Elements Named After Planets, Moons and Space

Uranium, neptunium, plutonium, mercury, selenium, tellurium, cerium, palladium and helium all have cosmic names. Here is who named them and why.

At least nine elements carry a name taken from the sky: uranium, neptunium and plutonium from planets, mercury from a planet and god, selenium from the Moon, tellurium from the Earth, cerium and palladium from early asteroid discoveries, and helium from the Sun. Chemists, it turns out, have always liked a good view. Pull up a chair, because the stories are better than you would expect, including one element named after a planet that later lost its planet badge.

Which elements are named after planets, moons and space?

Here is the full line-up. The last column is a plain, general-knowledge summary of what each one is used for, so treat it as a taster rather than a full list.

Atomic number, if that term is new, is the count of protons in an atom's centre, and it fixes which element you have. Our guide to reading the periodic table shows where to find it on each square.

Who gets to name an element?

The discoverers propose a name, and the International Union of Pure and Applied Chemistry, known as IUPAC, checks it and approves it. That is the modern rulebook. In the old days there was no committee, so whoever found the stuff simply announced a name and everyone else shrugged and got on with it. Several cosmic names below date from that cheerfully informal era.

Names for places and people have their own stories, which we tell in the sister article on elements named after places and people. This one stays firmly above the clouds.

Why are uranium, neptunium and plutonium named after planets?

Because chemists spotted a pattern and could not resist it. In 1781 the astronomer William Herschel discovered a new planet, which was later called Uranus. In 1789 the German chemist Martin Klaproth found a new element in a heavy black mineral and named it uranium in honour of the planet that had caused such a stir eight years earlier. A new world and a new element, both in one decade. Show-offs.

The story picks up in the twentieth century. Uranium has atomic number 92, and for a long time it was the last element anyone knew of. Elements heavier than uranium are called transuranic, meaning "beyond uranium". In 1940 a team at Berkeley in California, led by Edwin McMillan and Philip Abelson, made element 93 and named it neptunium, because Neptune comes after Uranus in the sky. Then plutonium, element 94, was first made at the same laboratory by a team led by Glenn Seaborg in 1940 to 1941 (the exact date depends on whether you count making it or proving it), and it took its name from Pluto, the next planet out.

It is a tidy trick: the table follows the solar system. The uranium family is part of the actinides, the row of heavy elements that usually sits as a strip beneath the main table, as explained in our article on lanthanides and actinides. If you want to know how Berkeley's chemists actually conjured up elements that do not exist in nature, how new elements are made has the details. (Spoiler: it involves firing very small things at other very small things at very high speed.)

All three are radioactive, which means their atoms are unstable and gradually break apart, giving off energy. The article on which elements are radioactive goes into that, and our uranium versus plutonium page lines the two up side by side.

Did Pluto really lose its planet status but keep its element?

Yes, and it is the funniest bit of naming bad luck in chemistry. In 2006 the International Astronomical Union decided that Pluto counts as a dwarf planet rather than a full planet. Pluto was demoted. Plutonium, meanwhile, shrugged, kept its name and carried on being element 94.

There is no rule that an element must update its name when the astronomers change their minds. A name records what people were thinking when it was chosen, rather like an old street name that outlives the shop it came from. In 1940 Pluto was the ninth planet, so the name made perfect sense at the time.

Is mercury named after the planet, or the planet after the metal?

This is the knotty one, and honest answers come with a hedge. The planet is named after the Roman messenger god Mercury, fast of foot, and quick to cross the sky. The metal is liquid, silvery and fast-moving, so the god's name suits it. The most common explanation is that alchemists, the early experimenters who were part chemist, part mystic, linked each of the metals they knew with one of the seven bodies they could see wandering the heavens. Mercury, the metal, got Mercury, the planet.

So the element and the planet share a name, but the element took it from the planet-and-god side of the family. The metal's older English name is quicksilver, meaning "living silver", which is lovely, and its symbol Hg comes from hydrargyrum, from Greek words meaning "water silver". Mercury is also the only metal that is liquid at ordinary room temperature, a record you can read about in the article on gases, liquids and solids at room temperature.

The alchemists' full list is a good quiz answer. By tradition gold went with the Sun, silver with the Moon, copper with Venus, iron with Mars, tin with Jupiter and lead with Saturn. Those metals kept their ordinary names, so only mercury ended up sharing its label with a planet.

What do selenium and tellurium have to do with the Moon and the Earth?

They are a matched pair of names, which makes them my favourite. Tellurium was named first, from the Latin tellus, meaning Earth. It was discovered in the 1780s by Franz-Joseph Müller von Reichenstein, and Klaproth later gave it its name in the 1790s.

In 1817 the Swedish chemist Jöns Jakob Berzelius found a new element that behaved very like tellurium and turned up alongside it. If the first one was named for the Earth, the new twin deserved the Moon. He called it selenium, from the Greek selene, the Moon. An Earth and a Moon, stuck together in the ground.

They are neighbours in the same column of the table. Selenium is a non-metal, while tellurium is a metalloid, an element with some metal-like and some non-metal-like properties. Our metalloids page shows who else sits in that borderline club, and metals, non-metals and metalloids explains the whole idea.

Selenium is also a trace nutrient: our bodies need a very small amount of it to work properly, and lots of it can be harmful, which is true of a surprising number of elements. See the article on elements in medicine for more on the balancing act.

How did cerium and palladium get names from asteroids?

The early nineteenth century was an excitable time for astronomers. On 1 January 1801 Giuseppe Piazzi spotted a new object between Mars and Jupiter and called it Ceres. It was first counted as a planet, then as an asteroid, which is a small rocky body orbiting the Sun, and today it is classed as a dwarf planet. Ceres has had more job titles than a start-up employee.

In 1803 a new element was discovered in Europe, and the credit is shared: Martin Klaproth in Germany, and Berzelius with his colleague Wilhelm Hisinger in Sweden, found it at about the same time. It was named cerium after Ceres, the object everyone was talking about. Cerium is one of the lanthanides, the soft, silvery metals tucked in the strip below the main table. You can meet the whole family on our lanthanides page.

The same fashion gave us palladium. In 1802 another asteroid, Pallas, was found, and the English chemist William Hyde Wollaston found a new metal around then and named it after the asteroid (sources differ on the exact dates of discovery and announcement). Palladium is a transition metal, and these days its most famous job is in the catalytic converters in car exhausts, which help turn harmful gases into less harmful ones. The transition metals page has the rest of its relatives.

Why is helium named after the Sun?

Because it was found there first. In 1868, during a total solar eclipse, astronomers studied sunlight with a spectroscope, a device that spreads light into its separate colours. A bright yellow line showed up that did not match any known element. The French astronomer Pierre Janssen is credited with spotting it, and the English scientists Norman Lockyer and Edward Frankland argued it came from a new element and named it helium, from Greek helios, the Sun. The ending -ium was used because scientists assumed it would be a metal. They were wrong on that point, but helium is a noble gas instead, a very unreactive one.

It was only in 1895 that William Ramsay found helium on Earth. An element discovered in the Sun before it turned up on Earth is a fairly good claim to fame. The idea of light fingerprints that spotted it appears in our article on flame tests and firework colours, and the noble gases article explains why it is so aloof. For a family portrait, see the noble gases page.

Honourable mentions

• Titanium. Named by Klaproth after the Titans, the giants of Greek myth. Saturn's largest moon, Titan, is named after the same crowd, so it is a distant cousin of the cosmic names rather than a true one. A near miss, but too good to leave out.
• The metals of the alchemists. As mentioned above, gold, silver, copper, iron, tin and lead each had a heavenly partner in alchemy, though none of them adopted the planet's name.

Everything else with a place in the sky is a stretch, and we would rather be precise than cheeky.

Try it on Periodic Bubbles. Pop uranium, then neptunium and plutonium in turn, and watch the atomic numbers climb 92, 93, 94, just like the planets going outwards. Then visit selenium and tellurium to spot the Moon and Earth pair sitting one above the other in the same column. The history behind the names is told in our history of the periodic table.

Frequently asked questions

Is there an element named after Mars or Venus?

No. There is no element named directly after Mars, Venus, Jupiter or Saturn. The nearest the table gets is the old alchemical link between those planets and metals such as iron, copper, tin and lead, none of which changed their names.

Is plutonium still named after a planet?

It is named after Pluto, which was counted as the ninth planet when the element was made, in 1940 to 1941. Pluto was reclassified as a dwarf planet in 2006, but the element's name did not change.

Did the Sun or the Moon really inspire an element?

Yes. Helium comes from the Greek word for the Sun and selenium from the Greek word for the Moon. Helium was detected in the Sun's light in 1868, nearly three decades before anyone found it on Earth.

Who decides if an element can have a cosmic name?

The discoverers suggest a name and IUPAC approves it. Today the rules say a new element can be named after a mythological concept or character (which can include an astronomical object), a mineral, a place, a property or a scientist, so a cosmic name is not forbidden, but all the current choices were settled long ago.

Fancy testing yourself on all these names? Take the Pop Quiz, or see what today's puzzle holds in the Daily Bubble.