Star Bright: Part I

Star Bright

..and The Sub-Brown Dwarfs

Click here for larger image. Brown dwarfs, such as Gliese 229B, lack sufficient mass (at least 75-80 Jupiters) to ignite core hydrogen fusion.
Credit: American Scientist/Linda Huff

What distinguishes a star from a planet?

This question seems so easy – a star is an enormous, shining-hot ball of plasma, while a planet is a much smaller, cooler conglomeration of rock, ice or gas that orbits a star.

But new discoveries in the universe have a lot of scientists scratching their heads, reconsidering the nature of stars, planets, and extrasolar planet discoveries.

The problem partly stems from the discovery of very low-mass objects called "brown dwarfs." The term "dwarf" refers to low or medium mass stars, and the color is an indication of temperature. Our own sun is a yellow dwarf, while stars cooler than the sun are red dwarfs. Brown dwarfs are even cooler than red dwarfs.

Brown dwarfs are often referred to as "failed stars," since they don’t have enough mass to fuse hydrogen into helium. In normal stars, hydrogen fusion is what makes a star shine.

Brown dwarfs also shine, but they do so from the burning of deuterium (2H, a heavy form of hydrogen). The deuterium burning is a shorter-lived energy source than hydrogen burning, so brown dwarfs only shine during the first 10 million years or so of their lives. Afterwards, just as an iron poker fades when pulled from a fire, a brown dwarf fades as it begins to rely on stored energy.

Theorists in the 1960s had suggested that brown dwarfs could exist, but the first brown dwarfs weren’t found until 1995. The International Astronomical Union (IAU) defines brown dwarfs as objects that are at least 13 times the mass of Jupiter, since that is the mass required to ignite the fusion of deuterium. Once an object reaches 75 Jupiter masses — the mass required to fuse hydrogen — it becomes a typically luminous star.

Gas giant planets like Jupiter were formed from gaseous material surrounding the young star.
Credit: NASA

Astronomers were beginning to discover extrasolar planets at the same time brown dwarfs were being found, and there was some crossover in the masses. Some reports had listed objects more massive than 13 Jupiters as planets. Although many scientists now adhere to the IAU’s brown dwarf definition, there is still some confusion over the objects of less than 13 Jupiter masses that have been found.

The IAU’s Working Group on Extrasolar Planets suggests that for objects less than 13 Jupiter masses, those orbiting around stars should be called "planets," while those that are solitary should be called "sub-brown dwarfs."

This location-based definition has not quelled the debate about how to define these low-mass objects, however. For one thing, an object’s location can change. Planets, for instance, may be gravitationally ejected from a solar system by larger planets with eccentric orbits, or, on rare occasions, by passing stars. Because many of the extrasolar planets discovered so far are gas giants with highly eccentric orbits, some suggest it is possible that solitary "sub-brown dwarfs" were once planets in unstable systems. But Iain Neill Reid of the Space Telescope Science Institute disagrees.

"There are some who would like to call isolated, very low-mass objects ‘planets,’ but I think that’s got more to do with press releases and funding than anything scientific," says Reid. "My own preference would be to define planets based on how they form — in a disk versus as a collapsing star-like object."

Many astronomers believe the circumstance of birth is what really distinguishes a star from a planet. Planets are thought to form from the disk of gas and dust that surrounds a young star. Gas giant planets like Jupiter are born first, gathering most of the gaseous material surrounding the young star. Terrestrial planets like the Earth form later, from asteroid-like clumps that collide into each other over time.

Terrestrial planets like the Earth formed after the gas giants, from asteroid-like clumps that collided into each other over time.
Credit: NASA

Stars, on the other hand, are created when clouds of gas collapse under their own weight. This can produce one shining star in the middle of the cloud, or several stars. Some clouds produce many stars of various sizes. Computer simulations conducted by Alan Boss of the Carnegie Institution in Washington have suggested that sub-brown dwarfs with as little mass as the planet Saturn (or one-third the mass of Jupiter) could form this way.

Most of the extrasolar planets found so far are much more massive than that. This mass crossover doesn’t mean that sub-brown dwarfs and extrasolar planets would be physically indistinguishable. Different circumstances of birth would result in very different internal structures. Planets should have solid metallic interiors, while the interiors of low-mass stars and brown dwarfs are chemically more similar to their atmospheres.

There’s no way to compare the interiors of sub-brown dwarfs and extrasolar planets, however, since these objects are so far away that we can just barely detect their existence. It would be highly impractical to send out probes over these vast distances in order to sample their interiors.

Even if they were closer, sampling would not be likely because of the difficulty of probing such massive objects. Astronomers still do not agree about what constitutes Jupiter’s core, for instance, because the extreme pressures and temperatures of that planet would destroy a probe sent deep into Jupiter’s interior.

"In Jupiter’s case, we can’t sample the interior directly — the conclusions are based on theoretical models," says Reid. "Those models rest on detailed observations, which we can only make for Jupiter because it’s so close."

The brown dwarf LP 944-20 (Digital Sky Survey).
Credit: European Southern Observatory

Another way of determining the composition of an object is to analyze the object’s light spectra. Planets and older brown dwarfs in planetary systems don’t shine, but they do reflect light from the central star. Analyses of reflected spectra could show what elements are in an object’s atmosphere, and this might indicate a place of origin.

"Planets are expected to have somewhat different chemical composition from brown dwarfs," says Reid. "Jupiter, for example, has about twice the fractional contribution from heavy elements — elements other than hydrogen and helium — than the sun. However, that’s not easily observable."

Some cool brown dwarfs do seem to share some properties with the less massive gaseous planets. The brown dwarf Gliese 229B, for instance, appears to have large amounts of methane and water, two molecules that also are abundant in Jupiter’s atmosphere. Some brown dwarfs also seem to have variable weather patterns similar to those of gaseous planets.

"This similarity is expected, since hydrogen, oxygen and carbon have very similar abundances in the atmospheres of both brown dwarfs and planets," says Reid. "They look alike because the atmospheric temperatures aren’t very different – about 250K for Jupiter, 950K for Gliese 229B."

Perhaps nature does not draw a sharp line between planets and stars the way we try to. Instead, there may be a natural mass continuum, where stars gather enough mass to "turn on," brown dwarfs only gather enough mass to smolder darkly, and gaseous planets never gather enough mass to ignite anything. Slap enough mass on something, and it will behave a certain way.

Although scientists have reported finding brown dwarfs and sub-brown dwarfs surrounded by a circumstellar disks of gas and dust, suggesting they are born and behave the same way as stars, brown dwarfs are unlike typical stars in many ways. Brown dwarfs tend to rotate faster than most normal stars, and some brown dwarfs have unusual amounts of radio emissions. The brown dwarf LP 944-20, for instance, has a radio emission 10,000 times stronger than was predicted based on its X-ray emissions. In regular stars, radio and X-ray emissions are related processes. Sub-brown dwarfs presumably also have characteristics that differ from normal stars.

Join us for Part II of this story on Wednesday August 6

Related Web Pages

IAU Working Group on Extrasolar Planets