Exploring distant stars
I WAS delighted to learn recently of the approval for an exciting space mission by the European Space Agency: the Atmospheric Remote-Sensing Infrared Exoplanet Largesurvey, whose acronym is ARIEL.
The spacecraft’s instruments are designed to reveal information about the composition of exoplanets.
Exoplanets are objects I have often written about in this column. They are planets orbiting other stars, the first of which, called 51 Pegasi b, was discovered in 1995. As of this week, the total number of known exoplanets has reached more than 3700, and this number is increasing all the time as the various methods of discovery of these objects yield their results.
There are several ways of detecting them, even though with rare exceptions, we can’t actually see them.
One method is to watch for a “wobble” in a star’s motion, caused by the planet moving in orbit around it. Another is to watch for dips in the light from stars as their planets move in front of them: this is called the “transit method”, and has been highly productive. In particular, the Kepler spacecraft, which used this method, was able to detect a huge number of them.
It is the planets of this type, called transiting exoplanets, that ARIEL will be targeting — and this is because of the method ARIEL will use to study them.
When astronomers examine the light from a star, they can learn a lot about it by studying the intensity and other details of specific wavelengths of the light. This is called spectroscopy, and it has been an extremely important method of learning about the stars and how they evolve.
If there is a planet’s reflected light mixed in with this, the light will be a combination of the two sources. However, because it is a transiting exoplanet, it will repeatedly move in front of, or behind, its star as seen from Earth. During both of these events, astronomers will learn more about the planet.
It is easiest to see why if we consider the period over which the planet is out of sight because of it being behind the star. When this happens, the light that astronomers observe will be coming from only the star itself, so examining the difference tells them about the contribution from the planet when it was “visible”.
A crude analogy is to imagine that a friend is shining a red and a green torch toward you, with torches that are close together but the friend being far away. To you, the light will appear to be a single yellow point, because red and green, when added together, produce yellow. However, if your friend hides the red torch or switches it off, you will see a green point of light—and measurements before and after could allow you to deduce how much red light had been removed.
However, astronomers can also make deductions about the composition of a planet when it passes in front of its star. When that happens, starlight is absorbed in the planet’s atmosphere, and the amount of absorption at different wavelengths depends on the constituents of that atmosphere.
ARIEL’s study will concentrate on hot planets orbiting relatively closely to their stars, because in those cases, more planetary material is expected to have risen to the planet’s upper layers.
An exciting possibility is that ARIEL may be able to reveal evidence of possible vegetation on extrasolar planets by making measurements of the infrared radiation from them. This will not prove the vegetation is there, but it would be a tantalising suggestion. Infrared light is outside the range of detectability by the human eye, but ARIEL will be able to “see” it.
ARIEL will be placed in space at a point known as L2, where it is protected from sunlight. Unfortunately, we will have to wait quite a while before the craft is in operation. The launch is planned for 2028 on an Ariane 6-2 rocket, which will come into use in 2020. THERE is a great opportunity for stargazing tonight, with the Astronomical Society of Tasmania Incorporated conducting a public astronomy night at the Mount Pleasant Radio Observatory just north of Cambridge.
Weather permitting, there will be great views through the telescopes, including craters on the moon and many fine sights of the Tasmanian night sky. Numbers are strictly limited and if you would like to come, it is essential to register with the Astronomical Society at www.astas.org.au/events.html. The fee is $10 per car, payable on arrival. For further information call 0419 341 469.