999 reported that 'Numerical simulations of the orbital evolution of dust particles from Kuiper Belt objects show that the four giant planets, especially Neptune and Jupiter, impose distinct and dramatic signatures on the overall distribution of Kuiper belt dust particles. The signatures are very similar to those observed in Epsilon Eridani. Numerical simulations of dust particles in Epsilon Eridani show that if the features on the dust disk are caused by a planet, its mass has to be smaller than that of Jupiter but much larger than that of the Earth."

In March 2000, a paper was presented at the 31st Annual Lunar and Planetary Science Conference which also compared observations of Epsilon Eridani's dust cloud with numerical simulations of the dust cloud. The authors reported that "micrometer-to-millimeter sized interplanetary dust particles in our Solar System are created primarily from asteroid-asteroid collisions, from comet disintegration near the Sun, and from mutual collisions between the Edgeworth- Kuiper Belt (EKB) objects. Similar processes are likely to exist in other planetary systems as well. What makes the Epsilon Eridani disk differs from other circumstellar dust disks is the azimuthal brightness variation along the ring. If one compares the pattern of the variation with the modeled structures of the EKB dust disk , one finds global similarities between the two. Therefore, a simple explanation for the observed structures in Epsilon Eridani is that they are caused by perturbations from objects (planets) orbiting the star."

There may yet be additional planets awaiting discovery. According to the UT press release "The irregular shape of this ring may be due to another, undiscovered planet. "If there is indeed a second planet, the asymmetry of the disk would suggest that the planet is orbiting just inside the ring, at a distance of 30 AU -- much farther out than the planet we have found and with a much longer orbital period than the one we've discovered," according to Hatzes. "Thus, it might also be responsible for the possible overall slope in our velocity measurements. And where there's one planet, there may be more."

Too young for life?

In 1960 radio astronomer Frank Drake and his team pointed a radio telescope at several nearby stars (including Epsilon Eridani) as part of Project Ozma and listened for signals from extraterrestrial intelligence. None were heard. What they did not know at the time is that Epsilon Eridani, while indeed similar to our sun, is probably much too young to have planets with life - much less intelligent life - upon them.

The first solid evidence of life on Earth has been found to date from 3.85 billion years ago. Our Sun is estimated to be 4.5 billion years old. As such, there was a time interval approaching 1 billion years before life managed to gain a foothold on Earth. Intelligent life took another 3.85 billion years to appear. Estimates of the age of Epsilon Eridani range from 500 million to a billion years. As such, were it to be assumed that events are occurring at the same pace and along the same path as they did in our solar system, conditions on such putative habitable planets would just now be ripe for life's initial appearance.

The dust cloud surrounding Epsilon Eridani is suspected of having as much as 1,000 times more dust than the inner regions of our own solar system. JAC researchers suggest that this may mean that "it has about 1,000 times more comets". As solar systems form, it is expected that rather intense bombardment of all planets occurs - and that this continues for quite some time.

In our solar system this period of heavy bombardment lasted for its first 600 million years. It was thought that during this time conditions on Earth (and elsewhere) would have been so chaotic that life would have had little chance to arise or (if it did) to thrive before being wiped out. Indeed, many think that life may have arisen more than once on Earth before finally taking hold. As a tribute to life's tenacity it would seem that the moment that life could arise on Earth - it did. The window between the end of the bombardment and the oldest known fossils (at least those that have survived) is only 50 - 100 million years.

Large planets - life's friend or foe?

The presence of large planets can have beneficial and detrimental effect upon the chances for habitable planets to form and for conditions amenable to life to be maintained for long periods of time. In a SpaceRef story earlier this year, "Hot Jupiters and Rare Earths: Planets are common. Are we?", the effect of the formation (and location) of large jovian class planets and the formation of habitable Earth-like worlds was examined.

According to one model, giant gas planets may push each other around. If, for example, you have 3 large gas planets forming in a solar system out in regions where ices are abundant, they will excite each other's orbital eccentricities. Over time this will cause orbits to shift around. Eventually planets will actually swap their relative order with respect to one another and their parent star. As this happens, the inner most planet moves outward, the outermost planet moves inward, and the planet in the middle is ejected from the solar system altogether While this model tends to explain why many of the hot Jupiters discovered thus far have high orbital eccentricities, this is not the leading explanation.

The other, more accepted model involves interactions between forming planets and with the dust disk from which they are forming. An analog to this process has been observed in Saturn's giant ring system. Small saturnian moons orbiting close to the rings tend to disrupt the organization of the rings. In so doing, spiral density waves are produced. These perturbations result in movement of the moons themselves via gravitational interactions. On the larger scale of a solar nebula, so the theory goes, spiral density waves clear out a space within the developing dust disk and locks the planet into a position in the cleared space. Later, as nebula moves inward and material falls into the sun, the planet moves inward, hence closer to the local star.

As is always the case with the universe one thing always affects another. The closer a large Jupiter-sized planet is to its local star during planetary formation, the smaller the chance that Earth-like planets will form. Evidence of this effect can be seen in our own solar system in the asteroid belt between Mars and Jupiter where Jupiter's gravitational influence has either hindered planetary formation or caused objects to collide and fragment before they achieved planetary size. As such, if the object is to find more potential Earths, then the emphasis should be to search within solar systems wherein large gas giants form further out than those currently known.

More to come

This discovery brings the total known collection of extrasolar planets to 41. Within moments of its formal announcement in Manchester on August 7th it is expected that a number of additional discoveries will be announced from the very same podium. Dr. Michael Mayor of Geneva Observatory will announce the discovery of 7 new planets. The veteran planet hunting team of Dr. Geoffrey Marcy (UC Berkeley) and Dr. Paul Butler (Carnegie Institution of Washington) will announce the discovery of 3 new planets (one of which is also cited by Dr. Mayor). Combined with the announcement of the planet circling Epsilon Eridani, this will cause the known collection of extrasolar planets to grow to 50.

In addition to announcing these 10 new worlds, it will also be revealed that one of these bodies is rather small (comparatively speaking) with a size very close to that of Saturn. In addition, research will be presented that suggests that many previously identified extrasolar planets appear to inhabit solar systems with more than one planet.

Persistence pays off

Suspicions have been held about planets circling Epsilon Eridani for quite some time. In 1974, Peter van de Camp published an article "Parallax and Orbital Motion of Epsilon Eridani" wherein he stated that "the star appears to have a perturbation with a period of 25 yr and a semimajor axis of 0."0191 + 0."0018; the mass of the unseen companion has a minimum value of 0.0006M_sol." This article was published after making 900 photos of Epsilon Eridani on 238 nights over the interval 1938-1972.

Planet hunting is not a new pursuit by any means. However, the tools have gotten better. None the less, persistence is still a prerequisite.

In a press release issued by the International Astronomical Union, William Cochran said "the discovery of the new planet circling Epsilon Eridani raises the tantalizing possibility of detecting planets with longer orbital periods and of detecting multiplanet systems like the solar system. Epsilon Eridani is located in one of the 10 nearest star systems and is bright enough to be seen with the naked eye. "You can go outside at night, even in Austin, and point at it and say that star there has a planet around it," Cochran said."

There will be a few extra people looking skyward tonight.