Stargazing in March: Snow-white Pluto and the eight dwarfs

Ninety years ago this month, the Lowell Observatory in Arizona sent out a telegram stating that they had found an “object which for seven weeks has, in rate of motion and path, consistently conformed to that of a trans-Neptunian body”.

In other words, a new planet, at the outer edge of the solar system. This cold distant world was discovered by a young assistant at the observatory, Clyde Tombaugh. “I was terribly excited,” Tombaugh told me when I talked to him before his death in 1997. “I don’t think I could ever top that thrill!”

When the news broke in England, an 11-year-old girl thought the dim world should be named after the god of the underworld. Her grandfather passed on her idea to Oxford’s professor of astronomy, who sent a telegram to the Lowell Observatory: “Naming new planet. Please consider Pluto, suggested by small girl Venetia Burney for dark and gloomy planet.”

Pluto turned out to be a runt of a world, far smaller than any of the eight other planets circling the Sun. Astronomers started to question whether it was a planet at all. The doubts became acute in 1992, when they began to discover other small objects out in the region beyond Neptune. Then, in 2005, American astronomer Mike Brown found a world out there that was even heavier than Pluto.

Named Eris, after the goddess of strife and discord, this object shook up the astronomical community. If Pluto was Planet Nine, should Eris be Planet Ten? And when astronomers discovered more similar worlds, should they be Planet Eleven, Twelve, Thirteen – maybe without limit?

In 2006, the weighty International Astronomical Union made a unique decision. They demoted Pluto. No longer would it be a planet, like the Earth or Saturn. It would be a “dwarf planet”. Into the same category went Eris, and also Ceres, the largest object in the asteroid belt between Mars and Jupiter.

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Today, there are eight dwarf planets beside Pluto, all but Ceres living out beyond the realm of Neptune. The most distant, Sedna, lies over 10 times further out than Pluto, and takes almost 11,000 years to complete one orbit around the Sun.

Our best guess is that these chilly worlds have a core of rock, surrounded by a thick layer of ice. On the surface, it’s so cold that substances we usually consider gases – like nitrogen and methane – are frozen into ice and snow.

We got our first – and so far only – close-up of a dwarf planet in 2015, when the New Horizons spacecraft sped past Pluto. It revealed mountains of ice and dark regions stained with tarry deposits formed from reactions between methane and nitrogen. Most prominent was a huge snow-white heart shape, formed from fresh deposits of nitrogen ice. It’s been named, appropriately, after Pluto’s discoverer: Tombaugh Regio.

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The first thing you’ll see after sunset is the glorious Evening Star over the west: Venus is more brilliant than all the stars and other planets, and by the end of the month it’s not setting until after midnight. On 28 March, the crescent Moon forms a stunning pair with Venus.

As the winter constellations, like Orion, begin to sink down towards the west, we are treated to the star patterns of spring. The familiar seven stars of the Plough, part of the larger constellation of Ursa Major (the Great Bear) lie almost overhead. High in the south crouches the celestial lion, Leo, his heart marked by the bright star Regulus. And below Leo is the long straggly shape of Hydra, the water snake – the largest constellation in the sky, though made up of some pretty faint stars. Its brightest member, in a region devoid of anything much else, is called Alphard – appropriately meaning “the lonely one”.

There’s a lot of planetary action going on in the early morning sky, with three planets performing a stately dance. Most brilliant is Jupiter, now rising around 4am. To its left is Saturn, some ten times fainter. The Moon is nearby on the mornings of 18 and 19 March. At the start of the month, you’ll find Mars (similar to Saturn in brightness) lying to the right of Jupiter; but the Red Planet is steadily moving leftwards, passing just under Jupiter on 20 March and below Saturn on 31 March.

If you have a moderate telescope, there’s a rare chance to easily locate the faint dwarf planet Pluto, as Mars passes right past the distant world. On the morning of 23 March, around 5am, point your telescope to the Red Planet on high magnification, and the faint “star” nearby is remote Pluto.