Nature Science Update

[Update]

Space: Disappearing planets
by Leslie J Sage

Just over a year ago, Dr Michel Mayor and Dr Didier Queloz of the Geneva Observatory in Switzerland announced the possible discovery of the first planet around a Sun-like star called 51 Pegasi. The evidence was based on observations showing that the star was wobbling systematically in space, in just the way that it should if it were orbited by a large planet about half the mass of Jupiter, by far the largest planet in our own Solar System.

In our own Solar System, giant planets such as Jupiter and Saturn orbit at distances relatively far away from the Sun. Hence the next surprise about the planet around 51 Pegasi: although a Jupiter-like planet, it appeared to be closer to 51 Pegasi than tiny Mercury is to our Sun. Everything astronomers believed they understood about the formation of stars and planets said that no planet should be that close to its star, especially not a 'gas-giant' planet similar to Jupiter. But there it was, zooming around 51 Pegasi once every four and a quarter days. Or so it seemed.

Over the past year, seven other planets have been detected, many of them orbiting surprisingly close to their parent stars. Three of these planets have 'years', known as orbital periods to astronomers, that are only about four days long, and one has a period of about two weeks. By comparison, Jupiter's period -- it's 'year' -- is about twelve Earth-years long.

Now, Dr David Gray, of the University of Western Ontario, Canada, reports in the 27 February 1997 issue of the science magazine Nature that all is not as it seems with 51 Pegasi. In fact, he has found that the star is pulsating in just the way needed to mimic the signature of a planet in orbit around the star. This is a new and unexpected type of oscillation, which is why Drs Mayor and Queloz discounted that explanation for their results, but their equipment did not have the ability to measure the subtle variations that Dr Gray has found.

A number of research groups have been searching for extra-solar planets, mainly looking for systematic shifts in the spectral lines in the atmospheres of stars like the Sun. Although we commonly speak of planets orbiting the Sun or a star, they actually orbit a common point, called the centre of mass. The centre of mass of the Sun-Jupiter system is just above the surface of the Sun, so the Sun doesn't move very much as Jupiter goes around it. (The other planets have much smaller effects on the motion of the Sun, and can for all practical purposes be ignored.) The larger the planet, or the closer it is to a star, the more quickly the star will move.

This motion is reflected in the positions of characteristic 'lines' in the spectrum of light from a star, which arise from elements such as iron, calcium and sodium, in the gas above the star's surface. The spectral lines are all shifted from their 'rest positions' in one direction as the star comes towards us ('blueshift'), and in the opposite direction as the star goes away from us ('redshift'). The size of the shift, combined with a measurement of the period of the orbit, tells astronomers how massive the planet is, and how far it is from the star.

This line-shifting is a manifestation of the Doppler effect. The usual example of the ambulance siren is still the best way of explaining the phenomenon: imagine you are standing at the kerb while an ambulance rushes past you on the street, siren wailing. The pitch of the siren sounds higher as the ambulance approaches you than when it recedes into the distance. As with ambulance sirens, so with the spectra of light from stars as they wobble around the centre of mass of the star-planet system, alternately approaching and receding from the Earth with each rotation.

Although simple in theory, the measurement is very difficult in practice. Dr Gordon Walker of the University of British Columbia, who developed the technique now in use around the world, spent much of his career looking for planets, but never finding one. He did find a number of stars which oscillated in just the right way to mimic the signature of a planet, and he has therefore always urged caution in leaping to the conclusion that Dr Mayor and Dr Queloz's data demonstrated the presence of a planet. His caution has now been vindicated.

Dr Gray has found that the spectral lines aren't really redshifted or blueshifted, rather the line shapes change just the right amount, and with just the right period, to explain the apparent shifts. A crude analogy is that of a boat on the ocean. As the waves pass under the boat, it rises and falls systematically. If you couldn't see the water itself, you might conclude that the boat is rising and falling because the general level of the water is changing, as it does with the tide: you needn't know anything about waves. This is like the situation where the presence of planets was inferred from the data. But, if you look more closely, you'll see that in fact it is the waves passing underneath the boat that causes it to rise and fall, not a general rise and fall in the level of the ocean. This is analogous to what Gray has done -- he's looked more closely at the spectrum of 51 Peg, and seen the line shapes changing.

No criticism of Drs Mayor and Queloz is intended or justified -- they did the best they could with their equipment, and realized that someone might eventually find just what Gray has seen. That's the way science works.

Another group, based at San Francisco State University, posted a rebuttal of Gray's paper on their world-wide web page (http://cannon.sfsu.edu/~williams/planetsearch/nrp.html) last week, before the appearance of Dr Gray's paper in Nature: meaning that astronomers were unable to assess the merits of Dr Gray's arguments at the same time as the rebuttal. A substantial part of the rebuttal is based on a paper to be published in the Astrophysical Journal by Dr Artie Hatzes, of the University of Texas at Austin.

In particular, a quote from the paper to the effect that Dr Hatzes did not see variation in the line shapes is given special prominence in the rebuttal. Dr Hatzes made measurements similar to those by Dr Gray, but he was very careful to explain in his paper that he had insufficient data to rule out the kind of oscillation seen by Gray. This important message was not reproduced in the rebuttal.

Dr Gray's work leaves a question-mark over the presence of other extra-solar planets. He suggests that the four candidate planets with orbital periods less than about two weeks need to be re-examined carefully, to see if they show the same effect he sees in 51 Pegasi. This will be a slow, arduous process. The existence of the four remaining planets with longer periods is not being questioned at this time, but astronomers undoubtedly will be looking more closely at them as well. While almost all astronomers agree that there probably are planets around most stars, it will be some time before their presence is demonstrated conclusively.

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