All about Planets

How old are the planets in our Solar System? Will they always be there?

All the planets in our Solar System formed at around the same time – about four-and-a-half thousand million years ago. The planets are made from material in the cloud of dust and gas that was orbiting the brand new Sun. The stuff stuck together and gradually grew, like gassy or rocky snowballs!

Scientists think that Jupiter may have been the quickest planet to form – so Jupiter is probably the oldest planet in our Solar System.

Even though all the rocky planets are about the same age, the rocks on them aren’t. Earth has something called plate tectonics which recycles older rocks into newer rocks. We think that Earth is the only planet with active plate tectonics so the youngest rocks in the Solar System are probably on Earth.

Planets don’t burn fuel like stars, so they could last a very long time, longer than our Sun. Jupiter probably swallowed up some potential planets while it was growing, but there is no chance of it doing that now everything has settled into place. Planets can get gobbled up by expanding stars. This happens when a star is starting to run out of fuel. We think the Sun has about another 5 thousand million years of fuel left, so in the future it could swell up and gobble up planets close to it, like Mercury and Venus. 


How many planets have been discovered?

There are five planets that have been known about on Earth for thousands of years. These are the planets that are bright enough to see without a telescope – Mercury, Venus, Mars, Jupiter and Saturn. Uranus and Neptune are much further away so were not discovered until people have invented telescopes. Uranus was discovered in 1781 by astronomer William Herschel and Neptune was discovered in 1846 by astronomer Johann Gottfried Galle (thought it turned out other people had seen Neptune but not realised it was a new planet).

The first exoplanet (planets outside our Solar System that orbit other stars) was discovered in 1992. Astronomers have now discovered over 4,300 exoplanets! About 200 of those have been discovered this year, in 2020. You can keep up to date by visiting the NASA website.

Some of these exoplanets are nicknamed ‘Goldilocks Planets’ because the temperature is just right for life! There are about 50 planets that could be Goldilocks planets, but only half of them are thought to be rocky like Earth. We don’t know how many of them have an atmosphere like Earth but it is likely to be a small amount (none of the other planets in our own Solar System have an atmosphere like Earth!). Astronomers use big space telescopes like the Hubble Space Telescope, Kepler Space Telescope and the new James Webb Space Telescope to study exoplanet’s atmospheres.


Why do planets orbit and spin?

Massive things like moons, planets, and stars have enough gravity that we can see its effects. We can observe things fall on Earth due to gravity. And gravity doesn’t stop at the surface of a planet or star, it extends out into space. All the planets (including our Earth) orbit the Sun because the Sun’s gravity pulls on the planets. The Moon orbits the Earth because the Earth’s gravity pulls on the Moon.

Our Solar System formed out a cloud of gas and dust, called a nebula, that clumped together to form the Sun and planets. All the stuff in the nebular was already moving. As it clumped together, the newly formed Sun and planets started to spin, and then spin faster. Our Earth spins (or rotates) once every twenty-four hours. It spins very quickly, at roughly one thousand miles per hour. We don’t notice we’re spinning so fast because we’re moving with it at the same speed.

If the Earth suddenly stopped spinning, anything not attached to the Earth would keep moving and be thrown sideways. That includes people, but also the atmosphere and oceans. This would be catastrophic.

But what would happen if the Earth gradually stopped rotating without catastrophic effects? The length of a year would stay the same, but the length of a day would also be as long as a year! So it could be daytime for 6 months and night-time for 6 months. Another difference would be that the sun would rise in the west and set in the east. The Earth would also change shape. Its spin creates a bulge at the middle around the equator. Without the spin, the oceans would move. The Earth could end up with most of its land around the equator and two gigantic oceans in the north and south.

Our Earth actually is slowly down a very little bit each day. A day on Earth gets 1 second longer about every 100,000 years. So not enough to notice, and the Earth certainly won’t stop spinning any time soon!


Why are planets different colours?

Objects appear to be different colours because the materials they are made of absorb and reflect different bits of the rainbow (or spectrum of light). Planets are made of different materials so appear to be different colours. Mars looks red because the dusty rocks on Mars contain a lot of iron oxide, which you know as rust. The hydrogen, helium and methane gas in Neptune’s atmosphere gives it its blue colour. Uranus contains more methane than Neptune so looks more greenish blue. Thanks to the NASA New Horizons spacecraft, we know that the Dwarf Planet Pluto also has a range of colours including white, yellow, dark red, brown and blue. This is because its surface is covered in ices that contain different mixtures of chemicals.


Why does it rain acid on Venus?

The clouds on Earth are made of water vapour, so they rain liquid water. The clouds on Venus are made of sulfuric acid gas so it rains acid. But actually, if you were on the surface on Venus, you wouldn’t get rained on. The rain evaporates before it reaches the ground because Venus is so hot. Surface temperatures on Venus are hot enough to melt lead!


Could humans survive on Mars? What would they see there?

Scientists have not found life on Mars. However, humans might visit Mars one day. If humans (or Santa) visited Mars they would need to take food with them because there are no plants or animals to eat on Mars and no Martian supermarkets! They’d also need to take water with them. There is some water on Mars but it’s all ice. There might occasionally be some liquid water flowing on Mars but it would be too salty to drink. Mars’ atmosphere is mostly carbon dioxide and that is toxic to humans so any astronauts would have to take oxygen with them. Also, it’s much colder than Earth. The average temperature is minus 60 degrees Celsius!

Any astronauts who do go to Mars may see some spectacular sights. Such as Olympus Mons, the largest volcano in the Solar System. They might spot a mile-high dust devil whirlwind. And one extraordinary sight would be seeing two moons in the sky.


What are dwarf planets and how many are there?

A planet must do these three things:

  1. Orbit the Sun
  2. Be nearly round in shape (the bigger an object in space is, the more gravity it has, so the rounder it gets because its gravity pulls everything towards the centre)
  3. Clear any smaller asteroids in its orbit out of the way (the bigger an object in space is, the more gravity it has, so nearby objects will get pulled in and destroyed or pushed further away)

Dwarf planets are not quite large enough to do all three things. Pluto orbits the Sun and is round but it is not big enough to clear its orbit of smaller asteroids. That’s why it is a dwarf planet. There are five official Dwarf Planets in our Solar System – Ceres (in the asteroid belt) and Pluto, Makemake, Eris and Haumea (out past Neptune). A NASA mission called Dawn visited Ceres and took photos of white spots on its surface. Scientists think these white spots are salty-ice that has erupted out of ice volcanoes on Ceres.

Scientists only started classifying things as ‘Dwarf Planets’ after discovering Eris. They realised that Pluto was more like Eris than the other planets and decided to call them both ‘Dwarf Planets’. Pluto was classified as a planet from its discovery in 1930 until its re-classification in 2006.


How many moons are there?

Astronomers have discovered more than 200 moons in our own Solar System. Saturn has the most with eighty-two moons. So that’s eight planets and over 200 moons!

Astronomers estimate that there are about one hundred thousand million stars in our galaxy, the Milky Way. If each of those stars is like our Solar System and has eight planets and about 200 moons that means there could be twenty million million moons in our galaxy!

We don’t know for sure how many galaxies there are in the universe. Astronomers have estimated that there could be two million million galaxies in the observable universe. That means there could be at least 40,000,000,000,000,000,000,000,000 (40 quadrillion) moons in the universe!


What is the different between a shooting star, a meteor, and a comet?

A meteor and a shooting star are the same thing. Astronomers don’t call them shooting stars because they’re not stars! Stars are huge balls of gas millions and millions of kilometres away. Meteors are pieces of space rock, space metal or space dust that are falling very quickly through the Earth’s atmosphere. Because meteors are travelling very fast (thousands of miles per hour) they heat up and start to glow and shine like a star. If a meteor is big and strong enough, it can reach the ground (smaller, more fragile ones burn up and disintegrate in the air). When it lands on the ground we call it a meteorite. Meteorites are usually pieces of asteroid but some have come from the Moon and Mars.

Comets are lumps of ice, rock and dust. Like planets, they orbit the sun. When a comet’s orbit brings it close to the Sun, it heats up and spews dust and gas out space. The dust and gas gets lit up by the light of the Sun and creates a giant glowing tail that stretches for millions of miles. Each year the Earth’s orbit takes it through clouds of debris left behind comets. This creates meteor showers.