A planet that never was

In the summer of 1991 I was in Buenos Aires, attending the triennial General Assembly of the International Astronomical Union, briefly and more frequently referred to as the IAU.

More than 2,000 astronomers from over 80 countries had descended on this South American metropolis in Argentina. The deliberations of the IAU were held in a conference centre and all of us, participants and accompanying persons were accommodated in nearby hotels.
In my hotel, my companion at breakfast was Sir Francis Graham Smith, from the Jodrell Bank. Not to be mistaken for the typical financial institution, the Jodrell Bank is a radio observatory run by the University of Manchester and the word “bank” in its name denotes the slight rise in ground level at its location. Our conversation would normally be small talk about astronomy in Cambridge, England, to which we once belonged, about astronomers we both knew and, of course, the weather which left nothing to be desired.
But on Wednesday, the 24th of July, I sensed that something was different. Sir Francis seemed in a state of suppressed excitement. Did he have some secret to keep? Although curious, I felt it proper to keep silent. And this strategy paid off. For, half way through his breakfast, Sir Francis told me to look at the daily newspaper that IAU published to inform the participants about news and views at the conference. The news usually contained announcements about stars and humans. “I have a little piece in there that will interest you,” said Sir Francis. “Indeed? You have tickled my curiosity. May I be privileged to get a preview?” I asked. After some hesitation, Sir Francis obliged on (the usual condition of) my swearing to silence till his press conference was over. Assured on that count, he revealed his secret which was indeed very significant news: “A Jodrell team has found a planet going round a pulsar.” The team consisted of three scientists led by Andrew Lyne, the other two members being Setnam Shemar and Matthew Bailes. It was Shemar who had done the main part of this work.
Till then planets circling stars other than the Sun were unheard of. Indeed, as one may easily imagine, a planet not being self-luminous would be difficult enough to spot through a typical optical telescope.
Till 1991 most astronomers were pessimistic about spotting extra-solar planets. It was against this background that the method used by the Jodrell team needed to be appreciated.
The radio observatory at Jodrell Bank had been successful in discovering many pulsars. Typically a pulsar is spotted by its highly regular pulsation: even the best man-made watch cannot compete with the pulsar’s time-keeping. Sometimes because of external disturbances the period is affected and the nature of irregularity of period can be identified with a specific cause. In this case the pulsar with catalogue number PSR 1829-10 showed some erratic effect in its period. The irregularity was noticed by Shemar who studied its occurrence carefully and found that it occurred periodically. Why did the irregularity occur? The simplest explanation was that a second body present near the pulsar was gravitationally pulling it.
Now we know from our school science that a planet like Venus is made to go round the Sun because the latter exerts gravitational pull on it. Although according to Newton’s third law of motion, the reaction of Venus on the Sun is equal and opposite, it is too small to have any noticeable result. Analysis of the effect on the pulsar suggested that it was being pulled by the gravity of a planet which was 10 times the size of the Earth, and the planet appeared to be moving at the same distance from the pulsar as the Sun-Venus distance. The period of revolution of the planet was given as six months.
As an example of planet around a star other than the Sun, this was a pioneering case. But it left astronomers with a couple of uncomfortable thoughts. Conventional wisdom said that a pulsar like PSR 1829-10 would form from the explosion of a star when it became a supernova, a highly catastrophic event. If the planet had existed prior to the event, it would have been shattered during the explosion. So one had to assume that the planet appeared there or was created in situ after the explosion. So far no one had contemplated such a scenario as a likely source of planets.
And the second aspect that looked suspicious was the near-six-month period of revolution of the planet. The natural variation of cosmic events observed from the Earth as it goes round the Sun has a period of one year. Some events may show a period of six months. But all these are linked with the Earth’s motion round the Sun. It has nothing to do with any periodic nature of the event observed. In short, after closer examination of the data, the Jodrell group came to the conclusion that the six-month period seen by them has nothing to do with the period of a planet going round the pulsar. What was observed was easily explained by our observing the pulsar from the moving platform of the Earth.
So it was a false alarm after all! At another international conference Andrew Lyne retracted the earlier claim. By a coincidence, at the very same meeting, Aleksander Wolszczan, an astronomer working with the large radio telescope at Arecibo, announced the finding of not one but two planets round another pulsar PSR 1257+12. This time, the observers had thoroughly checked their data and their conclusion stands. Of course it leaves the question unanswered:
How these planets came to be there despite a supernova explosion in the recent past?
As a postscript, one may add that in 20 years since then more than 500 planets have been discovered, most of them going round normal stars. Thus our Sun can no longer claim to be the only common type of star with planets.