An introduction to exoplanets
An introduction to exoplanets

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An introduction to exoplanets

1.4  What defines a planet?

Video 1 shows some of the key differences between planets and dwarf planets.

Skip transcript: Video 1  What is a planet?

Transcript: Video 1  What is a planet?

‘There is nothing so far removed from us to be beyond our reach, or so far hidden that we cannot discover it. To this end, we will explore and pose questions.’ (Descartes)

What is a planet? The ancient Greeks looking up at the night sky realised that some of the points of light moved against the background of stars. They called these lights ‘planetes’, meaning wanderer. For thousands of years this scientifically inexact word was sufficient to distinguish other objects in the sky from the stars. As the power of telescopes increased our ability to see into the depths of space, our understanding of the Solar System evolved.

By the middle of the nineteenth century, we listed 15 planets in the Solar System: Mercury, Venus, Earth, Mars, Ceres, Pallas, Juno, Vesta, Astraea, Hebe, Iris, Jupiter, Saturn, Uranus and Neptune. Within 50 years we had concluded that an additional term was needed to properly describe what we had discovered, the asteroid belt. At the beginning of the twentieth century, Ceres, Pallas, Juno, Vesta, Astraea, Hebe and Iris were referred to as asteroids and we believed that our Solar System included the eight planets clearly visible through our telescopes and at least one more, yet to be found. Pluto was discovered in 1930 and was hailed as the ninth planet, even though at the time some astronomers did not think the term planet accurately described the new find.

By 2005 many believed we had identified the tenth planet in our Solar System. Nicknamed Xena, later officially designated Eris, this distant icy object slightly larger than Pluto rekindled the debate over ‘what exactly is a planet?’

As we continue to explore our Solar System and our technology continues to improve, we are seeing more and more objects on the distant fringe of our Solar System that need to be defined and classified. Much like other sciences, such as biology, where one may discover a new species, classification is a common and needed process which groups like objects for purposes of comparison and further study.

And as in all sciences, classification can and should change based on new knowledge. The International Astronomical Union, a renowned organisation dedicated to promoting and safeguarding the science of astronomy, recently took up the debate over ‘what is a planet?’

While many accept the definition issued by the IAU, scientists worldwide continue to debate the issues. And, there are many national and international science organisations who have not weighed in. In this first attempt at scientifically defining a planet, the IAU said that a planet must have these three traits:

1 it must be a body that is in orbit around the Sun;

2 it must have sufficient mass so that its own gravity pulls it into a nearly round shape;

3 the object must clear away other objects in its neighbourhood.

The resolution further defined a new category called ‘dwarf planet’, which has these four following traits:

1 it too is in orbit around the Sun;

2 it also must have sufficient mass so that its own gravity pulls it into a nearly round shape;

3 however it has not cleared its neighbourhood

4 it cannot be a moon.

The second IAU resolution made Pluto the prototype of the newly created category called dwarf planet. Pluto as well as Eris, are not dominant enough for their gravitational fields to have incorporated or shoved aside all of their neighbours. In fact, based on current knowledge, they are only two bodies in a large field of thousands to millions of similar objects known as the Kuiper Belt.

To better visualise the location of the Kuiper Belt, let’s look at how far it is from the Sun. The earth is roughly 93 million miles from the Sun and the Kuiper Belt lies between 2.8 and 4.6 billion miles from the Sun. In fact, it takes more than 4 hours for the Sun’s light to travel this distance. How far is this really? Let’s imagine a road that took us from the Sun to the end of the Kuiper Belt. Now let’s imagine you’re in a sports car. Imagine speeding down the road at 100 miles an hour. You would pass the Earth after 106 years of driving. You would come upon Mars after travelling 162 years; reach Jupiter in 552 years and pass Saturn after 1011 years on the road. Finally at 3181 years, you would hit the beginning of the Kuiper Belt; drive by Pluto during year 4187. And at long last you would arrive at the outer reaches of the Kuiper Belt after being on the road for 5302 years straight.

It is not surprising then that we have only catalogued a very small percentage of the Kuiper Belt, and that only in the past 10 years. The scope for exploration is vast. In the future, Pluto, Eris and untold other objects in the Kuiper Belt, which have yet to be discovered, might be included in a new classification system as dwarf planets.

Although the IAU was the first to try to scientifically define a planet, many astronomers disagree with the definition as it classifies a planet in large part by what it is near, and not by its properties. Based on the IAU ruling, if the Earth were in the Kuiper Belt, it would not meet the current IAU definition of a planet. Also, Jupiter, with its shared orbital asteroids known as the Trojan asteroids, hasn’t cleared its orbital path, so it too might not be considered a planet. The topic of defining a planet is still being vigorously debated. What we know is that our technology will continue to improve and with it our understanding of the universe. We should be flexible in our interpretation of what we do know, as what we do not know will always surpass it.

End transcript: Video 1  What is a planet?
Video 1  What is a planet?
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As you have just seen, in 2006 the International Astronomical Union (IAU) met to decide what the definition of a planet in our Solar System should be. They came up with three criteria:

  1. A planet must orbit the Sun.
  2. A planet must have enough mass for its own gravity to cause it to be round, rather than being an irregularly-shaped large rock.
  3. A planet must have cleared the neighbourhood around its orbit.

The second and third points relate to the gravity that an object must have to be considered a planet. Planets have enough mass for their own gravity to be the strongest force acting on them, which pulls them into roughly a sphere shape. Smaller objects – rocky asteroids and icy comets – are often a variety of weird and wonderful shapes because they don’t have much mass and become distorted as a result of other forces acting on them.

The third point means that a planet must gravitationally dominate its own orbit. Large objects would either consume smaller bodies, capture them as moons or send them into different orbits. Only the most massive objects can ‘clear their neighbourhood’. This means that there shouldn’t be other independent objects of comparable size that also orbit the Sun in the same region of space as a planet. Moons don’t count because they orbit a planet rather than the Sun – they are controlled by the planet’s gravity. This definition also allows for other objects such as so-called Trojan asteroids, which are small clumps of asteroids pushed along or ‘shepherded’ by a planet’s gravity.

Point number three is where Pluto lost out. As you saw in Video 1, Pluto shares its orbit with many other objects. Pluto is now defined as a dwarf planet – an object that ticks points one and two of the planet definition, but not point three. Eris, and the largest object in the asteroid belt, Ceres, have also joined this new group.

Activity 2  Is the neighbourhood clear?

Timing: Allow about 5 minutes

Based on what you have just learned, which of the following planet candidates would astronomers say satisfy the third planet criterium?


a lone planet candidate


a planet candidate orbiting within the asteroid belt


a planet candidate with Trojan asteroids sharing its orbit


a planet candidate which shares the orbit of another, larger planet

The correct answers are a and c.


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