The Universe in the Classroom

What Have We Learned About Halley's Comet?

The Search for Planets Around Other Stars

As we live out our lives securely attached to the Earth, breathing its air, and using its water, it becomes all too easy to take planets for granted. After all, our solar system has nine of them, enough so students grumble each year about having to remember them all.

Yet the surprising fact is, we do not know for certain whether there are planets around other stars. We strongly suspect they are there; many of our best-established theories indicate that the creation of planets is a likely accompaniment to the birth of many kinds of stars. But as of today, we simply do not have any firm evidence that planets outside our own solar system exist.

Planets Are Hard To Find

The problem, as always in astronomy, is one of distance and brightness. While stars shine under their own power, planets simply reflect the light of their parent star. Furthermore, planets are generally much smaller than stars and the surface or atmosphere on which starlight can reflect is thus very limited. If a planet like the Earth orbited the nearest star to us, the light it would reflect would be too dim to be picked up with even the largest telescope on Earth!

A few years ago there was a flurry of excitement as some astronomers found wobbles in the motions of several nearby stars, suggesting the presence of planets. To understand how these observations were performed, imagine a star with just one good sized planet around it. When the planet is on one side of its star, its gravity pulls the star ever so slightly to that side. Then as the planet moves in its orbit to the other side, it tugs the star just a bit to that side.

From far away, we cannot see our hypothetical planet at all — it is just too dim. What we do see is the much brighter parent star. If we could make extremely precise measurements of its motion as the years pass, we might be able to detect a tiny wiggle in its movement, as the planet pulls it first to one side and then to the other. While this technique is fine in theory, in practice it turns out to be depressingly difficult.

Let us suppose that the star in our example is our nearest neighbor star Proxima Centauri and the planet in question is the largest planet in our own solar system, Jupiter, moving around Proxima Centauri in the same orbit in which Jupiter circles our Sun. How large a wiggle would we observe in the motion of Proxima Centauri over the course of decades? The answer turns out to be about 0.032 seconds of arc, or the size of a U.S. quarter as seen from 95 miles away!

Some years ago astronomers at one observatory, after decades of observations, reported the discovery of such wiggles in the motion of a few nearby stars. However, neither the discoverers nor other astronomers have been able to duplicate these findings. It may be that small changes in the telescope over the decades simulated wiggles. In any case, everyone agrees that we will need to watch the suspect stars carefully for another decade or so to pin down whether or not wiggles exist and if they are caused by planets (rather than, for example, small dark stars in orbit around the visible stars).

In the meantime, what can we say about the likelihood of planets existing around the hundreds of billions of stars in our Galaxy? Recently, a number of fascinating new clues in the planet puzzle have been found by astronomers working in a variety of research fields.

Discovering a Brown Dwarf

In 1984, astronomers Donald McCarthy and Ron Probst reported the discovery of an object which is halfway between a planet and a star — a stillborn star, if you will — too small to engage in the nuclear fireworks that sustain a star. The term astronomers use for such an object is brown dwarf and, although they had been predicted to exist by a number of astronomers, this would be the very first of this cosmic species to be found

The brown dwarf orbits one of the very faintest and coolest stars we know, called van Biesbroeck 8 (after the astronomer who found it) Located at about 21 light years away, in the direction of the constellation Ophiuchus, VB8 (as we affectionately and more pronounceably call it) was found in 1983 by astronomers at the U.S. Naval Observatory to show just a bit of a wobble.

Reasoning that a cool faint system like this might be more easily observed with infrared (heat rays) than visible light, McCarthy and Probst used the 4-meter telescope on Kitt Peak in Arizona to take many short exposures of the star in infrared light. They then combined these exposures using special computer processing techniques to bring out very faint details that might otherwise elude detection. Using 10,000 exposures taken over several nights (interrupted by the bane of astronomers — night rain), they found the star had a close companion about 16 times fainter in infrared light than the star.

Further observations indicated that the object had about the diameter of Jupiter and was located about the same distance from its star as Jupiter is from the Sun. But its mass and temperature make this object unusual. It weights in at an estimated 40 times the mass of Jupiter and has a surface temperature of 1400 degrees Kelvin (about 2000 degrees Fahrenheit) — much too massive and hot for a planet, yet too cool and underweight to be a star — and thus likely to be our first example of a brown dwarf.

However, we should add one important note of caution. As this issue was going to press, two teams of astronomers who had been trying to duplicate the observations of the dark companion around VB8 reported that they had been unsuccessful. Even the discoverers have urged caution about accepting their observations until more evidence is obtained.

For now, the object is known as VB 8B, hardly the most ear-catching of names. While school children have already written to Dr. McCarthy with suggestions for names ("Starbit'', "Farheat'', "Mongo'' and the like), the feeling now is that we must wait until we are certain that this object exists, what it is and how many more like it there are out there before a name is assigned that may set a precedent.

Should the observation be confirmed, its significance for the question we are considering will be enormous.

Hot news from IRAS

In 1983, a sophisticated orbiting telescope called the Infra Red Astronomical Satellite (or IRAS for short) spent 11 months observing the sky with "infrared eyes''. Among its many discoveries was that 12 nearby stars were surrounded by shells or disks of dark particles, not bright in visible light but glowing with the heat rays their warmth produced. At least some of these could be raw material from which planets could be forming or may already have formed.

Unfortunately, a vast swarm of smaller particles can scatter radiation much more efficiently than a single large body like a planet. Thus if there are planets in any of these systems we are unlikely to be able to see them with present technology. Still, the disk around one of these stars, called Beta Pictoris, has been photographed by two astronomers using specialized electronic detectors and a large telescope atop a Chilean mountain. The disk around Beta Pictoris stretches out some 10 times the size of Pluto's orbit.

Recently, astronomers have concentrated their attention an a star in the constellation of Taurus called HL Tau. Surrounding this youngish star, measurements made with radio waves have revealed enough gas and dust to make ten Jupiters, orbiting in a disk about three times the size of our own planetary system. While we cannot say that this system is producing planets, it certainly appears that the raw material for making them is there.

Thus we continue to be tantalized by hints that the process that led to planets in our cosmic "neck of the woods'' may have occurred and be occurring elsewhere. Should planets be discovered someday orbiting other stars, it will be difficult to resist speculating whether some of these planets may be capable of supporting life as our Earth does. This topic, so loved by science fiction writers and Hollywood, will be the subject of a future issue of The Universe in the Classroom.

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