What is an Exoplanet?
Exoplanets are bodies like planets, but with one key difference: they don’t go around the Sun; they go around other stars, apart from those known as ‘rogue planets’ which orbit galactic centres instead of stars [1].
First discovered in the 1990s, over 4,000 have been found to date [2], coming in all different shapes and sizes, but categorised into 5 main groups: ‘gas giants’, ‘super-Earths’, ‘Neptune-like’, ‘terrestrial’, and ‘unknown’ (well, nature is always more complex than we’d like it to be) [3].
Gas giants are common in the Solar System, with the planets Jupiter and Saturn falling into this category. They’re large (hence ‘giant’) and mostly made of hydrogen and/or helium (hence ‘gas’) [3], although they do have a solid core, since the pressure and gravity gets so great as to bring about the formation of one (as seen with Jupiter’s core being possibly solid iron in composition) [4]. They also provide some of the best conditions for finding them, with a subcategory of ‘hot Jupiter’ being born from how many gas giants are near their host star, and so, since they’re so massive, they tug on the star and make it wobble (more on that later) [3], leading to as much as 31% of all exoplanets discovered being gas giants (as of now) [2].
Super-Earths are next on the list. More massive than Earth; less massive than ice giants (Neptune and Uranus), but still enormous, at between 3 and 10 times the Earth’s mass, made of a combination of both gas and rock (like on Earth). That said, that’s where similarities with the Earth could stop, since they can be hot enough to vaporise metal, or even covered in a global ocean (I’d stick with this planet if I were you!) [5].
Artist's Impression of 55 Cancri e: a Super-Earth.
Credit: ESA/Hubble/M. Kornmesser, CC BY 4.0
Neptune-like is our next category, comprised of, you guessed it, planets with a similar size to Neptune! Often referred to as ice giants (since chemicals like water, ammonia and methane are frozen at the distance Neptune is from the Sun, although these chemicals aren’t frozen closer to a host star or in their interiors), these are exoplanets with often thick clouds of hydrogen, helium, water, ammonia and methane (often something that hides the composition of the atmosphere of these worlds, stopping any light entering to take a spectrum of -more on that later) [6].
Terrestrial is the last type of world I’m going to describe, made up of rock (mainly iron), silicate, carbon and/or water (a category fitting Mercury, Venus, Earth and Mars). These are the ones the tabloid newspapers prick their ears up for the most, since they’re the most Earth-like planets, with the best likelihood of being habitable, yet they’re also the ones with the least number of discoveries (3.1% of all discoveries to date), being the hardest to detect because of their size [7].
TRAPPIST-1 System: Largest collection of habitable terrestrial exoplanets.
Credit: NASA/JPL-Caltech
Where are they?
It’s not something for the average telescope user to look for, but instead entrusts the help of 5 methods of observation and some powerful equipment to locate and categorise them. Methods include: radial velocity, transit, direct imaging, gravitational microlensing and astrometry (no, not astrology!): the ‘Famous Five’ of exoplanet exploration techniques [8].
The radial velocity method, also known as doppler spectroscopy (try saying that quickly!), is the first method employed by people to detect exoplanets. It uses the fact that, when an exoplanet orbits a star, the tug of the exoplanet on the star moves the system’s centre of mass from the centre of the star to somewhere between the two objects (the exoplanet needs to be massive enough to get a measurable effect). This means that the bright star, seen from Earth, appears to wobble back and forth [9]. This wobble is so small that it’s not something that can be seen with the naked eye or even with a large telescope, so doppler-shift was employed: as light moves towards you, it’s squashed (it’s wavelength decreases) making it bluer (blue-shifted); whilst if light moves away, it’s stretched (it’s wavelength increases) making it redder (red-shifted) [10].
Radial Velocity Method. Credit: ESO, CC BY 4.0
The transit method also needs quite a large exoplanet, or for it to be near enough to the star so that, when it travels in front of the star relative to Earth, it periodically decreases the brightness of that star (like a transit of Mercury or Venus in front of the Sun) [9]. It’s also great for finding the diameter of the exoplanet, by studying the difference in brightness from the star’s normal brightness. However, it needs the exoplanet to cover part of the star visible from Earth: something that doesn’t always happen, meaning this method would also miss many exoplanets. Also, there’s that small problem that it’s hard to know if it’s a star passing in front of a brighter star or not [11]. That said, it’s the best method we’ve got, having discovered 3322 planets to date via this method [8].
Direct imaging really does what it says on the tin: you take a picture of an exoplanet. It’s not easy though! Stars can overwhelm the light from exoplanets by over a million times, and the planet must be massive (a few times that of Jupiter), young and far from their host star(s) to have a chance of being observed [9] (detecting just over 1% this way [8]). That said, it’s the most fruitful in data of all the five, showing everything from what the atmosphere is like, to whether there’s water on the surface and if that world is habitable [12].
Gravitational microlensing, our penultimate method, sounds very sci-fi, but it relies on some laws set out in the early 20th century by, you guessed it, Einstein. It uses the fact that, if an object has a large mass, light bends around it (since, famously, mass and energy bend space and time). Again, like the transit method, a perfect alignment is needed [9]. However, if this happens, light from a star with an exoplanet (“star A”) between you and another star (“star B”) amplifies star B’s light. Then, as the light from star B curves around star A, two images of star B are created, making what’s called an Einstein Ring. However, since it’s mass that bends light, the exoplanet also bends some light coming from star B, creating a third image. This creates a second peak of light intensity on top of the main smooth peak: a ‘bump’, characteristic of exoplanets (I hope you got all that!). This means very distant exoplanets (both from us and their host star) can be observed, but they can only be seen once (most likely), since this event is very unlikely to happen [13].
Astrometry is the last in our famous five, using the same premise as the radial velocity method of how a star ‘wobbles’ because of an exoplanet orbiting it . However, instead of using doppler shift to observe it (needing it to move back and forth relative to us), it uses the star’s position in the sky to detect visible, and regular, changes in its position in space: something which needs very precise measurements to yield results [9], leading to only one so far be observed this way: DENIS-P J082303.1-491201b (or VB 10b for short) [14].
Gravitational Microlensing. Credit: NASA/JPL
Who’s Looking?
There are countless missions and projects looking for them, but here are just a few of my favourites: SuperWASP (no, not from Dr. Who), HARPS, and TESS.
SuperWASP (WASP standing for ‘Wide Angle Search for Planets’) consists of two observatories, one in the Roque de los Muchachos Observatory in the Canaries (northern hemisphere) and one in an observatory in south west South Africa near Sutherland (southern hemisphere). It’s a UK run project using robotic camera arrays (8 cameras per telescope) to find exoplanets, employing the lucrative transit method to do this [15]. It has found 174 exoplanets so far [16] including discoveries such as WASP-76b, an exoplanet which rains iron [17], and WASP-107b, an exoplanet with a density of candy floss [18].
SuperWASP cameras, South Africa. Credit: David Anderson, CC BY-SA 3.0
HARPS, or the High Accuracy Radial velocity Planet Searcher (you can see someone wanted it to fit!), is located in the European Southern Observatory’s La Silla Observatory, in the southern Atacama Desert in Chile. It uses the radial velocity method to find its exoplanets, using spectroscopy to see if a star is blue-shifted or red-shifted [19].
And then there’s TESS (no, not from the TV): the Transiting Exoplanet Survey Satellite, using four cameras to detect exoplanets using the transit method. However, unlike the previous two projects, it has the added benefit of a view of the majority of Outer Space, dramatically increasing its probability of success [20] and leading to 120 confirmed exoplanets so far, in just a few years [21].
In fact, TESS (as well as telescopes in Spain, the USA, Chile and Hawaii) has even lead to a recent exoplanet discovery by the CARMENS project consortium (a group of Spanish and German scientists) that have got people listening: Gliese 486b. It’s a super-Earth with a temperature of 430ºC and nearly 3 times a large as Earth. However, it’s interesting mainly because of how its atmosphere expands due to its intense surface temperatures, meaning it’s easier to observe from Earth.
Then, with the added benefit of it transiting its host star every 1.5 days, the light from the star can be analysed using spectroscopy to see what makes up the exoplanet’s atmosphere, as well as how they help distribute energy across the planet [22], to see if super-Earths could be stable enough for life to survive [23].
Artist concept of TESS. Credit: NASA
These projects could bring up many more candidates for places to observe to find out if they could also harbour life, speeding up our search to find out if our Universe is as devoid of life as it seems. And even if Gliese 486b and others like it don’t turn out to be habitats for life, they could still tell us a lot about the evolution of planets and how our Solar System might have then evolved to become the perfect place for life to start, possibly leading us to the answer of the age old question ‘How did life begin?’.
by George Abraham, ADAS member.
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Click here for NASA's catalogue of exoplanets, and here for the exoplanet encyclopaedia's catalogue.
Click here for NASA's orbit simulations for various famous exoplanets
Click here to explore artists' impressions of some famous exoplanets
References
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