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, stars and the Solar System
Dr Melanie Johnston-Hollitt, from Victoria University of Wellington, explains the difference between planets and stars, and the place of the Solar System in our galaxy.
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.
Planet hunting telescope
The 1.8m telescope at the University of Canterbury Observatory at Mt John, near Lake Tekapo, is used by the Microlensing Observations in Astrophysics (MOA) group to search for microlensing events.
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.
Hunting for planets
Professor Denis Sullivan, from Victoria University of Wellington, explains why we can only detect extrasolar planets indirectly. He outlines the ‘wobble’ method of detecting planets around distant stars.
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!
The transit method
Professor Denis Sullivan, from Victoria University of Wellington, explains the transit method for detecting planets in orbit around stars other than our Sun.
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.
Planets in transit
This animated video demonstrates the transit method for detecting extrasolar planets.
A graph shows a temporary dip in the brightness of an observed star. This is explained by showing an orbiting planet passing across the face of its star.
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.
The microlensing method
Professor Denis Sullivan, from Victoria University of Wellington, explains how microlensing can be used to detect planets in orbit around stars other than our Sun.
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.
A planet caught by microlensing
This graph uses data from 6 telescopes, including one in New Zealand. A ‘lens’ star magnified the light from a star behind it (main peak). A planet near the lens star caused the smaller peak.
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.
Nature of science
Before astronomers announce the finding of an extrasolar planet, they must make sure they are right. The two main ways of doing this are to either use at least two different detection methods, or get other scientists to make measurements using the same methods. Having a network of scientists around the world, ready to help, is vital. Some methods, such as microlensing, measure one-off events, so they cannot be repeated. It is important to get as much information as possible at one time.
Since this article was written in 2009, thousands of probable extrasolar planets have been found. Check out the latest findings here.
Related content
Find out more about exoplanets using the citizen science projects Planet Hunters and Agent Exoplanet.
Find out how intermediate school teacher Matt Boucher incorporated planet hunting into a unit on light.
Activity idea
In Hunt the planet, students plot graphs of light measurements from stars, searching for dimming that indicates the presence of a planet, and calculate its size.
Useful link
This video is an introduction to SuperWASP extra-solar planet detection system. This system uses the wobble and transit methods.
