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It's all in the wobble!

Extra solar planets

Astronomers have just announced the discovery of another 32 extrasolar planets. This brings the number of extrasolar planets to over 400.

What is an extra solar planet?

Extrasolar planets orbit stars and not the sun. Most of the planets so far found have been massive, much bigger than Jupiter even.

Others have been very small though, only a few times bigger than the Earth. These are the ones that exobiologists are interested in because there might be life on them.

How do you detect an extrasolar planet?

It’s not easy. You cannot actually ‘see’ any extra solar planets even with the world’s most powerful telescopes.

Hubble Space Telescope

There are two reasons for this:

  • Planets are very faint, they shine by reflecting the light of their parent star, they don’t make light of their own.
  • They appear very close in the sky to their parent star and are drowned out by the glare.
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      How close are they?

      Well if you looked at them from typical stellar distance it would be like trying to see the separate headlights of a car parked on the moon. This is much too close for any telescope on Earth to see - yet! But we may be able to do it in the not too distant future!

      So how do we know they are there at all?

      The clue is in the wobble!

      Usually we think about the Earth orbiting the sun, which for most purposes is fine, but it’s not strictly true. The Earth and the sun orbit about a common centre of gravity.

      We normally think of the sun using its gravity to hold the Earth in orbit around it. In fact, the Earth pulls just as strongly on the sun as the sun does on the Earth (Newton’s third Law and all that!). The problem is that the sun is about 330,000 times more massive than the Earth so the pull causes it to move about a 330,000 times less. But, it does still move – a bit!

      The easiest way to think about it is to imagine the Earth was nearly the same mass as the sun. How would they move now?

Centre of gravity

They would orbit around their common centre of gravity which would be almost half way between them. We can see that the sun would move too. If we were to look at this situation from a long way away and in the same plane as the orbit, we would see the sun wobbling backwards and forwards. The Earth would be invisible because it would be too close and too dim to see.

So if we can see a star wobbling we know there must be an invisible planet in orbit around it. What’s more, by studying the amount and timing of the wobble we can work out the mass of the planet and how far away it is from the star. Dead clever!

How can we spot a wobble?

If there is a source of waves, like a star producing light, then we can see a series of wave fronts arriving at his telescope. If the source and observer are moving with respect to one another then the rate of arrival of the wave fronts can change. This can make the frequency of the source appear to change. Look at the diagram below:

How movement changes wave frequency

We can see that the waves arriving at B appear to be 'bunched up' due to movement of the source towards the observer. We can see the opposite effect at A where the waves appear to be 'stretched out', this increase in apparent wavelength causes a decrease in apparent frequency.

This is the effect that gives rise to the apparent change in pitch of a police car siren as it approaches and then recedes. It is called the Doppler effect. When these effects are seen in the spectra of stars we know that they are wobbling. If the wavelength of the light is shortened then it said to be 'BLUE SHIFTED', showing that the source is moving towards the Earth. If the wavelength of the light is stretched it is said to be 'RED SHIFTED' showing that the object is moving away from the earth. By looking for tiny blue and red shifts we can measure the wobble very accurately.

What does all this mean?

What it means is that planets around other stars are really common. Wherever we look, stars wobble. Our galaxy probably contains millions of Earth-sized planets! The Spitzer space telescope has even picked up a cosmic collision that looks identical to the one that formed the Earth moon system about four billion years ago. People had always puzzled how the Earth had managed to end up with a single large moon. It was always assumed that the moon and the Earth were made in different places at different times and the Earth ‘captured’ the moon with its gravity sometime later.

The surprise came when the Apollo astronauts returned rock samples back to the Earth for analysis. The rock samples weren’t just a bit similar to rock sample from the Earth, chemically, they were absolutely identical. How could this be if the Earth and the moon were formed separately?

The answer came later when scientists used a computer model to look at how planets might form. They realised the Earth had been hit by another planet about the size of Mars way back in its early history. The impact had been so violent that the two objects had completely melted and mingled. Lots of bits were spattered out into space and orbited the new Earth. Over time gravity pulled these left over pieces together to form the moon. That was why the Earth and the moon were so alike; they were actually made from the same stuff at the same time.

An event almost identical to this one has just been seen by Spitzer. It occurred around a star romantically called HD 172555 about 100 light years away. This is cosmically about two doors down the street, not far from the paper shop. So the Spitzer had a grandstand view.


What it saw through its infra red sensors was the heat signature of vaporised rock and bits of molten glass called tektites. Tektites are not as they sound, leggings worn by the employees of computer companies, but fragments of glassy rock thrown into the air when large meteorites impact on a planet. Millions of them can be found scattered across various regions of the Earth, remnants of cosmic impacts with the Earth from long ago. Some look almost manmade because they have a symmetrical button shape. In fact, it shows how they heated and then cooled during their high speed journey through the air.

All this data shows us that our ideas about the way the Earth was formed billions of years ago might be right! People think that rocky planets form and grow in size by colliding and melting together.

Today, the solar system is a much less violent place, but collisions do still happen. A few months ago a large lump of something, probably a comet or a small asteroid, smashed into Jupiter. The scar was big enough for amateur astronomers to see it clearly with their back yard telescopes. In fact, it was discovered by an amateur from his back garden in Australia. Good on ya cobber!

Jupiter after the impact in July 2009

The big questions:

How many extrasolar planets are there?

What sort of planets are they?

Are there likely to be many like the Earth?

How do you find extrasolar planets?

How many extrasolar planets do we know about?