Planet hunting

Hundreds of extrasolar planets (around other stars) have been found using several methods, including transit, ‘wobble’ and microlensing. New Zealand scientists are part of the hunt for extrasolar planets.

Planets outside the Solar System

Do other stars, as well as our Sun, have orbiting planets? Yes, nearly 350 planets have been found around other stars since the mid 1990s. Planets that orbit around other stars than our own Sun are called extrasolar planets. Professor Denis Sullivan of Victoria University of Wellington is one of several New Zealand scientists involved in the search for extrasolar planets.

Extrasolar planets are so distant, so small and so dark that they are almost invisible against the bright light of their stars. It’s hard to see the actual planets, but we can detect them by the effect they have on their nearby star.

There are three common methods scientists like Professor Sullivan use to find them. Scientists look for:

• the transit of the planet
• speed wobble (properly known as radial velocity
• microlensing.

Transit method

Professor Denis Sullivan went to Hawaii in 1999 to use a telescope at the Mauna Kea Observatory. Near the end of his 2 weeks there, he was told by some other astronomers that his instrument could be used to detect an extrasolar planet that night. He would need to change his work programme, but if successful, it would be worth it!

What would you have done?

Denis agreed to help. He aimed the telescope at a star called HD209458, 150 light years away in the constellation of Pegasus. A photometer measured the brightness of the star very accurately.

At first, the light stayed steady, then it dimmed very slightly. After a while, the brightness returned to what it had been before.

What had caused the star to dim? An orbiting planet had moved across the face of the star, in what is called a transit, blocking out a little of the light. Denis had helped identify a planet outside our own Solar System. He repeated his observations the next year and showed that the planet orbited the star every 3.5 days.

You can find out more about the transit method and help scientists identify exoplanets by joining the citizen science projects Planet Hunters and Agent Exoplanet.

Speed wobble method

When measuring the light from a distant star, scientists sometimes find that wavelengths change – sometimes they are shorter, sometimes longer. This tells them that there is something causing the star to ‘wobble’ or change its speed, probably a planet. In our own Solar System, the gravity of the massive planet Jupiter causes our sun to wobble.

The wobble method has been used to find most of the nearly 350 extrasolar planets discovered so far. If other methods can confirm the discovery, we can be very confident we have found a planet. This is what happened with Professor Sullivan and star HD209458. Astronomers in America had measured the star wobbling and predicted that there was an unseen planet in orbit around it. Denis confirmed their prediction with the transit method – the scientists had enough evidence to confidently announce the discovery of a new planet outside our solar system. They called it HD209458b.

Microlensing

As part of his planet-hunting work, Denis belongs to a group of astronomers called MOA – Microlensing Observations in Astrophysics. They use a telescope at Mt John, in the South Island, for this.

MOA and other worldwide networks monitor light from millions of stars. They look for stars that show temporary brightening. If one star passes directly in front of another, its gravity can act a bit like a lens, making the star behind it look brighter – this is called gravitational microlensing. The chance of two stars lining up like this is very small – it happens for about 1 in 10 million stars surveyed.

When a MOA scientist thinks they have found a star that is brightening, they alert others in the group. Each microlensing event only happens once, so everyone joins in to collect as much information as possible. Data from several telescopes is combined to form a graph of light coming from the target star – a peak in the graph shows a microlensing event.

The astronomers get very excited if the graph from a microlensing event has two peaks. It shows that the lens star has two parts – it is probably a star with a planet orbiting it.