The right size, and the shield of ozone

Distance from the Sun isn't the only thing that makes Earth habitable. Start with the shape of its orbit. In our solar system most planets, Earth included, have almost circular orbits. That keeps the sunlight and heat Earth gets nearly steady all year, so most places don't get extreme summers and winters.

Next, size. The Earth is the right size to hold on to an atmosphere, the layer of gases around it, which plays a major role in keeping life going.

You learnt in chapter 5 that Earth's gravity pulls things towards it. If the Earth were much smaller (with the same average density), its gravity would be too weak to hold the gases of the atmosphere, and they would escape into space.

That really happens. The atmosphere on Mars is 100 times thinner than Earth's, and Mercury has no atmosphere at all.

But a planet can also be too big. If it were much larger, with much stronger gravity, it might pull on us so hard that our bones could be crushed. The Earth's right size lets it hold an atmosphere without crushing what lives on it.

The oxygen in that atmosphere lets us breathe, and almost every form of life on Earth needs it. But oxygen has a second job.

Some of the oxygen in the air turns into another form called ozone: a molecule of three oxygen atoms (ordinary oxygen has two). It forms an important part of the atmosphere called the ozone layer, which acts like a shield, blocking harmful ultraviolet (UV) rays from the Sun that can damage living cells. Meanwhile India's Mangalyaan, the Mars Orbiter Mission launched by ISRO in 2013, carried instruments to study Mars's atmosphere, surface and signs of past water, and showed the world that India could do space science with smart, low-cost technology.