Dwarf planets, asteroids and comets

A slightly lumpy grey globe, covered in craters, with a light, bright spot on the right. — Ceres in near-true colour, photographed by the Dawn probe on 4 May 2015; the bright crater on the right is called Haulani. Justin Cowart, Public domain, source

There is an object in the solar system that has changed category twice without ever moving a metre.¹

It is Ceres. Its story closes a question that we opened elsewhere on this site.

The small bodies of the Solar System

The eight planets are not alone in circling the Sun: a host of smaller objects also orbit it, and they fall into three families.

Asteroids are rocky bodies, without an atmosphere, most often irregular in shape. The great majority occupy a region lying between Mars and Jupiter, the main belt. Contrary to what films show, there is no need to zigzag your way through it: the objects there are separated by millions of kilometres on average.²

Comets are made of ices and dust. Far from the Sun, a comet is nothing more than a dark nucleus a few kilometres across. As it approaches, its ices turn directly into gas and form a luminous envelope, the coma, then two tails that behave differently.

The ionised gas tail is thin, bluish, and points away from the Sun: it is swept along by the solar wind.
The dust tail is broader and brighter; the grains, much larger, remain partly on the comet’s path, so that it generally appears curved and not pointing strictly away from the Sun.³

A counter-intuitive consequence: these tails are not behind the comet. When it moves away from the Sun after its passage, it travels tail first. Comets come from the outer reaches: the Kuiper belt, beyond Neptune, and much farther still the Oort cloud, which surrounds the whole Solar System.

Dwarf planets are massive enough for their own gravity to have made them nearly round, but they have not cleared the neighbourhood around their orbit, and they are not satellites. Pluto and Ceres are among them.⁴

It is this last criterion that cost Pluto its status as a planet in 2006. Bear it in mind, because this whole page examines what it is worth: the question is not what Ceres really is, but who decides on the boxes, and by what criteria.⁵

1 January 1801

An Italian priest, a mathematician and astronomer, is observing the sky from Palermo. He is looking for a known star, and he notices next to it a point that was not there before.⁶

The next day, he observes it again: the point has moved.

He first thinks it is a comet. Then he observes it twenty-four times over the following month, and its motion convinces him that it might be a planet.⁷

He sends his observations to Paris in April; they are published in September. But the object is by then too close to the Sun to be seen again: it is not until December that someone else confirms it.⁸

It is named Ceres, after the Roman goddess of the harvest.⁹

Status number one: planet, for fifty years

For half a century, astronomy textbooks list Ceres among the planets.¹⁰

Between 1800 and 1806, other similar objects are discovered in its neighbourhood. Astronomers begin to wonder whether they have stumbled upon a whole family of objects whose existence no one had suspected.¹¹

Status number two: asteroid, for a hundred and forty years

The word “asteroid” is adopted as early as 1802 by the astronomer William Herschel, who borrows it from a classical scholar, Charles Burney Jr.¹²

It becomes untenable to call them all planets. In the 1860s, when it is understood that Ceres has several hundred similar neighbours, the distinction between planets and asteroids is widely accepted — even if what exactly a planet is remains unclear.¹³

Ceres was thus demoted, and this status lasted about a hundred and forty years.

Status number three: the reversal

In 2006, the International Astronomical Union adopts a definition of the word “planet”. Our page on the Solar System tells how this was decided: yellow ballot cards raised in the air, on the last day of a congress, and Pluto ceasing to be a planet.

Yet the same text creates a new category: dwarf planet. Pluto becomes its first member. Ceres is the second — and remains to this day the only dwarf planet in the asteroid belt.¹⁴

The same day, the same text, the same decision demoted Pluto and promoted Ceres.

Neither had moved.

In March 2015, the Dawn probe went into orbit around Ceres: the first visit to a dwarf planet.¹⁵

What to conclude from this, and it is not what one might think

One might conclude that these people change their minds all the time and that their categories are worthless.

The conclusion is tempting, but wrong, as a closer look shows.

A summary by the American Astronomical Society puts it in a way that we find very apt: astronomers have reclassified Ceres twice, and each time it found itself among the very first members of a wholly new category.¹⁶

The same summary adds the essential point: the way an object is classified is linked to the advancement of knowledge. Things are not reclassified on a whim; they are reclassified because enough has just been learned to need a new box.¹⁷

In the 1860s, it was not “demoted” out of pettiness: it had just been realised that there was a whole family of objects whose existence no one had suspected in 1801. A word was needed for them.¹⁸

In 2006, it was not “promoted” out of kindness: a whole population of round objects that do not dominate their orbit had just been discovered. A word was needed for them too.

Ceres is not the victim of a capricious administration. It is a marker. Each time it changes boxes, it is because enough has just been learned to need a new box.

Three boxes, the same question

Our page on the Solar System already presented two of these three cases. Let us bring them together one last time, because Ceres sheds a different light on them.

A shop category invented at a meeting in London, on 29 June 1987, because people in the record business did not know where to shelve certain records — and the minutes of their meeting exist.¹⁹

A word that the United Nations deliberately refuses to define, so that each people may decide for itself who it is.

A decision taken by a show of hands, one day in August, at the end of a congress.

A small ball of rock and ice that has been a planet, then an asteroid, then a dwarf planet, without ever doing anything other than circling the Sun.

Categories are not in nature. They are made by people, at a given moment, to solve a problem they had.

That does not make them arbitrary. It means that the same question must always be asked: who made this box, when, and to solve what?

The comet of 1910, and how a fear is manufactured

Words and categories shape our view of the sky, but hasty interpretations of science can also manufacture real panics.

In 1910, Halley’s comet passed close to the Earth. People bought gas masks. Others sealed up their windows.

This story is a textbook case, because each of its stages can be reconstructed.

Stage 1. In February 1910, an observatory publicly announces that the light from the comet’s tail shows the presence of cyanogen, a toxic gas. This fact is accurate.²⁰

Stage 2. A very prominent French astronomer suggests that this gas could snuff out all life on the planet. He presents it as a theoretical possibility.²¹

Stage 3. Other astronomers, quoted in the same newspaper article, reject the idea: a comet’s tail is extremely rarefied, and there is nothing in it to poison anyone.²²

Stage 4. The newspaper’s headline, however, keeps only the poison. The other newspapers pick up the sensational part without the contradiction.

The contradiction was there. In the same article. It did not travel.

Hence the maxim: corrections never travel as far as headlines.

What people did, and why we should not laugh at it

The newspapers of the time report that gas masks, “anti-comet pills” and anti-comet umbrellas were sold.

Some people, it is said, made costly decisions: not sowing, not repaying debts, since they were going to die before then.

These decisions would be perfectly reasonable if the premise were true.

These people were not foolish. They had been misinformed by a source they had good reason to believe: a famous astronomer, relayed by the newspapers.

The question to ask, then, is not “how could anyone believe that?”, but rather: how, today, can we tell an alarming headline from an established fact for ourselves?

And a caveat, which compounds all the rest

The wording of the previous paragraph deserves attention: “the newspapers report that”, “it is said”. This is not caution for the sake of style.

We know the newspapers’ panic very well. We know much less about the panic of the people. Almost all our sources on this alarm are… the newspapers that produced it.

The source that documents the panic is the one that manufactured it. That is the trap within the trap.

Halley’s comet will come back. Most people see it only once in their lifetime.

Note 1

As we have learned more about the objects in our solar system and beyond, astronomers have re-classified Ceres twice, each time being one of the first of a wholly new category

Wilson, Teresa, “This Month in Astronomical History: The Discovery of Ceres”, American Astronomical Society, Historical Astronomy Division, 5 January 2018