By Belinda Smith for the ABC
They sound like distant destinations ripped from the pages of science fiction: a deep blue gas giant where it rains molten glass, or a little iron "cannonball" world which cuts a lap around its star every eight hours.
Exoplanet and galactic nebula, illustration. Photo: AFP / Wladimir Bulgar / Science photo library
In recent years, astronomers have not only discovered these extra-solar planets, or exoplanets - and more - they can also paint often incredibly vivid planetary descriptions.
Yet with very few exceptions, exoplanets can't be seen directly.
They're too small or far away to see with a telescope, or the brilliance of their sun overwhelms what little light they give off.
So with this in mind, how do astronomers know what an exoplanet is made of - and if it might be a contender for harbouring life?
How to spot an exoplanet
For a start, astronomers need to detect these far-away solar systems. There are two main ways they do this.
The first relies on the fact that stars with planets don't sit motionless in the centre of their solar system. They're pulled into their own small circular or elliptical path by the gravitational tug of their planets.
It's generally too hard for telescopes to see a star's subtle side-to-side shimmy.
But as a star moves away from and towards Earth, its light stretches and becomes redder, then compresses and becomes slightly bluer - a change that telescopes can pick up.
This way of detecting exoplanets is known as the radial-velocity method, or, as University of Southern Queensland astronomer Jonti Horner calls it, the "wobble method".
And it was the dominant way of finding exoplanets for a decade or so after the first confirmed detections in the 1990s.
But spotting these stellar wobbles is labour-intensive, sometimes requiring years of staring at a single star to see if its light appears to get redder and bluer over time.
Enter: the transit - or "wink" - method, the second primary way to detect exoplanets.
If an exoplanet's journey around its star is in Earth's line of sight, we'll see that starlight momentarily, but periodically, dip in brightness.
"The beauty of that is that you can use a wide-field camera and simultaneously measure the brightness of a large number of stars at a time," Professor Horner said.
"When you're looking at enough stars, you're playing a numbers game.
"Some of them will have planets that are lined up just right that they pass between us and their star every time they go around, and you'll see that star periodically winking."
Illustration of the view from the innermost of the two exoplanets orbiting Gliese 667 C (largest star, a red dwarf) in the Gliese 667 system. Photo: AFP or licensors
The winking method's clocked up more than 3,700 exoplanets; the wobble method's found around 900.
Add to these another 120 exoplanets that were spotted using a technique called gravitational lensing, where light from a star far, far off in the distance bends around an exoplanet.
Only around 50 exoplanets have been directly imaged.
How do astronomers calculate exoplanet size and mass?
An exoplanet's girth and heft are calculated from its star's wink and wobble too.
A larger exoplanet produces a more substantial star wink, purely because it blocks more light, and an exoplanet's mass is given away by the amount it makes its star wobble.
Factor in how quickly an exoplanet travels around its star, measured by the time between winks, and astronomers can start to get some idea of what it's like out there.
For instance, a sizeable wobble and strong, fast winks points to something like a hot Jupiter - a gas giant that loops around its star quickly and closely.
What about what an exoplanet is made from?
Once astronomers calculate an exoplanet's mass and size, they can calculate its density, and from there, start to work out what kind of stuff the planet is made from, Professor Horner says.
"Perhaps we can also work out what its internal structure is."
For instance, a small but massive planet has little choice but to be mostly made from dense material, such as iron.
The transit or wink method of exoplanet detection also has the added bonus of providing clues to atmosphere composition and chemistry.
When an exoplanet momentarily passes between us and its star, a little of that starlight filters through its atmosphere (if it has one), and some of the light is absorbed, UNSW computational chemist Laura McKemmish says.
"What colours of light get through depend on what molecules are there, as well as clouds, and what temperature that atmosphere is," she says.
Illustration of the view from an exomoon orbiting a ringed exoplanet in a polar orbit. Photo: MARK GARLICK/SCIENCE PHOTO LIBRARY
Once you've got the atmosphere, you can start to think about the biology of the exoplanet, she adds, "rather than it just being a spherical blob".
Telescopes on and orbiting Earth capture that light. Instruments, called spectrometers, split it into a spectrum from infrared to visible light, like a rainbow.
Astronomers can then see which colours are missing from the rainbow and start to model what the atmosphere is like, Dr McKemmish says.
And not just what molecules are floating around.
"Is there a temperature inversion, where the temperature decreases [with altitude] then gets hotter, like what happens on Earth?"
This temperature inversion has been seen in the atmosphere of hot Jupiters, she added: "It's caused by titanium oxide."
How can we find signs of life?
An exoplanet's atmosphere may also harbour chemical signs of life as we know it, or biosignatures.
For instance, oxygen, in the form of O2, is produced by plants and is a biosignature for life on Earth, Dr McKemmish says.
"If you killed all the life on Earth, all the atmospheric O2 would react [with other molecules] ... and just disappear."
This means the presence of O2 on an alien world within the habitable zone, the region around a star where the temperature is right for liquid water, sounds pretty compelling.
The problem with detecting O2 on exoplanets is it doesn't absorb much in the way of light.
So it's not impossible to see, but it's much easier if the oxygen makes up a high proportion of the atmosphere's gases, around 20 per cent, and you need a very sensitive telescope, McKemmish says.
"But you could look for a different molecule, like ozone.
"It's also dependent on life, but has a stronger signal - it absorbs more light than O2."
Still, if a biosignature is found, that doesn't necessarily mean it was belched or exhaled or excreted by a living organism.
Computer illustration of an extrasolar planet with three moons and gaseous nebula. Photo: AFP or licensors
Other chemical reactions or geological processes, such as volcanism or weathering rocks, may produce biosignatures on exoplanets completely independently of life.
New telescopes, new views
Astronomers think pretty much every star is accompanied by at least one exoplanet, but they can currently only delve into this amount of detail for a fraction of them.
That's because to get accurate wink and wobble measurements from our vantage point here on Earth, we ideally need to observe solar systems side on, with planets moving in our line of sight.
And so far, there's been a bias towards finding massive exoplanets which orbit close to their star and produce bigger winks and wobbles.
Of the 4,900-odd confirmed exoplanets found at the time of publication, around two-thirds are thought to be ice giants (like Uranus and Neptune) or gas giants (like Jupiter and Saturn).
Most of those, around 2,400, were found by the Kepler space telescope during its near decade of operation.
NASA's Transiting Exoplanet Survey Satellite, or TESS, has so far spotted another 170.
The long-awaited James Webb Space Telescope, which is earmarked to be in position mid-2022, will - among other jobs - probe exoplanet chemistry to a whole new level, McKemmish says.
In the next decade or so, other missions will launch that will focus solely on analysing exoplanets, such as the European Ariel Space Mission, currently scheduled to launch in 2029,
Such missions are ushering in a new phase of exoplanet discovery, Professor Horner says.
"We're almost entering a part of the exoplanet era where we're no longer just finding planets … but we're at the point where we can start learning a lot more about them.
"And that's a precursor to being able to do things like look for any evidence of life.
"That's the journey we're on, and we're not at the end yet."
- ABC
