IN 1781, William Herschel, a church organist and amateur astronomer, became famous overnight. He did it by discovering Uranus, the first new planet to be observed since mankind had noticed that planets existed. Herschel was lucky. He blundered across his discovery. But Johann Galle, who located Neptune in 1846, knew exactly what he was doing. Irregularities in the orbit of Uranus, caused by the gravity of Neptune, had allowed John Adams, an Englishman, and Urbain Leverrier, a Frenchman, to calculate (independently, but with many accusations of bad faith and cross-Channel espionage), where the telescopes should look.

Similar calculations, based this time on Neptune�s orbit, led to the prediction of a ninth planet �and Pluto was duly found in 1930. But that, too, turned out to be a fluke. Pluto is too light to influence Neptune�s orbit to the necessary degree. Ever since that was realised, planetary scientists have been wondering if a tenth globe is lurking out there.

At first, it was referred to as planet X, conveniently eliding the Roman numeral for ten with the mathematical symbol for an unknown quantity. Later, the possibility was floated that X might be a small star. In one version of this story, the star was called Nemesis, and was accused of causing mass extinctions of life on earth. It did this, it was supposed, by disrupting the Oort cloud of comets that forms the outer reaches of the solar system, sending waves of them crashing periodically into planetary space and so into planets, the earth included.

Over the past few days, echoes of these stories have come together. Two researchers (separated by the Atlantic ocean, rather than the English Channel), have used gravitational perturbations that affect comets, rather than planets, to predict the existence of something that might be a planet, or might be a star � depending mainly on how you define a planet or a star.

The first to get his retaliation in was John Murray, of the Open University, in Britain. His paper, published in the latest edition of the Monthly Notices of the Royal Astronomical Society, was leaked to the press on October 7th. John Matese, of the University of Southwestern Louisiana at Lafayette, had to wait until October 11th to present his version of the story at a meeting of the American Astronomical Society (AAS) that was being held in Padua, Italy. Both researchers drew on the same body of data, and both came to broadly the same conclusions.

The data are the orbits of comets that are likely to have been first-time visitors to the inner solar system. Such "long-period" comets have orbits that, if left alone by the gravitational pull of the planets, bring them close to the sun only once every 1m years or so. But the reason that they come near the sun at all is because their original orbits, which kept them in the distant realm of the Oort cloud, had been somehow disturbed.

Such disturbance has traditionally been ascribed to the combined gravitational effect of all the stars of the earth�s home galaxy, the Milky Way. At Oort-cloud distances, the strength of this combined gravity adds up to the same sort of force as that exerted by the solar gravitational field, enfeebled as it is by its remoteness from the sun. But that strength is exerted unevenly: the pull in the direction of the galactic centre (where there are a lot of stars) is greater than that in other directions. The resultant tidal wobble, so the story goes, dislodges comets from their historical orbits and sends at least some of them tumbling towards the sun.

The story told by Dr Murray and Dr Matese is, however, rather different. After looking at the orbits of almost 300 long-period comets, they have concluded that too many are coming from particular parts of the sky. The galaxy�s tidal wobble is, they think, being modulated by the gravity of something big within the Oort cloud itself.

Dr Matese�s reception at the AAS meeting was, it has to be said, not altogether enthusiastic. Some astronomers seemed to think that he and his colleagues had been too selective about the sample of comets they studied. But, according to Dr Matese, once you start including comets with shorter orbital periods, you lose sight of the wood for the trees. Short-period comets have their short periods precisely because of their interactions with closer-by planets, which are already known to astronomers. They have, in essence, "forgotten" where they originally came from.

If the researchers are right, their putative new object lies somewhere on a so-called "great circle" that stretches all the way around the heavens, and represents the view, from earth, of the object�s orbital plane. Unfortunately, they disagree about exactly which great circle that is (this appears to be because Dr Murray was even more discriminating than Dr Matese about which comets he included in his analysis), and only Dr. Murray is prepared to have a stab at where abouts on his great circle X actually lies.

Choosing which comets to put into some grand synthesis, and which to leave out, will no doubt exercise astronomical minds over the next few months. On the other hand, the two researchers do more or less agree about distances. Dr Murray reckons that the object is about 32,000 times as far from the sun as the earth is (a distance known as an astronomical unit, or AU). Dr Matese thinks it is perhaps a little closer�around 25,000AU.

Whichever turns out to be right (if either does), the new object must be very faint, otherwise it would have been spotted already. That means it cannot be a proper star. The light from even the smallest possible star would shine brightly enough to be noticed at such a distance. This, in turn, means that the thing cannot be all that much bigger than Jupiter, the largest known planet. Dr Matese reckons it could weigh as little as three jovian masses. Dr Murray notes that it could not be much bigger than ten times the size of Jupiter, or it would reflect so much sunlight that it would have been seen already.

Nor can it be Nemesis. That object, if it exists, is predicted to have an orbital period of 26m years. Even at a distance of 32,000AU, something going round the sun would complete its orbit in a sprightly 5.8m years.

So�always assuming Dr Murray and Dr Matese are right�what could they have discovered? According to Dr Murray, it is unlikely to be a proper planet, for two reasons. First, its orbit appears to run in the opposite direction from those of the nine known planets. Second, that orbit looks so unstable that he thinks it unlikely that it could have lasted for the 4.5 billion-year history of the solar system. It is more likely, then, that the object was floating free in space, and was captured by the sun�s gravity in what was, in astronomical terms, the fairly recent past.

That would be exciting. The past few years have seen the discovery of several apparent planets going round stars other than the sun. (Again, this was done by observing the effects of gravitational perturbation, this time on the light of the stars in question.) But argument rages, much as it once did over how many angels could dance on the head of a pin, about whether these new discoveries are "true" planets, or are star-like objects, too small to shine, known as brown dwarfs.

If the new object were a captured alien, it could give astronomers their first good look at an extra-solar planet, or a brown dwarf if that is what it turns out to be. And if it exists, they should be able to see it pretty soon. Even something too small to emit visible light will probably give off radio waves and infra-red radiation (Jupiter does). So a big radio telescope would probably be able to spot it. If not, the next generation of infra-red space observatories certainly should. And if they do, both Dr Murray and Dr Matese should enjoy rather more than 15 minutes of fame. For they will have been instrumental in putting an interesting new "X" on to the map of the night sky.


LINKS
An abstract of John Murray�s article

A summary of John Matese�s presentation in Padua appears on the
American Astronomical Association�s website

More details are published on
John Matese�s web pages