The planets of our solar system: a warm-up tour

The solar system is small by cosmic standards and unimaginably large by human ones. Eight planets, a couple of dwarf planets depending on who’s counting, all orbiting a single middle-aged star. Five of those planets are visible with the naked eye. With a telescope, you can see all but the faintest.

Here’s a tour, in order from the sun, focused on what makes each one interesting to a beginner and when you might see it.

Mercury, the messenger

The smallest, innermost, fastest-moving planet. Mercury zips around the sun in 88 Earth days. Rocky, airless, covered in craters, and swinging from over 400°C in the day to -170°C at night.

From Earth, Mercury is famously elusive. It never strays far from the sun in our sky, which means it’s only ever visible for a short window around sunrise or sunset, and never far above the horizon. Experienced amateur astronomers sometimes go years between naked-eye Mercury sightings. If you ever catch it as a pale, steady dot low in the dusk, say hello. You’ve seen something most people never consciously do.

Venus, the jewel

Venus is roughly Earth-sized and catastrophically different. A runaway greenhouse effect has made its surface hotter than Mercury’s, despite being further from the sun. The atmosphere is crushing carbon dioxide, with clouds of sulphuric acid. No life, no hope of any, no easy way to visit.

From Earth, Venus is the brightest thing in the sky after the sun and the moon. When it’s the “evening star,” it dominates the western sky for months. When it’s the “morning star,” it’s the brilliant dot that beats the sun up. Through even a small telescope, Venus shows phases like a tiny moon, because we’re seeing varying angles of its sunlit side. Galileo’s observation of Venus’s phases in 1610 was one of the first hard confirmations that Earth isn’t the centre of everything.

Earth, the blue one

Our home. Four and a half billion years old, mostly water, irritatingly well-suited to life. The only planet where stars and moons are watched by beings who look up, wonder, and write articles about it. Worth preserving.

Mars, the red one

Smaller than Earth. Drier. Colder. A thin atmosphere and rusty iron-oxide dust covering everything. The orange-red tint is visible even without equipment. Mars has had more robotic visitors than any other planet and has been front-and-centre in human imagination for centuries.

From a back garden, Mars is best at opposition: the point in its orbit when Earth is directly between it and the sun. Oppositions come roughly every 26 months. Around opposition, Mars outshines nearly everything except Venus and Jupiter. Between oppositions, it fades and can be hard to distinguish from a moderately bright star.

Through a telescope, even a small one, you can see Mars’s polar ice caps and sometimes the bright-and-dark surface features. Dust storms occasionally obscure the whole disk.

Jupiter, the giant

The solar system’s largest planet. Eleven Earths would fit across Jupiter’s diameter. It’s a gas giant, no solid surface, just a deep atmosphere of hydrogen and helium with storm bands that dwarf our entire planet. The Great Red Spot, a storm that’s been raging for at least several centuries, has shrunk in recent decades but is still visible through amateur telescopes.

Jupiter has at least 95 known moons. Four of them, Io, Europa, Ganymede, Callisto, are big enough to see with cheap binoculars. These are the Galilean moons, named after Galileo, who first saw them in 1610. Their nightly dance around Jupiter was the observation that convinced Galileo not everything orbits Earth.

In the sky, Jupiter is creamy white and brighter than any star. It spends about a year in each zodiac constellation as it orbits.

Saturn, the ringed one

Saturn is the second-largest planet and has the most striking feature in the solar system: its rings. Made almost entirely of water ice, in particles ranging from dust to boulders. The ring system spans about 280,000 km wide but only a few tens of metres thick. Proportionally thinner than a sheet of paper stretched across a football pitch.

Saturn itself is another gas giant, similar to Jupiter but softer. Gentler yellow colour, less dramatic cloud bands. It’s famously less dense than water, which means if you could find a bathtub big enough, Saturn would float.

In the sky, Saturn is a steady yellowish point, dimmer than Jupiter. Easy to mistake for a moderately bright star. Through even a small telescope, the rings are unmistakable. For most amateur astronomers, the first clear view of Saturn’s rings with their own eye is the moment they get converted for life.

Uranus, the tilted one

An ice giant. Colder and smaller than Jupiter and Saturn. Pale cyan-green from methane in its atmosphere. Its most peculiar feature is its tilt: Uranus lies almost on its side, with its rotation axis in the plane of its orbit. Something enormous hit it early in its history and knocked it over.

From Earth, Uranus is, in theory, visible to the naked eye. But only just, only from a truly dark site, and only if you know exactly where to look. Most observers need binoculars. Through a telescope, it shows as a small featureless pale disk.

Neptune, the windy one

The outermost planet. Another ice giant. Deep blue from more methane. Its winds are the fastest in the solar system, clocked at over 2,000 km/h. That’s surprising for a planet so far from the sun. Neptune clearly has internal heat sources we don’t fully understand.

Not visible without equipment. A small telescope shows Neptune as a tiny blue disk, barely larger than a bright star. A destination for patient observers, not casual ones.

The dwarf planets

Pluto was demoted from planet to dwarf planet in 2006. Not because it shrank, but because we got better at spotting similar objects and had to draw a line somewhere. Pluto is now one of at least five officially recognised dwarf planets, along with Eris, Haumea, Makemake, and Ceres (which sits in the asteroid belt between Mars and Jupiter). None are visible to the naked eye.

What to do with this list

Open the Tonight’s Sky tool and see which planets are above your horizon right now. Chances are one or two are. If you can see Jupiter, try binoculars. You should spot at least some of its moons. If Venus is in your evening sky, you’ll have no trouble noticing it. If Saturn is up and you have a friend with a telescope, ask them to aim it that way.

The planets, unlike the stars, are neighbours. Close enough to visit one day. Close enough already that the same sunlight lighting your afternoon is, right now, lighting them too.