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  • Our Solar System, and all other star systems, form from a collapsing nebula. Often called stellar nurseries, nebulae are the birthplace of stars. They are made up of mostly hydrogen but also contain other matter like gases, dust, ice and rock. The gravity of the nebula pulls this matter into the centre, and the nebula experiences a gravitational collapse. If the compression raises the core temperature enough to reach thermonuclear fusion, the centre mass generates a protostar. This process takes about 10 million years.

    A protostar is a very young star, early in the process of gathering dust from the cloud in which it formed. What elements are found around this baby star depends on where the nebula came from. Our star formed from a previous generation of stars, which created the heavy elements (like iron, titanium and nickel) found on Earth.

    Find out more about how elements formed in the cores of different stars in the interactive Universal element formation.

    Understanding the evolution of the planets is important

    Understanding the evolution of our Solar System and star helps to understand why finding other Earth-like planets is so difficult and so exciting when it does happen. By studying other planetary systems, we can better understand the life cycle of stars. At the beginning of learning about our star system, scientists thought there was an explosion that pushed the gas elements away from the central mass, which is why the gas giants are further away from the Sun. While the gas elements were thrust far away, the rocky elements remained near the Sun. Scientists are now able to study other star systems and have learned that not all star systems form in the same way. They think that, when our Solar System formed, there just happened to be more rocky material nearer the central mass and more gas elements further away. Studies continue to discover exactly how star systems form.

    A star system begins with a nebula – a cloud of gas and dust. This collapses around a central mass, which pulls the rest of the mass of the system around itself. All this occurs because of gravity. Gravity is why we stay on Earth, why the ocean doesn’t float away and why things fall to the ground when we drop them. Gravity is also responsible for the orbital pattern of planets and moons, it’s why Jupiter’s gas doesn’t dissipate into space and it’s the cause of the collapsing process that creates stars. In fact, gravity is one of the key determining factors in planet classification.

    The classification of planets

    You may recall that Pluto was reclassified as a dwarf planet in 2006, changing its classification to dwarf planet and changing our Solar System planet count to eight. One of the reasons this occurred was because of the discovery of other small, rocky objects like Pluto. The International Astronomical Union (IAU) is an organisation that, among other things, is responsible for naming planets and stars and setting the classification requirements for a planet. The IAU definition for a planet in our Solar System is a celestial body that:

    • is in orbit around the Sun
    • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes hydrostatic equilibrium (a nearly round shape)
    • has cleared the neighbourhood around its orbit.

    It is the last point that Pluto fails, along with other similar-sized planets like Ceres (located within the asteroid belt), Eris, Haumea and Makemake (located near Pluto). The gravitational pull of the object isn’t strong enough to clear its neighbourhood. Pluto is in the Kuiper belt, an area filled with frozen asteroids and other dwarf planets – leftovers from the formation of our Solar System. If the IAU definition was changed to include Pluto, other planets would need to be included in our Solar System as well, making for a large family of planets.

    Searching for other planets

    For as long as we’ve been able to, humankind has searched for another Earth. In February 2017, scientists announced the discovery of the TRAPPIST-1 system, which has a record-breaking seven planets, three of which are in the Goldilocks zone – the area around a star where the temperature is not too hot and not too cold. Planets that are in this area are thought to be good contenders for having liquid surface water and, potentially, life. A system like this has never been seen before, and scientists say it’s the best bet today for finding a planet that could support life.

    Look up at the stars and not down at your feet. Try to make sense of what you see, and wonder about what makes the universe exist. Be curious.

    Stephen Hawking

    Related content

    Our astronomy topic covers a vast range of articles and activities, from space junk, satellites and rockets to navigating by the stars and the modern-day star compass (kāpehu whetū).

    Activity ideas

    In this activity, students complete a true or false worksheet that highlights common alternative conceptions about gravity and satellites. This activity is useful as a pre-test to establish student understanding.

    In this activity, students explore the transit method of searching for planets, plotting graphs of light measurements from stars and searching for dimming that indicates the presence of a planet and calculating its size.

    Useful link

    This YouTube clip explains the formation of the Solar System.


    This article has been written by Stardome Observatory and Planetarium, which has been operating since 1967. It is a place of exploration, research and sharing of knowledge and hosts New Zealand’s first and still largest planetarium theatre. Stardome Observatory and Planetarium celebrates its 50th anniversary in 2017.

      Published 20 March 2017 Referencing Hub articles
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