What's a planet? New riddles beyond the solar system.
"I found a planet!" Caltech astronomer Mike Brown remembers exclaiming during a phone call he made to his wife early in 2005. Little did he know that he'd have to eat his words just 18 months later. Brown had found an outer-solar system object heavier than Pluto, so it seemed reasonable to call the object the tenth planet.
But last August, the International Astronomical Union “IAU” redirects here. For other uses, see IAU (disambiguation).
The International Astronomical Union (IAU) unites national astronomical societies from around the world. approved the first formal definition of a planet since the Greeks coined the term some 2,000 years ago. Pluto, got the boot, and Brown's proposed tenth planet, a body now called Eris, was disqualified dis·qual·i·fy
tr.v. dis·qual·i·fied, dis·qual·i·fy·ing, dis·qual·i·fies
a. To render unqualified or unfit.
b. To declare unqualified or ineligible.
The group of astronomers decided to call Pluto and Eris "dwarf planets'--a class that the scientists say is separate from the solar system's eight official planets. That decision remains controversial, although many astronomers say that there's merit in the demotion de·mote
tr.v. de·mot·ed, de·mot·ing, de·motes
To reduce in grade, rank, or status.
[de- + (pro)mote. . Both Pluto and Eris are considerably smaller than the first eight planets discovered. These two orbs ought to be grouped with the swarm of other icy objects in the Kuiper belt Kuiper belt: see comet; Kuiper, Gerard Peter.
or Edgeworth-Kuiper belt
Disk-shaped belt of billions of small icy bodies orbiting the Sun beyond the orbit of Neptune, mostly at distances 30–50 times Earth's distance , a region beyond Neptune that may contain millions of such bodies, Brown says.
Although Pluto's planethood is etched in cultural memory--and on lunch boxes, on postage stamps This is a list of postage stamps that are especially notable in some way.
The best-known stamps:
"The new definition essentially corrects an astronomical mistake from 76 years ago and shows that astronomy can move forward in the face of new information about the solar system," Brown wrote recently on his blog (http://www.gps. caltech.edu/~mbrown/eightplanets/). But even as scientists and the public grapple with Pluto's demotion, new findings from beyond the solar system are eroding the meaning of planet.
Over the past few years, astronomers have found several extra-solar objects that by weight would qualify as planets, yet they lack what would seem to be the most basic of planetary prerequisites--a parent star. Many of these free-floating orphans are surrounded by disks of dust and gas with enough mass to coalesce co·a·lesce
intr.v. co·a·lesced, co·a·lesc·ing, co·a·lesc·es
1. To grow together; fuse.
2. To come together so as to form one whole; unite: into their own miniature solar systems. One of the orphans, which some researchers call planemos, may even have a planetary-mass object orbiting it.
The discoveries are blurring the line between planets and stars--and may bring about a revolution in thinking about planets that goes far beyond the Pluto debate.
OLD THEORIES, NEW DATA In the good old days, circa 1990, stars were stars and planets were planets. For the most part, astronomers accepted that stars arose from the gravitational collapse gravitational collapse
1. The implosion of a star or other celestial body under the influence of its own gravity, resulting in a body that is many times smaller and denser than the original body.
2. of giant clouds of gas. They shined by fusing hydrogen nuclei into heavier elements.
Planets, on the other hand, arose from disks of gas and dust surrounding stars. In the most popular theory, planets were spawned by bits of material in the disks gradually coalescing coalescing (kōles´ing),
n a joining or fusing of parts. into larger and larger objects. In a competing scenario, sections of the disk suddenly fragmented, forming planets wholesale. In either case, planets orbited their parent stars and weren't massive enough to trigger hydrogen fusion.
But in the early 1990s, researchers discovered the first hybrid objects--failed stars known as brown dwarfs The first free-floating brown dwarf discovered is Teide 1 in 1995. The first brown dwarf discovered that orbits a star is Gliese 229B, also discovered in 1995. The first brown dwarf to have a planet is 2M1207, discovered in 2004. . These bodies, theorized to form as stars do, tip the scales at up to 75 times the mass of Jupiter, the most massive planet in the solar system.
Although the minimum mass of a brown dwarf brown dwarf, in astronomy, celestial body that is larger than a planet but does not have sufficient mass to convert hydrogen into helium via nuclear fusion as stars do. remains controversial, some researchers peg it at 13 Jupiter masses. That's hefty enough to initiate fusion but not to sustain it. After these failed stars stop that process, they spend the remainder of eternity cooling.
The discovery of planets that orbit sunlike stars further muddied the standard picture of planet formation. For starters, many of these extrasolar planets are several times as heavy as Jupiter. Also, around the time of the discovery of these massive orbs, several researchers, including Gibor Basri of the University of California, Berkeley The University of California, Berkeley is a public research university located in Berkeley, California, United States. Commonly referred to as UC Berkeley, Berkeley and Cal , were finding dwarfs with lower and lower masses. Inevitably, note Basri and Brown in the 2006 Annual Review of Earth and Planetary Science planetary science or planetology, study of planets and planetary systems as a whole. Planetary science applies the theories and methods of traditional disciplines such as astronomy, geology, physics, chemistry, and mathematics to the study of , extrasolar planets and brown dwarfs became harder to tell apart. About 5 years ago, astronomers began finding objects in Jupiter's mass range that weren't orbiting anything (SN: 5/19/01, p. 312).
"Astronomers and planetary scientists were forced to admit that they didn't really have a clear-cut definition of 'planet,'" Basri and Brown say.
PUZZLING FINDINGS "The fact is there is ambiguity ... and this ambiguity will be with us for a while," says theorist Adam Burrows Adam Burrows (born August 31, 1972, Birmingham) is a British hockey player. Early days
Burrows' hockey career started at the age of 15 for Olton & West Warwickshire H.C. in Solihull. Moved up through the ranks until reached the 2nd XI towards the end of his 18th year. of the University of Arizona (body, education) University of Arizona - The University was founded in 1885 as a Land Grant institution with a three-fold mission of teaching, research and public service. in Tucson.
Consider, for instance, the star HD 3651, which is slightly less massive than the sun and just 36 light-years from Earth. By measuring the back-and-forth motion of this star, astronomers deduced several years ago that HD 3651 hosts a planet smaller than Saturn that whips around the star in a close, 62-day orbit. Now astronomers have directly imaged a much more distant object orbiting HD 3651. The new find is one of the faintest and coolest brown dwarfs ever discovered close to Earth. The object lies about 1,500 times farther from HD 3651 than the star's planet does.
The image of HD 3651 demonstrates for the first time that brown dwarfs and planets can form around the same star--perhaps in the same way--notes Markus Mugrauer of the University of Jena in Germany. He and his colleagues describe their findings, from several ground-based telescopes, in an upcoming Monthly Notices of the Royal Astronomical Society Monthly Notices of the Royal Astronomical Society (MNRAS) is one of the world's leading scientific journals in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes peer-reviewed letters and papers reporting original research in relevant . Another team, led by Kevin Luhman of Penn State University in State College and using NASA'S orbiting Spitzer Space Telescope Spitzer Space Telescope: see infrared astronomy; observatory, orbiting. , describes its study of HD 3651 in an upcoming Astrophysical Journal The Astrophysical Journal, often abbreviated to ApJ, is a scientific journal covering astronomy and astrophysics. It was founded in 1895 by George Ellery Hale and James E. Keeler. It currently (October 2006) publishes three issues per month, with 500 pages per issue. .
In separate work, reported in the Oct. 1 Astrophysical Journal, Luhman and his colleagues used the Hubble Space Telescope Hubble Space Telescope (HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble) was expected to provide the clearest view yet obtained of the universe. to photograph an object, about 10 Jupiter masses, orbiting the star CHXR 73. The orbiting object's mass alone would typically identify it as a heavy planet spawned from a disk that once surrounded this young star.
However, the newfound object lies about five times as far from CHXR 73 as Pluto's average distance from the sun. Theory suggests that a gas-and-dust disk isn't likely to contain enough material that far from a star to make a planet. "Although the object seems to be [of] planetary mass, we argue that it's not a planetary companion, just a very small brown dwarf companion," says Luhman.
Distinguishing between brown dwarfs and planets is important, says Luhman. A brown dwarf could have its own retinue of planets, which astronomers could search for. In contrast, a planet at such a great distance from its parent star would most likely be a loner loner Psychiatry A single young man estranged from society and family, who suffers from psychogenic pain, and tends to live 'on the edge', vacillating between aggression and depression; loners often have unrealistic goals, but are unable to work towards those goals .
In a report presented in August at the meeting of the International Astronomical Union in Prague, researchers suggested that brown dwarfs might form in a different manner from both stars and planets. According to the new hypothesis, brown dwarfs are much smaller than bona fide [Latin, In good faith.] Honest; genuine; actual; authentic; acting without the intention of defrauding.
A bona fide purchaser is one who purchases property for a valuable consideration that is inducement for entering into a contract and without suspicion of being stars because a dwarf arises from the sudden fragmentation of a ring or disk of material surrounding a pair of brand new stars.
Planets might also arise in this manner, but the fragmentation of a large disk surrounding two stars favors the formation of a brown dwarf, according to study coauthor Greg Laughlin of the University of California, Santa Cruz The University of California, Santa Cruz, also known as UC Santa Cruz or UCSC, is a public, collegiate university, one of the ten campuses of the University of California. .
In many cases, the stellar pair kicks away the new dwarf but doesn't give the body enough oomph to escape a surrounding cluster of stars. That would account for the multitude of brown dwarf loners found in young stellar clusters, such as the Orion star-forming region. In other eases, the dwarf remains in orbit around the two stars. Laughlin, Doug Lin, also at Santa Cruz, and Ing-Guey Jiang of the Central University of Taiwan recently posted their ideas online (http://xxx.lanl.gov/abs/astro-ph/0610814).
It might be that planet formation proceeds at different rates and in different ways at different distances from stars, says extrasolar-planet hunter Ray Jayawardhana of the University of Toronto Research at the University of Toronto has been responsible for the world's first electronic heart pacemaker, artificial larynx, single-lung transplant, nerve transplant, artificial pancreas, chemical laser, G-suit, the first practical electron microscope, the first cloning of T-cells, . Moreover, determining just how little mass an object can have and still qualify as a star provides critical information about the history of star formation throughout the universe.
RIDDLE RESOLUTIONS Given the shifting definition of stars and planets, what are astronomers to do?
Basri and Brown each suggests a different way out of the planetary puzzle. Basri proposes lumping together all objects massive enough to have ever undergone nuclear fusion--whether they did so for less than a million years as a brown dwarf or billions of years as a sunlike star. He would call all such objects fusors, which he maintains are fundamentally different from objects that never had enough mass to fuse nuclei.
Basri would call the lower-mass objects planemos. Only those planemos that orbit a fusor would be considered planets.
Brown, Luhman, and some other astronomers prefer to categorize objects on the basis of how they formed, not how big they are. A low-mass object would qualify as a planet only if it formed from a gas-and-dust disk surrounding a star, even a brown dwarf.
There's a hint that nature itself distinguishes objects on the basis of how they form, Brown says. The universe appears to have separate methods and locations for forming star companions, depending on their mass. Within the immediate neighborhood of a star--a few times the Earth-sun distance--orbiting objects tend to be either less than five or more than 60 times Jupiter's mass.
The gap in mass suggests that nature has two distinct ways to make an object that will orbit a star, Jayawardhana notes. One mechanism, the traditional planet-formation scenario, may dominate at low masses, while another, which tends to make brown dwarfs, may dominate at high masses.
Determining how an object formed, however, is a daunting daunt
tr.v. daunt·ed, daunt·ing, daunts
To abate the courage of; discourage. See Synonyms at dismay.
[Middle English daunten, from Old French danter, from Latin task, notes Luhman. Telescopes take snapshots but not movies following a body back in time to its origin.
Using the proposed successor to Hubble, the James Webb Space Telescope This article or section documents a scheduled or expected spaceflight. Details may change as the launch date approaches or more information becomes available. , as well as sharpened optics soon to be installed on the ground-based Gemini North and South telescopes, astronomers hope to determine whether planets exist farther away from stars' neighborhoods. This may provide additional clues about planet formation. The sharper resolution will also give astronomers a chance to directly glimpse extrasolar planets like those in our own solar system.
"At some level, I don't' really care" what people call planets and stars, says Jayawardhana. "To me, the main sort of motivation and fascination with this business ... is that we've learned in the last couple of years that nature seems to be able to make planetary-mass objects in more than one way." He adds, "We haven't figured out details of the story, but it's saying something really fundamental" about the universe.
For now, Burrows suggests that astronomers hold back on a rigid definition of planet. "People just want ... to name things or to stuff objects into cubbyholes," he says. "I prefer a bit of ambiguity to the illusion of clarity and the pretense of certainty. When there is fog, to acknowledge its presence is more honest than to try to ignore it."