A leap forward in the hunt for Earth-like planets

Artist’s impressions of how exoplanets may appear are based on data that is difficult to collect. – NASA

Twenty years ago Swiss astronomers discovered we may not be alone when they detected 51 Pegasi b, the first exoplanet. But the team couldn’t see 51 Peg b (as it is known), only infer its presence by the way it makes its star wobble. Now a new technique to measure the sunlight glinting off the planet is giving an unprecedent view of 51 Peg b itself,  revealing new details about the alien world.

The technique was developed by astronomy PhD student Jorge Martins from the University of Porto in Portugal and reported in the journal Astronomy and Astrophysics.

“It is a real gift to get a direct measurement of the light from this famous planet at the 20th anniversary of its detection,” says Didier Queloz, who was part of the team that first discovered 51 Peg b.

“This pushes a very difficult technique to a new limit,” says Chris Tinney, an exoplanetary scientist at the University of New South Wales. “It’s a ridiculously hard experiment to do.”

Since finding 51 Peg b, which orbits a star in the Pegasus constellation, scientists have discovered nearly 2,000 other distant worlds.

Until now, astronomers have had two ways of tracking them down through telescopes. The most widely used is the radial velocity technique, which relies on picking up the ever-so-slight wobble stars have from the gravitational pull of their orbiting planets. It’s how 51 Peg b was discovered and the technique has since uncovered hundreds of exoplanets, and told us something about these worlds. By measuring the magnitude of its star’s wobble, astronomers have calculated 51 Peg b’s mass must be approximately half that of Jupiter’s.

The second is the transit method, in which astronomers measure the dip in a star’s light as a planet passes in front of it. One benefit of this technique is that astronomers can sometimes make out the exoplanet’s atmosphere as the star’s light filters through it.

But as both are indirect ways to “see” a planet, there are problems. A lot depends on Earth’s perspective. We can easily detect a star if it wobbles toward us, as this motion causes a characteristic blue shift in the star’s light, then a red shift as it moves away again. But if the star wobbles side to side or up and down, we won’t see it as clearly. Similarly, the transit method relies on an exoplanet passing directly between the Earth and its star – and we may have to wait years for it to hit that sweet spot if it ever does at all.

In 1999, University of St Andrews astrophysicist Andrew Cameron and colleagues suggested looking for exoplanets directly, by picking out the light it reflects from its star – the same way we can see other planets in our own solar system by the sunlight they reflect. This approach would have two benefits over existing techniques, Cameron predicted. Astronomers could watch the planet move through space, and so establish its orbit. And from that they will be able to calculate the exoplanet’s mass precisely – this can only be calculated approximately using the radial velocity technique. 

The wavelengths of the light and how much is reflected should also give some idea of an exoplanet’s atmosphere. Reflectivity – also called “albedo” – changes depending on a planet’s surface and atmosphere. Venus, for example, has a very high albedo from its highly reflective clouds of sulfuric acid. But Mercury’s albedo is low due to its dark surface.

The challenge with Cameron’s technique is picking up the little bit of light reflected from an exoplanet. It tends to get swamped by the intense light streaming from its star – especially for planets with a close orbit.

So Martins decided to try the technique using the 3.6-metre telescope at La Silla Paranal Observatory in Chile. The telescope is equipped with one of the world’s most sensitive spectrographs – an instrument that can analyse a distant source of light by splitting it into its component wavelengths and filter out unwanted “noise”. What better target than 51 Peg b to try out the new technique?

"Maybe we'll find we're not alone in the Universe."

Martins pointed the telescope towards it when the bright, daytime side of the planet faced Earth. And when he enhanced the signals using a computer algorithm he had developed, he was left with an albedo signature typical of a type of exoplanet known as a “hot Jupiter”. Although the planet has around half the mass of Jupiter, as it orbits much closer to its star than Jupiter orbits the Sun its gaseous atmosphere is “puffed up” by the intense heat.

Queloz, who was a PhD student on the team that discovered 51 Peg b in 1995, says he is “thrilled” by the work.

Even so, the amount of light Martins was able to collect was tiny. Luckily, he has more powerful hardware up his sleeve – the 8.2-metre Very Large Telescope, also in Chile, will be able to suck in more light from 51 Peg b, and its spectrograph will be able to filter out more noise to give a clearer view.

Martins hopes to one day find an exoplanet like the Earth orbiting a star like the Sun. “Who knows?” he says. “We’ll maybe find we’re not alone in the Universe.”

This artist’s rendering shows a gas-giant exoplanet transiting across the face of its star. We can learn much about distant planets through the transit method, which relies on it passing directly between Earth and its star. – NASA/ESA/C.Carreau

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