COULD THERE be a tenth planet in our solar system? The idea is not as bizarre as it seems. For years, scientists have been scanning the skies for evidence of "Planet X," as it is called, in the hopes that it might explain such diverse phenomena as the irregular motion of the outer planets, the sudden demise of the dinosaurs and the influence of comets. {See box.} So far, the object -- if there is one -- has eluded them. But the hypothesis remains enticing, as do the celestial mysteries that prompted the search.

The Widening Horizon

Mercury, Venus, Mars, Jupiter and Saturn have been known since antiquity. Visible to the naked eye, their motion was seen by the ancient Greeks, who called them planetes -- "wanderers." Uranus was discovered by Sir William Herschel in 1781, because he detected that it was fuzzy, not stellar, in appearance and moved with respect to the stars.

Planetary motion was at first presumed to be regular. But in 1846, French astronomer Urbain Le Verrier and Englishman John Adams independently compared observations of Uranus with its ephemeris -- a computed table describing a body's position at a given time -- and found significant discrepancies. To explain the variation, they predicted that there had to be another planet whose gravitational force was affecting the motion of Uranus. The differences between the calculated and observed positions were approximately 100 seconds of arc -- large by astronomical standards. (One second is 1/360 of 1 degree; the sun and moon have apparent diameters of about 1800 seconds of arc.) Both men had trouble getting their fellow astronomers interested; but finally LeVerrier convinced German astronomer Johann Galle to look. The prediction was so accurate that on the first night Galle detected the planet now known as Neptune.

A similar perception later led to the discovery of Pluto. Early in this century, Percival Lowell and W. C. Pickering analyzed the observed and predicted positions of Uranus and Neptune, and found differences for Uranus of about 5 seconds of arc. They concluded that there had to be yet another planet, or planets, in the solar system. In 1930, after Lowell's death, a new and more intensive search was initiated and Pluto was found near the position he had predicted.

But the discovery generated a mystery. Lowell's prediction was based on a planet much farther from the sun and much more massive (six times the Earth's mass) than Pluto is. The gravitational influence of one planet on another is determined by mass and distance. The easy and accurate way to determine the mass of a planet is from the motion of a satellite. But the only known satellite of Pluto was not discovered until 1978. Based on the knowledge we have today, Pluto is only about one five-hundredth the mass of Earth -- much too small to be capable of detectably perturbing Uranus. Nevertheless, Pluto was discovered very near the predicted location, whether by serendipity or by proximity to an unnoticed and much heavier body.

Pluto, too, varies from its predicted position; but the data are too scant to be conclusive. Pluto is a 15th-magnitude object (a 2nd-magnitude object is 2.51 times fainter than one of the 1st-magnitude, and so forth) and therefore 10,000 times too faint to be seen without a telescope. Observations have only been available since 1914 -- 70 years out of an orbital period of 248. These observations have an uncertainty of approximately one second of arc. That range of uncertainty, combined with the short observation period, makes it impossible to calculate an accurate ephemeris.

The observations of Neptune, however, cover the period from 1846 to the present time, or 141 years out of the planet's 165-year period. These observations have been fit by ephemerides without difficulty -- but with a puzzling aspect: Approximately 10 years after an ephemeris has been calculated, Neptune no longer appears at the predicted locations. This has happened repeatedly since 1900 and continues today. No one knows why. Perhaps a Planet X is deflecting Neptune from its course. But it is also possible that the observations will match predictions when they have covered a full orbital period. There are three prediscovery observations of Neptune, one in 1613 by Galileo (possibly inaccurate) and two in 1795 by J.J. Lalande, who recorded differences in right ascension (i.e., location in the plane of the equator) of approximately 12 seconds of arc but no differences in declination (position perpendicular to the equatorial plane).

More important, Uranus continues to display irregularities in right ascension. No single ephemeris will fit the nearly 300 years of observations -- or even those since 1830, when significant improvements in astronomy made data much more accurate. Since Uranus has an orbital period of about 80 years, almost two complete orbits are covered by the data. If observations from only one orbital period are used, an ephemeris fitting those observations can be calculated. But the same computations will not fit both orbits. Again, this could be the result of some external force. But it could also be that observations before 1900 were subject to systematic error.

There are two other puzzling anomalies in the outer solar system. One is Neptune, which has two satellites: one large and close, known as Triton, and the other small and more distant, called Nereid. Nereid's orbital motion is in the same direction as that of planets and most other satellites. But Triton moves in the opposite direction. It is generally believed that planets are formed when a cloud of swirling gas and dust collapses and condenses in the force of its own gravity; hence they all spin the same way. Conversely, it is assumed that if a satellite is moving in the reverse direction, it must have been "captured" by the planet's gravitational field. Another anomaly is Pluto. Its motion is so eccentric that its orbit comes inside the orbit of Neptune. (At present, Pluto is closer to the sun than Neptune.)

Ominous Orbits

Might a Planet X have disturbed Neptune's satellite system and caused Pluto's unusual motion? And if so, how could we determine its position?

The problem is difficult. The effect of one planet on another is proportional to the mass and inversely proportionate to the square of the distance; so the farther away an object is, the more mass is necessary to cause the same effects. Thus, when scientists attempt to calculate the possible location of a planet, they cannot unambiguously determine its distance and mass. Moreover, because angular motion decreases with the distance, we cannot precisely determine its velocity or orbital period. Thus observational data can only be used to determine an approximate location, mass, distance or motion. {see illustration}

Still, we can extrapolate an orbit for Planet X in such a way that it will explain the irregularities in the outer planets' motions. Such an orbit would not have to be highly eccentric -- that is, non-circular or elliptical.

The plane of the orbit of Planet X could be similar to the other planets, or it could be more inclined (tilted at an angle) to the ecliptic, which is the plane of the Earth's orbit. The lack of discrepancies in the declination observations argues for a planet in the plane of the ecliptic. The history of previous searches indicates the possibility of an inclined orbit, which would also increase the separations between Planet X and the outer planets. Such a planet could explain the systematic errors between the observed and computed locations of Uranus. It could improve our capability of predicting the motion of Neptune. Depending on its orbit, it might provide another piece in the puzzle of the motions of Neptune, its satellites and Pluto.

It is not likely, however, to explain the motions of the long-period comets or the extinction of the dinosaurs. An orbit large enough to disturb the comets would have to be extremely eccentric -- carrying Planet X far away from the sun at one end of a long ellipse. Still, some scientists consider it possible.

Nemesis and Enigmas

Others hypothesize a star, called Nemesis, which is a binary companion of our sun and in a very eccentric orbit. It could come close enough to perturb the comets and cause bunches of them to enter the inner solar system at a given time -- thus explaining the demise of the dinosaurs. Nemesis would have a very long orbital period so that its effect on the outer planets would be very slight over the 200-year period for which we have dependable data. Its principal effect would be on the barycenter of the solar system, the point around which the sun and planets move. The effect is sufficiently small that it could not be detected in current observations. Thus, Nemesis would not give us an explanation for discrepancies in the motions of the outer planets.

Another possible hypothesis is incompleteness in our knowledge, or application, of the theory of relativity or the universal law of gravity. There is an old question concerning gravitational effects: Are they instantaneous like a field effect, or is there a time delay proportional to distance? This has been considered a settled issue in favor of the field effect, but recent evidence from precise Earth-based measurements reopens the issue.

About 1900 it was recognized that the observed motion of Mercury was different from the calculations. In that case a planet inside of Mercury was hypothesized, and searches were made during eclipses of the sun. A discovery was even claimed, although it was never confirmed. The real explanation was Einstein's Theory of General Relativity, which explained the observed motion of the perihelion of Mercury. Similarly there could be something which causes the unexplained motion of Uranus and Neptune. This might explain the observed effect, but it is not likely that it would explain the appearances of comets nor the disappearance of the dinosaurs.

Can the location of Planet X be predicted? Groups at the U.S. Naval Observatory in Washington and at Teledyne-Brown Engineering in Huntsville, Ala., are attempting to calculate the orbit and present location of the planet, using observations of both Uranus and Neptune. These computations are much more difficult than those predicting Neptune from its effect on Uranus. The systematic variations now observed are much smaller than those of Uranus 140 years ago; in fact they are not much larger than the observational uncertainties. In addition, the orbital period of Planet X is expected to be much larger than Neptune's, probably 500 and possibly 1,000 years. This in turn requires a much longer period of observations.

Another problem: Any solution actually produces two positions in opposite directions in the sky. Just as you cannot tell if the moon is overhead or underfoot when it is high tide, so there is an ambiguity in looking for missing planets. The resolution of this question depends on other factors than which solution gives the best formal numerical result. (For example, one result puts the planet in the area searched at Lowell; the other does not.) Nor do we know exactly what the mass of Neptune is, and without that it is much more difficult to apportion the influences of Neptune and Planet X on Uranus.

What is the likelihood of finding Planet X? In 1929, Clyde Tombaugh began a search at the Lowell Observatory which resulted in the discovery of Pluto in 1930. He continued that search until 1946 covering a large part of the sky for objects as faint as the 16th magnitude. There are parts of the sky he did not cover, and it is possible that the planet could be fainter than 16th magnitude. It might be very dark. Iapetus, a satellite of Saturn, has a dark side and a light side. The rings of Uranus are much darker than expected. Comet Halley is darker than coal.

Evidence may come from one of various NASA spacecraft now underway. The Infrared Astronomical Statellite (IRAS), launched in 1983, surveyed the sky for objects radiating infrared light -- indicative of solid matter. Since a planet would be far nearer and warmer than most other sources, it should appear relatively bright. The only way it could be detected, however, is by its motion with respect to the stars; and given the time difference between observations and the accuracy in determining positions, the possiblity of detection is marginal. Nonetheless, the IRAS data are still being studied.

In addition, two Pioneer spacecraft are now traveling in different directions in the outer part of the solar system. Monitoring radio signals from these spacecraft, scientists are able to calculate their motions and velocities using knowledge of the positions and masses of all the planets. As these spacecraft go further out into the solar system, they may have greater sensitivity to the distance, direction and mass of any additional planets (provided, of course, they are traveling in the correct direction). NASA has recently announced that, so far, the Pioneers have not detected any unexplained motion.

And traditional ground-based searches for Planet X continue. Astronomers at Lowell Observatory are now looking using the Teledyne results, and a survey has already been carried out from Palomar Observatory in California. Several small searches have been carried out from the Naval Observatory in Washington; another is underway from the Naval Observatory station at Black Birch in New Zealand. So far only one thing is certain: There is a lot of sky to cover, even with a good guess, and nobody thinks it's going to be easy.

If it has a relatively normal orbit, Planet X's gravitational field could explain the perturbation of the outer planets. If the orbit is extremely eccentric, it could travel far enough to affect the flight of comets.

1. Gas and dust, spinning slowly in a huge cloud, are drawn together by their gown gravity.

2. As the cloud collapses, it revolves faster and solid elements form into clusters.

3. The mass condenses into a disk shape. Heat at the center increases until a star is formed. Solid clusters grow larger, forming planets in the disk plane.