by Kevin Apps

October 6^th 1995 is not a day etched firmly in the minds of many but perhaps it should be. It was on this day that a Swiss team of astronomers announced to the world the first definitive detection of a planet orbiting another sun.

People had wondered about the existence of extra-solar planets for centuries. But finding them was to be no easy task. Unlike the stars, planets do not generate any visible light of their own but merely "shine" by reflecting their own sun's light. Imagine looking at our sun from the nearest star, a distance of about 40 trillion kilometres. The distance between the Sun and Jupiter would be appear equivalent to the distance between a car's headlamps, 50 kilometres away! Combine this with the problem that the visible light from Jupiter is 250 million times fainter than that from the Sun. Trying to find extra-solar planets is like trying to see a firefly sitting on a searchlight from many kilometres away! This is why extra-solar planets have not been seen directly - they are known to exist because of the gravitational effects they have on their parent suns.

How do you find an extra-solar planet?

The first technique tried was Astrometry. This involved precise measurement of the stars' positions in the sky and looking for the telltale 'wobbles' in their orbits caused by the gravitational tug of an orbiting planet.

Lynette Cook Detecting the 'wobble' these planets cause in their sun's motion requires very precise measurements.

For the last 50 years astronomers have looked for these wobbles and despite a handful of false claims have not found any. The blurring effects of the earth’s unsteady atmosphere makes accurate measurement of so small a motion difficult.

There is however another way to look for these 'wobbles'. Instead of measuring motion in the plane of the sky you can measure the motion along the line of sight. This is known as the Radial Velocity method and is how all the extra-solar planets have been found to date.

It works by utilising the Doppler effect. This effect can be heard on sirens. When a police car is moving towards you the pitch of its siren sounds slightly higher, and then lower as the car passes you and moves away. The same effect happens with light waves. When a star is approaching the Earth the light it emits appears to shift towards shorter wavelengths, and equivalently to longer ones if it is moving away. This can be measured by splitting the starlight into its constituent wavelengths and measuring these shifts. The effects are tiny, for example the effect of Jupiter's pull on our sun would appear as a change in the wavelengths of Sunlight of less than 1 part in 10 million when viewed from a distant star.

This is extremely difficult to measure and requires both the finest equipment and also a lot of ingenuity and patience. Many measurements of a star’s velocity need to be taken over time to see the periodic motion caused by an orbiting planet. From these measurements you can calculate the orbital period of the planet and the eccentricity (which indicates if the orbit is circular or elliptical) and estimate further properties of the planet such as its orbital distance and mass.

Expectations

Planets are thought to form as a product of star formation. Newly born stars are usually surrounded by a rotating disk of material several times the size of our own solar system. This disk, called a protoplanetary disk is mainly composed of gas but also contains a mixture of both dust and ice grains. These grains are thought to grow, over hundreds of thousands of years,  by process known as collisional accretion into planetary mass bodies. Close to the star where temperatures are too high for ice to exist, rocky planets like the earth are expected to form. Further out ice is far more abundant so huge icy protoplanets can quickly grow and then gravitationally capture the surrounding gas from the disk before it dissipates away. It is by this process that giant planets like our own Jupiter and Saturn are thought to have formed.

This is what we expected to find. Like in our own solar system, small rocky planets close in to the star and more massive giant planets like Jupiter and Saturn orbiting beyond 4 or 5 AU (one AU is the distance between the earth and the sun). We also expected roughly circular orbits. All of the four giant planets in our own solar system as well as Venus and Earth have orbits that are close to circular. Indeed the theories of planetary formation tend to have planets forming on circular orbits due the damping influences of the protoplanetary disk.

Lynette Cook Only the giant planets of ice and gas can be detected

Unfortunately with the current radial velocity technique only the most massive planets like Jupiter and Saturn are detectable. Small earth-like planets do not have the gravitational muscle to be detectable. In our solar system our two giant planets orbit at about 5 and 10 AU in near circular orbits and they take about 12 and 29 years to do so respectively. The various radial velocity surveys have only been recording precision data for about 5 to 6 years. Hence they are only sensitive to planets with orbital periods shorter than this (a complete orbit of a planet needs to be observed before its existence is confirmed). Therefore all the planets announced so far have orbital periods shorter than 5 years and are much closer to their star than Jupiter is to the Sun.

The startling diversity

Time and time again in the history of science, nature has turned out to be more diverse and fascinating than scientists had predicted - the realm of extra solar planets is certainly no exception. The most unexpected find has been the location of these giant planets in their systems. We have found a range of orbital distances, from 0.05 AU which is eight times closer in than our closest planet Mercury, right out to the current detection limit of about 2 - 3 AU which is similar to the distance of the asteroid belt in our system, just beyond Mars. Indeed some of these gargantuan gas balls are located at a distance where liquid water, the necessary ingredient for life could well exist on the surface of any moons they might posses.

Recall that it is thought that gas giant planets form out where ices are abundant in the protoplanetary disk (at distances of about 4 AU or greater). That they are seen much closer in suggests they might migrate inwards over the lifetime of the protoplanetary disk (thought to be in the range of 1 to 10 million years). This migration could be caused by gravitational torque interactions with the viscous protoplanetary disk. This is one possible explanation for the wide range of orbital distances we see. There also appears to be a pile up of planets in very small ~0.05 AU orbits. This may be due to the young star clearing a hole in the inner disk, perhaps by the influence of it’s strong magnetic field, so that the inwardly migrating planet halts when it reaches this hole and is stopped from spiralling into the star. Another idea is that the planet may be halted by tidal effects at small orbital radii.

Lynette Cook Virtually all the extrasolar planets discovered have more eccentric orbits than our own giant planets.

The other big surprise has been the generally non-circular orbits of the newly found planets. Those planets in very small (~0.05 AU) orbits are expected to have circular orbits because the gravitational influence of their parent star should tidally circulise the orbit and cause the rotation period of the planet to match its orbital period. This latter effect is seen in our Earth/Moon system in the way that the Moon always keeps the same face pointed towards the Earth. Amongst the more distantly orbiting planets (those where this process doesn’t occur) the majority of orbits are quite eccentric. Indeed virtually all are more eccentric than the orbits of all the giant planets in our own solar system.

Since planets are expected to form in circular orbits this may indicate building solar systems is a chaotic process. Perhaps several giant planets are formed which then gravitationally interact with each other. Simulations of this process show that planets can often be either ejected from their solar system in this way, or have their orbits altered in size and shape, with wildly eccentric orbits being a common outcome. Whatever the case, it seems that our stable solar system of widely spaced circularly orbiting planets may be an unusual outcome.

The most massive planets found are somewhere around 8-10 times Jupiter’s mass. However the number of planets increases towards lower masses, right down to the current detection limits of around 0.5 times Jupiter’s mass. This trend may well carry on beyond this current threshold - the planets that have been found might be just the tip of the iceberg. Even extrapolating from the ones found so far indicates at least 2 to 3% of sun like stars have giant planets; there must be over a billion planets in our galaxy alone!

The parent stars themselves are also of interest. Most planets have been found around G type stars (like our Sun) but this could be partly due to the fact that more of these stars are being targeted. Only one red dwarf star has been found to have a planet though, despite many being searched. This may indicate that these faint low mass stars which are the most common in the galaxy may not be as commonly attended by large planets as the more massive Sun-like stars. The planet bearing stars also tend to be richer than average in metallic elements as well as carbon, nitrogen and oxygen (which are the elements from which the planet-forming grains in the protoplanetary disk are composed of).

The icing on the cake

Two systems deserve special mention. The first is the multiple planetary system around the star Upsilon Andromedae. This system is composed of three giant planets. The first one is in a circular orbit very close to its parent star, the second in an eccentric orbit at a distance similar to Venus in our solar system and the most massive one also in an eccentric orbit somewhat further out than Mars. Several other systems are showing evidence of additional planets, which may indicate that given the right conditions multiple giant planets are often formed in the same system.

Lynette Cook We can see the shadow this planet casts on its sun.

The planet around the star HD209458 is also an interesting case. It also orbits very close to its sun and fortuitously its orbit is aligned almost edge on to our line of sight. We know this because the planet has been seen to pass in front of its sun, causing the star to dim slightly. From precise measurement’s it has been determined that the planet is 60% the mass of Jupiter but has a diameter 50% greater. The planet orbits so close to its parent star that its temperature is over 1000^oC, its outer atmosphere swells up, causing this bloating effect. These measurements also confirm it is made primarily of gas like our own Jupiter and Saturn.

The future

As more planets will be found over the coming years from these surveys we will be able to understand the distribution of planetary masses, eccentricities and orbital distances in these alien systems better. We will learn more about multiple planetary systems and the relations between the properties of the parent stars and the types of planets they host. Within 5 to 10 years we will know about the rate of occurrence of planetary systems that have giant planets that resemble our own Jupiter, i.e. in more distant circular orbits. These types of systems may have habitable planets like our own.

However, to find Earth sized planets we will need to place arrays of telescopes in space above the distorting effects of our atmosphere. This is being planned, both Europe and the US are aiming to launch a mission around 2010 that will not only look for Earth-like planets but also search for evidence of life on them by looking for the signature of ozone in the feeble glow they emit. Only then will we know if our solar system is a unique or unusual example, or whether billions of solar systems like ours exist in the vast depths of space.

You can see more of Lynette Cook's space illustrations at www.spaceart.org/lcook/.