The Sciences

Earth Has Many Natural Satellites – but How Many Can Be Called 'Moons'?

It's not fair to call a dust cloud a 'moon' because it is important to use words whose meanings are generally agreed upon.

A National Geographic article published on November 6 carried a surprising headline:

Earth has two extra, hidden ‘moons’

The lede followed through:

Earth’s moon may not be alone. After more than half a century of speculation and controversy, Hungarian astronomers and physicists say they have finally confirmed the existence of two Earth-orbiting “moons” entirely made of dust.

This sounds strange because there has been little else in the news about dust-moons in the last few years. No major discoveries are made in one instant, and can often be anticipated many years in advance through discussions among scientists. However, the rest of the article put paid to the doubt.

The ‘dusty moons’ National Geographic alludes to are in fact the Kordylewski dust clouds. Late last month, a group of Hungarian astronomers confirmed the presence of these clouds, located in two different directions at about the same distance Moon is from Earth.

Astronomers have been debating the existence of these clouds since the 1950s. In that decade, an astronomer named Kazimierz Kordylewski climbed a mountain and photographed parts of the night sky where these clouds had been predicted by other astronomers before him to exist. The dust clouds have since been called Kordylewski clouds in his honour.

However, confirming their presence has taken so long even though they’re so close to Earth because of their brightness – or lack of it. They are too faint to spot because the stars in their background far outshine them, even at this distance. But they aren’t completely obscured either: they reflect sunlight in feeble amounts, giving themselves away to the persistent observer.

Although Kazimierz Kordylewski found the dust clouds this way, the Hungarian group was more sophisticated. According to their two published papers (here and here), they took advantage of dust’s ability to polarise light. Waves of light are in fact waves of electric and magnetic fields undulating through space at right angles to each other.

Credit: Onde electromagnetique/Wikimedia Commons, CC BY-SA 3.0

Credit: Onde electromagnetique/Wikimedia Commons, CC BY-SA 3.0

The electric fields of different waves point in different directions. But when they hit a dust particle, they get polarised: the electric fields all line up. This is how sunglasses work: the lenses are filters that don’t let light of certain polarisations pass through, cutting glare.

Like all astronomical discoveries, their finding will have to be validated by independent observers before the community reaches a consensus. But in the meantime, the claimed discovery is a matter of concern because of where the Kordylewski clouds are located: at two Lagrange points.

The Lagrange – or libration – points are places in space where the gravitational fields of the Sun, Moon and Earth tug at each other such that an object at that point will be in an Earth-synchronous orbit around the Sun. The five such points are located as shown below:

An illustration showing the relative position of the Lagrange points as Earth moves around the Sun. Credit: Anynobody/Wikimedia Commons, CC BY-SA 3.0

An illustration showing the relative position of the Lagrange points as Earth moves around the Sun. Credit: Anynobody/Wikimedia Commons, CC BY-SA 3.0

Scientists like stationing satellites at these points because they can stay in orbit with much less fuel spent than if they were stationed elsewhere. However, now we (may) know the Kordylewski clouds are located at the points labelled L4 and L5. This means satellites stationed there will have to carry protective shielding. Otherwise, dust particles could  damage sensitive instruments and end the mission before its time.

However, the Kordylewski clouds can’t be classified as moons, although they can be as natural satellites. Judit Slíz-Balogh, a coauthor of the current study and an astronomer at the Eötvös Loránd University, calls them “pseudo-satellites”. The distinction is important because, even when bracketed between single- or double-quotes, the label of moon can’t be applied to a dust cloud.

The International Astronomical Union (IAU), which decides the meaning of astronomical terms like planet, star, etc., defines a moon only as a planet’s natural satellite. However, that isn’t license to call every natural satellite a moon. (In fact, one of the definitions of a planet would make our Moon a planet, too.)

But a size-based organisational paradigm would imply that an object much smaller than the moon would have to be called a moonlet. For example: Saturn’s moon Pan, which is 35 km at its widest. Something even smaller will have to make do with the catch-all label ‘particles’. Then again, the paradigm falters with the overall form of the satellite. For another example: the dust, ice and rocks that make up Saturn’s rings are called ‘ring particles’ even though some of them weigh a few quintals.

Carolyn Collins Petersen, a former member of the Hubble Space Telescope instrument team, wrote for ThoughtCo. earlier this year, “There is no official definition of ‘moonlet’ and ‘ring particle’ by the … IAU. Planetary scientists have to use common sense to distinguish between these objects.”

Importantly, it would be counterproductive to argue that anything goes because there is no technical definition. To the contrary, especially with science communication, it is important to use words whose meanings are generally agreed upon. ‘Natural satellites of dust’ would have helped that cause better than ‘”Moons” made of dust’.