Time passing every second:
Following a planet shows its big moons (their distances are squeezed). Slow time to 1 hour or 1 day per second to watch them move.
Easy to see squeezes the outer planets in. True distances puts every planet at its real distance (Neptune is 30 times farther out than Earth); the planets and the Sun are still drawn hundreds of times too big, or they would be invisible.
Pick your city and a date. Every number is calculated in your browser: when each planet is up in a dark sky, where to look, how bright it is, and when it rises and sets.
| Planet | When to look (dark sky) | Where (best time) | Brightness | Rises / sets |
|---|
Dark sky = from about 40 minutes after sunset to 40 minutes before sunrise, with the planet at least 7° above a flat horizon. Magnitude (mag): smaller numbers are brighter; the faintest stars you can see from a city are about mag 3–4. Times are for the city’s own time zone.
Tap a world to see it up close with its key facts. The pictures are made from real maps of each world (the giant planets show their cloud tops).
Tap a column title to sort the table, for example by size or by temperature.
True sizes: the planets to the same scale, next to the edge of the Sun (109 Earths wide).
True distances from the Sun in astronomical units (1 AU = 150 million km). The four rocky planets all fit within about the first 1.5 AU.
| Ball size | Distance from the Sun ball |
|---|
Your mass stays the same everywhere, but gravity is different on each world, so a bathroom scale would show a different number. And each planet takes a different time to go round the Sun, so your age in its years is different too.
| World | The scale would show | You could jump | Your age in its years |
|---|
The giant planets have no solid surface: their numbers are for the gravity at the top of the clouds. Jumping height compares with the same jump on Earth.
The diagram you label in a test: the Sun, the eight planets in order, the asteroid belt and the Kuiper belt. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a label worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Ten questions: name the planet from a picture, plus questions about orbits, sizes and the parts of the solar system.
Tap the planets in order, starting with the one closest to the Sun. Tap a filled box to take a planet back.
Tap a world, then tap the box it belongs in. Check when all twelve are placed.
The solar system formed about 4.6 billion years ago from a spinning cloud of gas and dust. Most of it fell to the middle and became the Sun, a star that holds 99.86% of all the mass. What was left formed a flat disc, and in it the planets grew. That is why all the planets go round the Sun in the same direction and almost in the same flat plane.
Close to the hot young Sun only rock and metal could stay solid, so the four inner planets (Mercury, Venus, Earth, Mars) are small and rocky. Farther out, beyond the asteroid belt, it was cold enough for ice, and the planets grew big enough to pull in huge amounts of gas: the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune. Beyond Neptune is the Kuiper belt of icy bodies, home of Pluto.
A planet farther from the Sun has a longer path and feels a weaker pull, so it moves more slowly. Kepler found the rule in 1619: the square of the year is the cube of the distance, $T^2 = a^3$ (with $T$ in Earth years and $a$ in AU). For Jupiter $a = 5.2$ AU, so $T = \sqrt{5.2^3} \approx 11.9$ years. Earth moves at about 30 km/s, Neptune at only 5 km/s.
Set the speed to 1 month / s and watch Mercury lap the other planets. Press True distances and Whole system to see how empty space is. Tap Jupiter, set 1 day / s, and watch its four big moons (they really are where the lab shows them). Then find out which planets you can see from your city tonight.
In 2006 the International Astronomical Union (IAU) decided that a planet must (1) orbit the Sun, (2) have enough gravity to pull itself into a round shape, and (3) have cleared its orbit of other objects. Pluto passes the first two tests but shares its orbit with many Kuiper belt objects, so it is a dwarf planet, like Ceres, Eris, Haumea and Makemake. A moon is a natural satellite that orbits a planet or another body, not the Sun directly.
| Part | Where | What it is like |
|---|---|---|
| Sun | centre | a star, 109 Earths wide; 99.86% of the mass |
| Inner (rocky) planets | 0.4–1.5 AU | Mercury, Venus, Earth, Mars: small, rocky, few moons, no rings |
| Asteroid belt | about 2.1–3.3 AU | millions of rocky bodies; the largest is the dwarf planet Ceres |
| Giant planets | 5–30 AU | Jupiter and Saturn (gas giants), Uranus and Neptune (ice giants): huge, no solid surface, many moons, rings |
| Kuiper belt | about 30–50 AU | icy bodies and dwarf planets such as Pluto, Haumea and Makemake |
| Oort cloud | thought to be about 2,000–100,000 AU | a huge, far-away shell of icy bodies where many comets come from (never seen directly) |
Distances in the solar system are measured in astronomical units: 1 AU is the average Earth–Sun distance, about 150 million km. Sunlight takes about 8 minutes 20 seconds to reach the Earth and about 4 hours to reach Neptune at 30 AU. The spacecraft Voyager 1, launched in 1977, is more than 160 AU away and has left the Sun’s bubble of solar wind, but it would still need tens of thousands of years to cross the Oort cloud.
Planets move on ellipses (slightly squashed circles) with the Sun near the middle. Most are almost circles: Earth is only about 3% closer to the Sun in early January than in early July. Mercury’s orbit is the most stretched of the planets (its distance changes from 46 to 70 million km), and Pluto’s is so stretched that for 20 years of its 248-year orbit (most recently 1979–1999) it is closer to the Sun than Neptune. They can never collide: Pluto goes round twice for every three Neptune orbits, which keeps them far apart.
They are falling all the time, but they are also moving sideways so fast that they keep missing it. Earth travels about 30 km every second. Without the Sun’s gravity it would fly off in a straight line; without its sideways speed it would fall straight in. The balance of the two is an orbit, the same idea as a satellite around the Earth.
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A memory trick: “My Very Educated Mother Just Served Us Noodles.” The first four are small rocky planets; the last four are giant planets.
Key takeaway: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.In 2006 the International Astronomical Union said a planet must orbit the Sun, be round, and have cleared its orbit of other objects. Pluto is round and orbits the Sun, but it shares its region with many other Kuiper belt objects, so it is called a dwarf planet. The discovery of Eris, almost the same size, made the decision necessary.
Key takeaway: Pluto has not cleared its orbit, so it is a dwarf planet.Venus is the hottest, about 464 °C on average, because its thick carbon dioxide atmosphere traps heat; it is hotter than Mercury even though it is almost twice as far from the Sun. The coldest are the ice giants: Neptune has the lowest average (about −200 °C), and Uranus holds the record for the coldest measured temperature in a planet’s atmosphere, about −224 °C.
Key takeaway: hottest: Venus; coldest: Uranus and Neptune.Five: Mercury, Venus, Mars, Jupiter and Saturn. They look like bright stars that do not twinkle much and slowly move against the stars from week to week. Uranus is just at the limit of the eye in a very dark sky, and Neptune needs binoculars or a telescope. The Planets Tonight section shows which are up from your city and where to look.
Key takeaway: five planets are easy to see with the naked eye.Gravity. The Sun pulls every planet towards it, and every planet is moving sideways very fast. The pull bends the planet’s straight-line path into a curve around the Sun, again and again: an orbit. The farther away a planet is, the weaker the pull and the slower it moves.
Key takeaway: gravity plus sideways speed gives an orbit.Neptune is about 30 AU (4.5 billion km) from the Sun and the Kuiper belt reaches about 50 AU. The solar wind fills a bubble that reaches about 120 AU from the Sun (Voyager 1 left it in 2012), and the Oort cloud of comets is thought to stretch to about 100,000 AU, more than a third of the way to the nearest star. Light from the Sun takes about 4 hours to reach Neptune.
Key takeaway: from 30 AU (Neptune) to perhaps 100,000 AU (Oort cloud).Not in a perfect line: their orbits are tilted a little and they move at different speeds. But sometimes several planets are on the same side of the Sun, so they appear spread along one line across the night sky (the ecliptic). This is often called a “planet parade”. It has no effect on the Earth.
Key takeaway: they can share the same part of the sky, but not a straight line in space.A planet orbits the Sun, is round, and has cleared its orbit. A dwarf planet orbits the Sun and is round but has not cleared its orbit (Pluto, Ceres, Eris). A moon orbits a planet or another body instead of the Sun directly: our Moon orbits the Earth. Some moons, like Ganymede and Titan, are bigger than the planet Mercury.
Key takeaway: what it orbits and whether it has cleared its orbit.About 4.6 billion years. The oldest solid grains found in meteorites are about 4.57 billion years old, measured with radioactive dating, and the Earth and the Moon formed soon after.
Key takeaway: about 4.6 billion years.The 3D model at the top of this page puts every planet where it really is on today’s date, calculated in your browser. Press Today to go back to now, or pick any date in the past or future. For where to look in your own sky, use Planets Tonight.
Key takeaway: the 3D lab shows the real positions for any date.