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Skywatching

Planet still missing

In the 18th Century, Johann Bode and Johann Titius came up with a strange procedure for predicting where to look for planets.

It seems more like a numbers game than a scientific tool.

Make a little table with four columns. A spreadsheet program provides a nice way to do this.

  • In column 1, write the numbers 1 to 8.
  • In column 2, put zero at the top, then a 3 below it, and below that just keep doubling the number above:
    3, 6, 12, 24 etc.
  • In column 3, add 4 to the numbers in the second column: 4, 7, 10, 16, 28, etc.
  • Divide the numbers in the third column by 10, and put the results in the fourth column, which gives:
    0.4, 0.7, 1, 1.6, 2.8, 5.2, 10, 19.6.and 38.8.

In order to make the distances in the Solar System easy to comprehend, we define the average distance between the Earth and Sun as 1 astronomical unit (1 au).

Using this convention, the distances of the planets from the Sun are:

  • Mercury (0.4 au),
  • Venus (0.7 au)
  • Earth (1 au)
  • Mars (1.5 au)
  • Jupiter (5.2 au)
  • Saturn (9.8 au),
  • Uranus (19 au)
  • Neptune (30 au).

That strange series of numbers concocted by Bode and Titius are either exactly or close to the actual distances the planets lie from the Sun.

Uranus was discovered after Bode and Titius put together their number series, and was exactly at the predicted distance. This gave that list of numbers huge credibility.

Fascinatingly though, the numbers also suggested there should be a planet orbiting at 2.8 au, between the orbits of Mars and Jupiter, and there wasn't one. The search was on.

Finally, in 1801 Piazzi found something. Initially it was believed to be the missing planet, and it was named Ceres.

However, it turned out that this new discovery has a diameter of less than 1,000 kilometres, which is far smaller than the Moon, and definitely not big enough to qualify as a planet.

A year later another object turned up. Named Pallas, it was even smaller, with a diameter of a bit over 500 km.

By 1807, the count was up to four, with Vesta and Juno added to the list. Vesta is about the same size as Pallas, and Juno a bit more than half the size.

Instead of a planet, there were multiple small bodies sharing similar orbits between Mars and Jupiter. These new objects came to be called planetoids, or, less accurately, asteroids. The less accurate name is the one that stuck.

As telescopes got bigger, and cameras started to be attached to telescopes, the number of new discoveries rocketed. Astronomers carefully stabilized their telescopes over hours to collect enough light to image distant galaxies, and then found asteroids had made unsightly tracks across the images as they drifted past.

Because of this, asteroids soon became known as the "vermin of the skies.”

However, until recently, nobody knew why, instead of a planet between Mars and Jupiter, there is an enormous collection of rubble, ranging from dust and gravel to a body the size of Ceres.

Today, we have a possible explanation. The story starts back when the Solar System formed.

A great cloud of gas and dust collapsed into a rotating disc, and then as the disc got smaller, denser and rotated faster, the core formed the Sun, and in the surrounding disc, "diskettes" formed, each of which formed a planet, except for one.

The problem was that one of those diskettes had accumulated a large amount of material and formed the giant planet Jupiter. Then, the strong gravity of that planet interfered with the neighbouring diskette toward the Sun, preventing it forming a planet, and leaving it as a huge collection of smaller bodies.

We have an explanation for the missing planet, but we still have no good explanation for that series of numbers Bode and Titius put together.

  • Jupiter is conspicuous in the south overnight
  • Saturn is to its left.
  • Mars rises around midnight
  • Venus, shining even brighter than Jupiter, appears shortly before dawn.
  • The Moon will reach Last Quarter on the 11th.




The world that never was

Neptune was the first planet 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 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 fire. 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 that 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 six-kilometre diameter, other than asteroids with orbits taking them close to the Sun and then far out into the Solar System.

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 rises around midnight and Venus at 3 a.m.
  • Mercury might be discernible low in the dawn glow, rising about 4 a.m.
  • The Moon will be full on the third.


Hairy star passing through

Through binoculars, a few nights ago comet NEOWISE looked like a blurry star that had been smeared upward by the artist before the paint was dry.

At 3:45 a.m., the sky was already brightening and almost twilight. Against the deep blue, the comet was a beautiful sight, even to eyes that were still half-asleep.

What is a comet, and why do astronomers find them so interesting?

When our Solar System formed some 4.5 billion years ago, there was a lot of construction material left over. This is sitting out at the edge of the Solar System as a collection of millions or billions of lumps of dust and ice, typically a few kilometres in diameter, except that few, if any of them, are actually spherical in shape.

Occasionally, a collision or near collision between two of these bodies can result in one of them going into a new orbit, taking it into the inner Solar System.

Comet NEOWISE is one of those deflected objects.

In the outer Solar System, far beyond Pluto, it is dark and very cold. Bodies lying out there are in a cosmic deep-freeze, which can preserve them as unchanged pieces of Solar System construction material for billions of years.

It is still extremely difficult to send spacecraft into the outer Solar System. This is one of the reasons we are so interested when a lump of construction material comes into our neighbourhood, which is exactly what is happening now.

The new orbit of one of these deflected objects may take it close to the Sun, maybe closer than Mercury or Venus, and then back out to where it came from, and then back into the inner Solar System again.

The object starts its inward trip as a dark lump of dirty ice, containing trapped organic chemicals and other things from the Solar System's birth cloud.

As it gets closer to the Sun it warms up. The more volatile chemicals start to evaporate and the ice begins to melt. In the vacuum of space the ice converts directly to water vapour. Blasts of vapour erupt through the surface, blowing dust into space.

Light pressure from the Sun and the solar wind blow the vapours and dust outward from the object. This is lit up by the Sun, and that dark, almost invisible lump of dirty ice becomes surrounded by a glowing envelope and trailing a glowing tail millions of kilometres long. It has become a comet, or "hairy star,” one of the most beautiful objects we see in the sky.

In the past, the search for comets has been mostly the province of amateur astronomers. Many of them have had their names attached to the comets, following the tradition of Halley's Comet, Comet Arend-Roland, Hyakutake and Hale-Bopp.

The advent of large-field, survey and search instruments over the last decade or so, deployed on the Earth and in space, means that today the amateur comet hunters face some very strong competition. Most new comets are found using these instruments.

This new comet was discovered on March 27, by the Near Earth Object Wide-Field Survey Explorer, NEOWISE. The instrument's primary purpose is to search for near-Earth asteroids, but it is also a good comet finder.

Comets are usually discovered when they are far from the Sun, before they have got into full production of jets of vapour and dust.

At that time it is hard to tell whether it will only release a little, in which case it will remain hard to see, or if it will produce huge amounts, possibly making a spectacle visible to our unaided eyes.

Comet NEOWISE was widely expected to be a fizzler. It turns out to have become quite a spectacle.

Search for the comet with binoculars between 11 p.m. and midnight by scanning the sky below and either side of the Big Dipper.

Otherwise look in the northeast between 3 and 4 a.m.

  • Jupiter and Saturn rise around midnight and Mars in the early hours.
  • That searchlight in the east before dawn is Venus. Scan the sky to the left of Venus to find the comet.
  • The Moon reaches Last Quarter on the 27th.




The search for planets

How we managed to discover planets orbiting other stars is quite a story.

Actually, how we came to find the planets in our own Solar System is an interesting story too: one that covers our entire history, dating back to when we first started to look hard at the sky and to record what we saw.

At night our ancestors saw the stars wheeling across the sky as the hours passed, saw different constellations at different seasons. They also deduced that what constellations they saw was dictated by whether the Sun was in the sky.

The stars were still there, but invisible in the glare. People did note that under certain circumstances, they could see stars during the day, for example, looking up from the bottom of a dark hole or a hollow tower.

They also noted that the patterns of the stars did not change. However, there were some bright star-like objects that moved against the starry background over days, weeks and months.

The Greeks called them wandering stars, or planets. These wandering stars moved to and fro along a strip of sky called the zodiac. They named them Mercury, Venus, Mars, Jupiter and Saturn. All of these planets are relatively easy to spot.

The scenario of seven objects moving around the sky (including the Sun and Moon) remained for a long time. After all, seven was a number of mystical importance. Then, the telescope was invented.

The early telescopes, like the ones used by Galileo, had relatively poor light-collecting power, and even worse, could only easily observe a small piece of sky.

It was a point-and-observe instrument, not good for searching the sky for new things, although, obviously, Galileo and his contemporaries must have done that. What was needed was a revolution in telescope design.

This happened in 1668, when Isaac Newton made the first reflecting telescope. Instead of using a convex lens to collect the light and form the image, he used a concave mirror.

In addition to not having the false colours and other problems of the lenses available at the time, mirrors could be made big, because they could be supported from behind. Large mirrors mean collecting lots of light and detecting fainter objects.

William Herschel made several large reflecting telescopes and used them to search the sky for comets. In 1781, he found a new planet, which was named Uranus.

Newton's theory of gravity made it possible to precisely analyze planets' orbits, and showed that in addition to planets being pulled at by the Sun, they also tug and tweak at each other, slightly changing their orbits.

Measurements of the motions of the outer planets by Urbain Le Verrier led him to conclude there was another large planet out there. In 1846, Johann Gottfried Galle looked at the predicted position and found the culprit, now known as Neptune, a similar body to Uranus.

In 1906, Percival Lowell, who had dedicated most of his life to the study of Mars, did similar studies of the orbits of the outer planets, including Neptune, and concluded their orbits were being perturbed by an unknown, ninth planet.

Lowell failed to find it. Ironically, he had recorded faint images of this unknown planet in 1915, but hadn't noticed. Finally, in 1930, Clyde Tombaugh did.

However, it turned out that Pluto was too small to explain the orbit perturbations used to find it, so this discovery might have been more due to serendipity and dogged searching than science.

Today, we have more powerful and sophisticated instruments than astronomers in the past could have dreamed of. We can search large areas of sky, look for very faint objects and measure small perturbations in orbits.

We are finding many unknown bodies at the outer edges of the Solar System, but so far, no more planets.

  • In the early hours, Venus lies low in the northeast
  • Mars in the southeast
  • Saturn and Jupiter low in the south.
  • The Moon be new on the 20th.


More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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