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Extrasolar News - 6/4/99

Recent Findings & Hypotheses:

Methane Brown Dwarfs - While brown dwarfs have too little mass to fuse "regular" hydrogen (which has a single proton nucleus), virtually all of the ones discovered until 1999 were too hot -- that is "young" -- to show evidence of methane which is destroyed by stellar temperatures. In fact, while methane is a atmospheric characteristic of giant gas planets like Jupiter, the only brown dwarf found to even have a trace of methane was Gliese 229 B, which orbits a reddish, M-class dwarf located about 20 light-years away from Earth.

This Spring, however, two very dim and reddish brown dwarfs were found as solitary objects (one 30 light-years away in Ophiuchus and another also relatively nearby in Virgo). Analysis of their spectra indicated that both have atmospheres that are rich in methane. In addition, four similar objects that are too cool to be observed in visible light were found using near-infrared telescopes also to have the methane fingerprint of extremely cool (that is "old") brown dwarfs. These discoveries represent strong evidence that faint brown dwarfs which have had billions of years to cool, although hard for astronomers to detect, may represent a significant population of the universe. Some astronomers speculate that these objects may well be as numerous as the stars, reviving theories of stellar formation that suggest the existence of uncountably numerous brown dwarfs, rather than the relatively few easy-to-detect, bright ones found thus far.

Brown Dwarf or Planet? - When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.

Caltech astronomer Ben Oppenheimer, who helped to discover the apparent brown dwarf, Gliese 229 B, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "regular" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.

Other prominent astronomers, such as San Francisco State University astronomer Geoff Marcy who has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. There may also be many that form brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.

Hot Jupiters - One of the most surprising findings thus far is the detection of giant, Jupiter-class planets in orbits very close to their host stars (three within the range of tidal interaction with their stars). Under standard theories of planetary formation, such giant planets should not form in such close-in orbits because it's too hot there for dust grain condensation, with too little solid material in the vicinity to build a protoplanetary core of 10 Earth-masses quickly enough and too little gas to build a massive envelope, before the developing star's T-Tauri wind blows too much of its disk of dust and gas away. First proposed in the 1980s, the main theoretical mechanism used to explain the presence of these "hot Jupiters" is protoplanetary in-migration during the relatively brief time (within three million years) that a protostar has a significant circumstellar disk.

Two main types of in-migration have been proposed. Because a "small" protoplanet (up to a few Earth masses) lacks sufficient gravitational pull to open a gap in the protostar's disk, it will move inward as the disk lying inside the protoplanet's orbit transfers angular momentum to it at a slower rate than it is transferring angular momentum to the outer disk (through interactions with Lindblad resonances induced in the disk). This movement is relatively rapid and can cause the protoplanet to move into its host star before the disk dissipates enough to halt migration. Even if a gap in the disk is created by a sufficiently massive protoplanet (0.1 Jupiter-mass), most planets produced may still fall into the star (as "torques" between the planet and the inner and outer gap edges causes the planet and its gap to flow inward along with the viscous disk material towards the star). In theory, giant protoplanets may be constantly forming and migrating inwards during the lifetime of a protostar's disk. Whether or not terrestrial planets like Earth develop and survive in Solar System-type inner orbits beyond the first few million years may depend on the original size (and possibly metallicity) of the disk and where the in-migration of any surviving giant planets are halted.

Metallicity & Giant Planets - A three-year spectroscopic survey of 12 stars orbited by giant planets shows that they have high abundances of elements heavier than helium, typically two to three times that of our sun, Sol. Two opposing explanations have been ventured. On one hand, more heavy-element-enriched interstellar clouds of gas and dust may be more likely to create giant planets during stellar formation. On the other hand, many of these star systems also may have experienced more protoplanetary in-migration, whereby newly formed, giant planets move in toward their host stars and drag a lot of the remaining inner, circumstellar disk material (heavy-element-rich dust and smaller planets) into these stars, thus enriching them.

Superflares - According to one recent hypothesis, unusually intense stellar flares from a sun-like ("Sol-type") star could be caused by the interaction of the magnetic field of a giant planet in tight orbit with that star's own magnetic field. Some Sol-type stars of spectral class F8 to G8 have been found have been observed to produce "superflares" that release between 100 and 10 million times more energy than the largest flares were observed on the sun. These "superflares" last from one hour to one week and increase the normal luminosity of a star as much as one thousand times. If our sun were to produce a large superflare, Earth's ozone layer would be destroyed, and ice on the daylight side of moons as far out as those of Jupiter or even Saturn would be melted, producing vast floodplains that refreeze after the flare subsides. No traces of past superflares have been detected in our solar system. In 1998, nine Sol-type stars (naked-eye objects Omicron Aquilae, Kappa Ceti, and Pi1 Ursae Majoris, as well as MQ/5 Serpentis, UU Coronae Borealis, S Fornacis, MT Tauri, BD+10 2783, and Groombridge 1830 -- now believed to be a single, Halo subdwarf without a "flare star" companion) were found to have produced superflares, on average, about once per century. None of these yellowish stars rotate particularly fast, have close binary companions, or are very young. Superflares have also been observed in a group of dim main-sequence, reddish M dwarfs known as flare stars.

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