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Will there ever be a time when all the planets of our solar system line up in a row, one behind the other, as seen from Earth?

Ian Christie
Melbourne, Australia

The short answer is no. The long answer is: Assuming that we don鈥檛 need this alignment to include the sun, the limiting factor is that all of the planets must align with Mercury. We will also ignore the possibility of the sun obscuring the view of some of the planets.

This gives us two chances for each orbital period of each of the other seven planets. To a first approximation, this will occur once every 1.2 脳 10¹⁹ years. This time is just the product of all of the half-orbital periods of the seven planets (with a little rounding off and ignoring any long-term changes in orbits, not to mention possible cosmological events such as the big rip, when the universe may tear itself apart).

My workplace, the Victorian Space Science Education Centre, has a primary school programme that helps students understand that the pictures in educational texts and websites that show the planets all lined up are misleading, with wrong relative diameters, wrong relative spacing and an alignment that never happens.

David Craig
Edinburgh, UK

Mutual alignments, when one planet overlies another as seen from Earth, occur about once every 30 years. A rough calculation shows that an alignment of all seven planets as seen from Earth is therefore likely to occur once in 22 billion years, i.e. longer than the 14-billion-year age of the universe.

A conjunction of the five planets visible to the naked eye 鈥� when they align within 10 degrees of each other in the sky 鈥� happens roughly every 500 years. The last was on 17 September 1186 and the next will be on 8 September 2040. Unless it is cloudy.

Eric Kvaalen
Les Essarts-le-Roi, France

It depends what you mean by lining up. They will never line up exactly, unless there is some very fine tuning, but they can all be within a certain angle.

We can estimate how often Venus, Mars, Jupiter and Saturn will be within 1 degree of Mercury. This will happen around the time of a 鈥済reat conjunction鈥�, when Jupiter and Saturn line up, more or less.

Great conjunctions occur on average every 19.86 years and are sometimes triple, in which the two planets line up three times within a period of several months. When Jupiter and Saturn align, it can be anywhere from 0 to 360 degrees away from Mercury, but in every great conjunction one alignment occurs within about 80 degrees of the sun. Mercury stays within 23 degrees of the sun, so the probability of that alignment of Jupiter and Saturn being less than 1 degree from Mercury is about 1 in 80 (1.25 per cent).

Venus stays within 46 degrees of the sun, and the probability of it being within 1 degree of Mercury at the moment in question is about 1.6 per cent. Finally, the probability of Mars being in that window is about 0.9 per cent. So the probability of all four being within 1 degree of Mercury is about 0.00018 per cent. This would occur on average once in 10 or 11 million years.

This is the frequency for Venus, Mars, Jupiter and Saturn being within 1 degree of Mercury when Jupiter and Saturn line up exactly. However, the four might be within 1 degree of Mercury a couple of days before or after the exact alignment of Jupiter and Saturn, so this pattern would occur more often than once in 10 million years.

All this only considers how far apart the planets are in the east-west direction. It doesn鈥檛 account for how far north or south they are relative to each other, nor does it take Uranus and Neptune into consideration, each of which would increase the time interval for all planets to be within 1 degree of Mercury by a factor of 180.

By that time, the sun would have expanded and swallowed up Mercury and Venus.

To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.

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