There are about 100 billion stars in the Milky Way galaxy, and main sequence dwarves (such as our Sun) are one of the most common types. Which is to say there are some tens of billions of stars having similar chemistry to the Sun’s, and formed in very much the same way.
It strains credibility, therefore, to believe that the Sun is the only such star with attendant planets. One would expect there to be millions of planetary systems orbiting stars of this type.
With such large numbers of possibilities, spotting planetary systems should be a doddle, but this is not so, for the scale of space conspires against us. Viewed from the nearest star we can see with the naked eye, alpha Centauri A (4.3 light years or 40 x 10′3 km distant), our Sun would appear the same size as a 50-cent piece 370 km away. Seen from the same viewpoint, the giant planet Jupiter—only a tenth the diameter of the Sun—would be no bigger than a red lentil about seven metres away from the 50-cent piece.
To make matters worse, while stars emit light, planets are merely reflectors, and generally poor ones at that. Indeed, the inner or rocky planets are better described as absorbers rather than reflectors.
Mercury and the Moon reflect only 11 per cent of incoming sunlight, cloudy ice-capped Earth averages almost 37 per cent, and Venus, brightest of all (being completely covered with cloud), scores 65 per cent.
Even the gas giants, with surfaces formed from clouds, are still far from mirrorlike. Jupiter reflects only 52 per cent of visible light because of the chemical composition of its atmosphere.
For an observer, surface reflectivity is only one of five basic factors affecting the apparent brightness of a planet. The observer must take into account:
- the brightness of a planet’s sun, which can range over more than ten million fold,
- the distance of the planet from its sun—double the distance and the intensity of incident radiation is decreased fourfold,
- the albedo or reflectivity of the visible surface,
- the planet’s diameter, since apparent brightness will be proportional to the visible surface area,
- the distance of the observer—the perceived brightness will diminish proportional to the square of the observer’s distance.
These factors place all but the very closest possible systems beyond direct observation, because even the largest planets will be too small and faint to see with the most powerful of telescopes.
As a result, we must rely on inferences drawn from the data generated by other techniques. We can do this with confidence because the laws of physics and the properties of matter do not change from place to place.
Measurements made here are valid samples of what happens elsewhere, and the value of such fundamental constants as the speed of light, the mass of the proton, the Laws of Gravity and Thermodynamics and the behaviour of electromagnetic radiation are the same anywhere in space.
Although astronomers have discovered many strange phenomena—stars which spin hundreds of times per second, objects so massive that even photons are trapped forever, as the speed of light is less than their escape velocity—so far we have found nothing to undermine the generality of application of physical laws (known as the Cosmological Principle).
One clue in searching for planets is to observe the effects that a massive planet has on its sun as it orbits it. In reality, a planet does not orbit its sun, nor the sun its planet, but both orbit their common centre of mass, termed the barycentre. In a system with only one planet, its sun’s motion would trace out an ellipse.
Due to the effects of the various planets in our solar system, our Sun’s path is an open-looped curve.
If, however, the mass of all the planets in our solar system were concentrated in a single giant planet, Jupiter+, the Sun would be seen to orbit in time with it. Since the orbit would be small, the Sun would appear to wobble rather than describe an orbit. However, this wobble would be in time with the orbital motion of the planet, for the two bodies would be inextricably linked by their mutual gravitational attraction, and so the slight reflex motions of the bright primary would be evidence of the existence of an invisible, less massive and distant partner.
Movements of stars measured along the line of sight of the observer are called radial velocities, and may be away from or towards the observer. Refinements in the design and construction of spectroscopes now enable us to measure the Doppler shifts caused by line of sight motions as little as 3 metres per second.
Over the years there have been a number of claims to have discovered planets by such means. Unhappily, meticulous scrutiny of the data has revealed mistakes or experimental errors, and these claims have had to be withdrawn. For example in the early 1970s van de Kaamp became convinced that irregularities in the motion of Barnard’s Star indicated the presence of a pair of Jupiter-sized planets. Photographic plates made on another refractor telescope going back to 1911 not only failed to confirm the data, but showed that the ageing objective of that telescope was showing a slowly changing colour effect.
Subsequently, the objective of the telescope with which the “discovery” had been made was shown to be suffering from the same deterioration, which had a subtle effect on star positions measured off the plates, and so Barnard’s star remains bereft of planets.
The first planetary system confirmed was that around the millisecond pulsar PSR 1257+12, discovered by Aleksander Wolszczan in 1990 from an analysis of the cyclic variations in the timing of its basically very regular radio pulses. This was an entirely unexpected result, as pulsars are rapidly rotating neutron stars which are the hugely compressed cores remaining from supernova explosions.
These explosions are the most violent events known in the galaxy, and are sufficient to vaporise nearby planets and blow more distant ones out into space. Yet it is now accepted that this pulsar has two approximately Earth-sized planets orbiting in 67 and 98 days at a distance less than that of Mercury from the Sun, and maybe a third further out.
The puzzle is, if the planets were orbiting the parent star how did they survive the explosion? If they have joined the pulsar after the explosion, where did they come from ?
In 1995, the existence of planets too faint to be seen in orbit about a Sun-like star was established. Mayor and Queloz, working with a high-sensitivity spectrograph at the Haute-Provence observatory, obtained radial velocities from 51 Pegasi. This is a main sequence star 40 light years away, and the regular sinusoidal variations in the data could only be convincingly explained by positing a planet half the mass of Jupiter orbiting the star at only 7 million kilometres out every 4.229 days.
As with the PSR 1257+12 system, the existence of a planet at this distance from its sun poses problems for current theories of stellar and planetary formation. 51 Pegasi is an exemplary main sequence dwarf star, stable and unvarying, giving not the slightest hint of anything unusual in its past. How did so large a planet form so close to its primary during those turbulent early aeons when the star was 100 times more luminous and losing matter with a massive solar wind?
What type of planet could this be—is it a ferrous clinker (like Mercury), or does it have a massive, Jupiter-type atmosphere, which, although theoretically possible, seems unlikely? But to say something appears unlikely seems to guarantee its existence in the contemporary astronomical bestiary.
Now the search for planets is a hot topic, and telescope time all over the world is being booked. With two surprises out of two, it looks as if for some time to come our models of both planet formation and possibly stellar formation will be under revision.
As we go to press, news has come in of two new planetary systems discovered by Marcy and Butler at the Lick Observatory, California. Like the planet about 51 Pegasi, the first discovery poses problems, because its markedly elliptical orbit is characteristic of a pair of stars, while planets typically have fairly circular orbits. It is possible that rather than a planet (an object condensed and accreted from the primordial circumstellar disc) this object is a mini-brown dwarf, a wannabe star too small to generate the pressures and temperatures for thermonuclear ignition at its core, and so condemned to be just a slowly cooling mass of hydrogen and helium orbiting a “proper” star.
The only body which appears to be a classical planet is that in orbit about 47 Ursae Majoris. Twice the mass of Jupiter and orbiting its primary at about the distance of the asteroid belt from the Sun, it has all the hallmarks of a classical gas giant. Although this planet is in no way a candidate to harbour any sort of life as we know it, nevertheless it may be that it is the only detected member of a system of planets akin to our own, including one capable of supporting life. Watch this space.