365DaysDate: April 4, 2009


Title: Census of the Solar System

Podcaster: Darren Bennett

Organization: South East Queensland Astronomical Society

Description: When I grew up, there were nine planets, a couple of dozen moons, a hundred comets, and six thousand asteroids. But that was thirty years ago, and things have changed. This podcast combines a look at how many of what objects is/are out there, with some historical digressions on the definitions of the objects involved.

Bio: As a child, Darren read the astronomy sections of two local public libraries. Thirty years later he spends his spare time observing the skies, participating in the activities of the local astronomy societies, and helping his friends take astronomy to schools and the public in Brisbane, Australia. As the Shallow Sky Officer of the South East Queensland Astronomical Society, he describes his beat as 0.1AU from the Sun to 100AU from the Sun.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by the AAS.


Hi, this is Darren Bennett, the Shallow Sky Officer of the South East Queensland Astronomical Society. Today I’d like to talk to you about my Census of the Solar System: a look at how many of what objects is/are out there, with some historical digressions on the definitions of the objects involved. Let’s start at the centre.

Stars: One and only one. Despite much conjecturing over the years, there is no compelling evidence that the Sun has any stellar companions. As visible and infra-red surveys scan for fainter objects, we’ll assume it’s becoming less likely all the time.

Planets: Variable, currently eight. When I grew up in the seventies, there were nine planets. There had been nine planets since Pluto had been discovered, and barring the surprise discovery of a planet X in the outer solar system, there were going to be nine in the future. There were no unexplainable motions of the existing planets that would lead to the Neptune-like discovery of another planet. Textbooks could be written with confidence. Before 1930, there were eight planets. One assumes that before the discovery of Neptune in 1846, there were seven planets, and before the discovery of Uranus there were six, and so on back. But the history of astronomy is rarely that simple, and certainly isn’t in this case. Perhaps we’d better start at the beginning.

If we go back to the origin of the word planet, from the Greek for wandering stars, this was used to describe all the objects in the sky that weren’t fixed, like the stars, or obviously atmospheric phenomena, like comets. So there were seven planets that orbited the earth: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. Some sources would not include the Moon and the Sun, which had obvious discs, and so gave the five planets as: Mercury, Venus, Mars, Jupiter, and Saturn.

In the early seventeenth century, with the increasing use of telescopes, and the adoption of the heliocentric model of the solar system, some changes obviously had to be made. The Sun was dropped from the list, and the Earth was added. Then, when the large satellites of Jupiter and Saturn were discovered, the Earth’s Moon was also dropped, and those other satellites added. In the late seventeenth century, a list of the planets might be given as: Mercury, Venus, Earth, Mars, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Titan, Iapetus, and Rhea – the thirteen planets. By the late Eighteenth century, general consensus had dropped the satellites, and a list of the seven planets would be: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Georgium Sidus, or Herschel, or Uranus, depending on who and where you were.

On January 1st, 1801, Piazzi discovered a new planet orbiting between Mars and Jupiter. Ceres was followed by Pallas in 1802, Juno in 1804, and Vesta in 1807. None of these showed a disk, and William Herschel, discover of Uranus, in 1802 declared of Ceres and Pallas: “From this, their asteroidal appearance, if I may use that expression, therefore, I shall take my name, and call them Asteroids; reserving for myself, however, the liberty of changing that name, if another, more expressive of their nature should occur.” However First Steps to Astronomy and Geography of 1828 lists the planets as: “Eleven: Mercury, Venus, the Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Herschel”.

No more planets were discovered until 1845, when Astraea turned up. But the cracks were starting to show: already in 1841, the Nautical Almanac and Astronomical Ephemeris listed the elements of the first four asteroids under “Minor Planets, Elements of”. Then, in 1846 the next “real” planet, Neptune was discovered. By 1851 there were fifteen known objects orbiting between Mars and Jupiter.

By the mid-1860s, European publications were referring to the asteroids as “small planets”. The US Naval Observatory historically seemed confused about these bodies, till 1868 it called them “asteroids”, from 1868 to 1892 “small planets”, from 1892 to 1900 “asteroids” again, from 1900 to 1929 “minor planets”, and since then back to “asteroids”. By the end of the nineteenth century we had settled down to the eight planets known today.

But the hunt for Planet X was on. Planet X was hypothesized to exist in the outer solar system to explain discrepancies in the orbits of Uranus and Neptune. In 1930, while searching for Planet X, Tombaugh discovered Pluto. Eventually, it became apparent that Pluto was too small to cause the discrepancies, so further searches for Planet X ensued. Finally, spacecraft data revised the planetary masses of Neptune and Uranus, removing the discrepancies. So, no Planet X was needed.

My university astronomy text, Abell’s Exploration of the Universe of 1982, says: “The nine known planets include the earth, the five other planets known to the ancients (Mercury, Venus, Mars, Jupiter, and Saturn), and the three discovered since the invention of the telescope (Uranus, Neptune, and Pluto).” Here things remained till 2006, when the IAU gave Pluto the boot.

Dwarf Planets: This is not the place to be going into the IAU deliberations on planets in 2006, so we’ll accept their ruling. Currently, there are five dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. Estimates for the total number of dwarf planets expected to be found range from two hundred to two thousand.

Planetary Moons: Lots, and growing. Back in the seventies, the list of planetary satellites was fairly static as well. Earth had its moon, Mars had the two discovered in the nineteenth century. Jupiter had thirteen, Saturn had nine, Uranus five, and Neptune two, mostly pre and early-twentieth century discoveries. Total: Thirty-two. In 1978 Pluto’s moon Charon was discovered, but as a dwarf-planet moon, I can’t count it now.

In 1979 the second golden age of planetary satellite discovery commenced, all because of a couple of space probes called Voyager. During the eighties, Voyagers I and II added three moons to Jupiter, seven to Saturn, ten to Uranus and six to Neptune. With a few ground-based discoveries thrown in, by the early nineties the satellite total had risen to sixty.

The third golden age of planetary satellite discovery started in the late nineties. This is provided by the new eight-plus-metre class of telescopes, with their adaptive optics systems and sensitive electronic cameras. As of early 2009, the totals stand as follows: Jupiter sixty-three, Saturn sixty, Uranus twenty-seven, and Neptune thirteen. This gives a grand total of one hundred and sixty-six planetary satellites.

Dwarf Planet Moons: Between them, the five current dwarf planets have six moons.

Comets: Probably not as many as you might think. As a bright comet can put on a spectacular display, we have records of these going back many thousands of years. Yet the average naked-eye observer is lucky to see a dozen easily visible comets in a lifetime. We break down the comets as follows:

Numbered periodic comets: 212 – these have been observed through a second perihelion passage, or have periods less that 200 years. Unnumbered and intermediate periodic comets: 245 – these have a defined periodic orbit, but do not yet meet the numbering criteria. Long period comets: 3,174 – these do not have a well defined period. Total: 3,631

The current champion comet hunter is the SOHO satellite – the Solar and Heliospheric Observatory. Although its prime mission is to study the sun, every day hundreds of keen observers worldwide download the latest images to look for sun-grazing comets. SOHO’s current comet count stands at 1558.

Minor Planets: More than you’d probably think, and growing almost exponentially. I suppose I didn’t really become aware of minor planets as a class until I got to university. This was the mid eighties, and back then the number you heard was six thousand. “There are currently six thousand numbered asteroids”, the books would say, but even then it was obvious that they had to qualify the statement with “current”. These vermin of the skies just kept being found.

As mentioned previously, the first minor planet was found in 1801, and the first four by 1807. Then there was a thirty-eight year gap until 1845, when the fifth was found. Since then only two years, 1846 and 1945 have gone by without a numbered minor planet being found. By the end of the eighteen-hundreds 463 numbered minor planets had been discovered.

You’ll notice I’m using numbered minor planets here for the counting. After discovery, a minor planet is given a provisional designation, and is numbered after the orbit is confirmed. For a main belt asteroid, this usually involves at least one orbit of the sun for the asteroid, and its recovery at the predicted location. Consequently, by counting only numbered minor planets, I’m sure we aren’t counting any duplicate observations of the same object.

Back to the count. In the first half of the nineteen-hundreds, another 1,658 numbered minor planets were found. In the next thirty years, up to 1979, another 6,753. The eighties brought along another 5,499.

The first five years of the nineties saw 8,031 numbered minor planets found. The second half of the same decade, a staggering 45,631 new bodies. And the first five years of the 2000s, an astronomical 123,336. Another thirteen-thousand odd since then, brings the grand total of numbered minor planets up to 205,172. And remember, the recent flattening of the curve is due to the time lag in numbering an asteroid after discovery, and does not represent a slackening off of the pace of discovery.

And unnumbered? About another 230,000, give-or-take, just waiting for confirmation of their orbits to join the ranks of the numbered. 14,971 numbered minor planets have been given names. 102,789 have been discovered by the LINEAR project at Lincoln Laboratory, New Mexico, making it the most prolific minor planet discoverer to date.

The numbers I have quoted today were the best I could put together on January 19th this year. Some of them will have already changed, because if we can be certain of one thing in this solar system, there’s always more out there to find.

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.

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