Night skies aglow with an array of bodies that defy definition
Now that the solar system has been officially trimmed to eight ``planets" by the International Astronomical Union amid much shouting and hoorah, you might think that the definition of a planet is all sewn up.
You would be wrong. Nature is too prolific in the weird astronomical bodies it creates, and the evening sky this month holds an object that warns of future planet-classification issues yet to be settled.
The new definition says, among other things, that a ``planet" has to be massive enough that its gravity clears most other objects out of the region where it orbits. Mercury through Neptune qualify in this regard; Pluto does not. Pluto is merely one of the largest ``Kuiper Belt objects" in the outermost solar system, none of which are big enough to have swallowed or flung away the rest.
Similarly, Ceres is the largest body in the asteroid belt between Mars and Jupiter, but it lacks the heft to send the other asteroids packing. So it too fails to make the cut.
But that only covers the small-critter wing of the planetary zoo. The astronomers who met last month carefully sidestepped how to define the largest planets -- fascinating objects that are now being found in abundance orbiting other stars.
In our own solar system, this is not an issue. The largest planet is Jupiter, king of the four gas giants. This week Jupiter is the brightest point of light shining low in the southwest at dusk. Even though it's gaseous and hot inside, Jupiter is clearly a planet and not a ``star" because it's not massive enough to generate its own heat and light by nuclear reactions.
Mass is what makes a star. Jupiter has 318 times the mass of Earth -- but the sun weighs 1,000 times more than Jupiter. That's why the sun lights up; the pressure and heat near its center are enough to make hydrogen fuse to form helium, as in a hydrogen bomb.
How massive can a ``planet" be before it becomes a star? The standard hydrogen-fusion reaction turns on if a gasball has about 80 Jupiter masses. That's just enough to set the dimmest red-dwarf star burning with a steady glow.
Below 80 Jupiter masses, but above the masses of ``planets," is a class of objects called brown dwarfs, also known as failed stars. They just cool down and go dark as they lose their heat of formation.
Most of them drift alone through deep space, almost unseen, but some of them orbit stars, planet-style.
And the line between brown dwarfs and planets?
Some astronomers draw this at 13 Jupiter masses, the point below which a gasball undergoes no glimmer of hydrogen fusion even early in life. Others differ; they say the difference between a ``planet" and a ``brown dwarf" ought to be based on how they form, not what they weigh.
To complicate matters, there are objects with fewer than 13 Jupiter masses drifting loose in interstellar space. Is it a planet if it doesn't circle a sun? Many astronomers call these weird creatures of the dark ``planet-mass objects," or ``planemos" for short.
The brightest star in the west is Arcturus, sparkling pale yellow-orange. To its lower right is a little triangle of fainter stars (you may need binoculars if your sky has light-pollution). The lower-right one of these is Tau Bootis.
At a distance of 42 light-years, Tau Bootis hosts one of the first planets discovered beyond our solar system.
The star's slight, periodic wobble tells that a body with at least four Jupiter masses orbits it in only 3 days, 7 hours, 31 minutes. To orbit so fast, this super-Jupiter must be so close to the star that it's roasted almost red hot. It's surely a gas giant, and according to atmospheric models, it should be roiling violently with black clouds of mineral dust and perhaps rainstorms of molten iron.
``Hot Jupiters" like this turn out to be common. Not to mention ``hot Neptunes" and ``super-Earths." As discoveries continue, there's no end of classification yet to be done.
Alan M. MacRobert is a senior editor of Sky & Telescope and Night Sky magazines in Cambridge (SkyTonight.com). His column appears the first Saturday of every month. 
