The world that wasn’t

The planet Neptune was the first planet to be discovered by detecting and measuring deviations of known planets from the predicted orbits. From these measurements astronomers calculated how big a body was causing it and where to look. The sky is a very big place, especially when we are looking through telescopes that can see only a tiny bit of sky at any one time. We need to know where exactly to look, and when.

After this method triumphed in finding Neptune, the search was on for other unknown planets. Almost immediately the attention of astronomers was drawn to Mercury, the closest planet to the sun. There was something funny about the planet’s orbit. All the planets have very slightly egg-shaped orbits, so there is one point where they are at their closest to the sun (the perihelion) and at the other end of the orbit, where they are at their furthest (the aphelion).

Strangely, instead of the orientation staying fixed compared with the stars, it was slowly slipping, or precessing. Something had to be causing it, maybe an unknown planet.

Astronomers made measurements, calculated, and then concluded the perturbations to Mercury’s orbit indicated there was a planet orbiting the sun even closer than Mercury. They were so sure that even before the planet had been found, it was given a name, Vulcan, after the Roman god of Àre. Temperatures on the surface of Mercury are high enough to melt lead. Vulcan would be far hotter.

Mercury is a hard planet to observe. Since it orbits close to the sun, most of the time it is lost in the sun’s glare. We only get to see it when it is at the eastern or western extremes of its orbit. Then it is at a large enough angle from the sun for it to rise in a reasonably dark sky before sunrise, or to set long enough after the sun for most of the glare to have gone. Vulcan was expected to be far harder to find, so even when astronomers searched the predicted positions and failed to find it, they were convinced it was probably there, somewhere, and continued searching. There was the odd report of someone actually finding it, but the observations could not be repeated and were not consistent. Finally, in the early years of the 20th Century, someone came up with a new branch of physics, which offered a completely different explanation for the problems with Mercury’s orbit. The scientist was Albert

Einstein and the explanation for the orbit problems was the distortion of space-time due to being close to the large mass of the sun.

All objects bend space-time, similar to the distortion of a trampoline when we drop a bowling ball or other heavy object on it. Because Mercury’s orbit takes it nearer to and further from the sun, it moves between its perihelion, where space-time is more distorted, out to its aphelion, where it the distortion is less. It is this that causes the orbit to precess, and Vulcan became the "planet that never was", or maybe not. What, if anything did those reports of sightings of Vulcan mean?

There are two satellites sharing Earth’s orbit around the sun, one leading and one trailing. The pair, called STEREO is being used to make high quality, 3D images of the sun and coronal mass ejections, and to provide a more complete view of our Star. A search through the STEREO database has come up with nothing, which means there is nothing there bigger than about 6km diameter, other than asteroids with orbits taking them close to the sun and then far out into the Solar System. So far the only planet Vulcan we know of is the fictional Mr Spock’s home world. That planet is said to orbit the star 40 Eridani A, which lies 16 light years away. Astronomers recently discovered a planet orbiting that star.

Guess what they called it.

• Jupiter and Saturn rise around 9 p.m., Mars around midnight and Venus at 3a.m.

• Mercury might be discernible low in the dawn glow, rising about 4 a.m.

• The moon will be full on the 3rd.