Exoplanet – a planet which orbits a star outside the solar system

Prior to 1992 not a single exoplanet was known to exist. Now, here we sit 24 years later having discovered 3,537 exoplanets in 2,653 planetary systems. The field of exoplanetary science has without a doubt been the booming area of astronomy in the last decade. So it’s only right Rationalising the Universe respects this big thinking field of science and we go on a little whistle-stop tour through the history of exoplanet exploration, explain how detections are made and discover just how strange some of these new worlds are. This is a topic which has a special place in my heart (brain) as when i’m not typing away furiously on this site I work for a UK Space Mission which will, come 2019, launch a satellite into low-earth orbit to look at exoplanets which have already been discovered in our galaxy to learn more about what they are made of, what their evolution history is and ultimately to answer the question of whether they are habitable.

Some History

“This space we declare to be infinite…in it are an infinity of worlds of the same kind as our own.” Giordano Bruno (1584)

Although thoughts go back centuries it was only in 1992 that Aleksander Wolszcan and Dale Frail announced the discovery of two rocky planets orbiting PSR B1 257+12, a complicated name for a pulsar in the constellation Virgo. Then shortly after in 1995 we entered more familiar territory and found the first exoplanet orbiting around a main-sequence star. (Main-sequence stars are in hydrostatic equilibrium, where thermal pressure form the core is balanced by inward pressure of gravitational collapse.) Our sun is a main sequence star and as such this discovery was pivotal to the understanding that our solar system is not that rare or unique after all. The star was 51 Pegasi, and the planet,  (roughly half the size of Jupiter) was named 51 Pegasi-b, as is the slightly bland but effective book-keeping convention of naming planets – add a letter after the star name.

Then in 1999 we got the discovery of the first multi-planetary system – excitement went up a notch in the community. Researchers from San Francisco State University published the discovery of two additional planets orbiting the star Upsilon Andromeda in the constellation Pegasus, which was already known to have one exoplanet. Planets had begun to pop up left right and center in the sky. Then the excitement was turned up another notch – 2001 the first planet found within the ‘habitable zone’! Geneva University astronomers discovered HD 28185 b, a planet that orbits its star at roughly the same distance the Earth does from the sun. The habitable zone differs depending on the size of the star, i.e. the heat it emits, it is the zone where it is not too hot yet not too cold so that an atmosphere can be maintained around a planet such that then life could possibly exist. In 2002 we got the first ‘normal’ solar system, with a Jupiter-like planet orbiting a star at similar distance to Jupiter in our solar system instead of taking an extremely close, or in colloquial terms, ‘roasting’ orbit. Then finally in 2014 we got the first Earth-sized planet in the habitable zone, where liquid water had the potential to exist on the planet’s surface – Kepler 186f.

So there we have the milestone discoveries in the field but now time to discuss how they are discovered before delving into the wacky, wonderful and downright weird different types of planets that exist.

The Detection

• Radial Velocity

The first method has the record for discovery of the most exoplanets, weighing in 536 discoveries. This method is the Radial Velocity method or ‘Watching for Wobble’ as NASA likes to call it. When the planet orbits around the star its gravitational force (due to its mass) causes the star around which it travels to ‘wobble’ a little bit. When the planet is on the left of the star, the star is pulled to the left a little bit and when its on the right it is pulled to the right. Astrophysicists can detect this wobble in the wavelengths of the light they receive from the star as when the star moves to and fro the light waves compress and then stretch out, repeating this pattern in a regular manner due to the regular orbital period of the planet.

• Transit Method

The second method has produced 306 discoveries, this is the transit method or ‘Searching for Shadows’. The telescope looks at light from a far away star, then every so often the exoplanet of passes in-front of the star whilst on its orbit and blocks out a little light from the star, making it dimmer. It is this periodic dip in the brightness of the star that allows us to detect the exoplanet. Much information can be obtained from the transit method – bigger planets block out more light and the father away a planet is the longer it takes to orbit and pass in front of the star.

This method can also characterise the planets as, when comparing the wavelengths of light received from the star when the planet was and was not in front of it we can deduce what molecules are present in the planet’s atmosphere. This is because, depending on which molecules are present in the atmosphere of the planet, different wavelengths of light are absorbed from the star. With this extra information we can ask questions like what are the exoplanets made of, what the weather is like there and the ask the ultimate question – are they are habitable?  If we see that key molecules such as H20 or C02 are present this would be a very good indicator of life on this planet!

• Direct Imaging

The third method weighs in at only 33 discoveries. It is the method of direct imaging or ‘Taking Pictures’ where astronomers can actually take pictures of the planets themselves by eliminating the brightness of their central star. Exoplanets are much much dimer than their star so it is extremely difficult to be able to block out this glare so we can see the planet itself but this method is at the forefront of future exoplanetary science. An analogy would be like trying to take a detect a fairylight in front of a floodlight. Instruments called Coronagraphs are added onto telescopes to act as light blockers or ‘Starshades’ that block the star’s light before it even enters the telescope. Direct imagining is a promising method of the future which would allow us to identify things like oceans and landmasses on the surface of planets.

• Gravitational Microlensing

Only 18 planets discovered here but it is a finicky one – nickname, ‘Light through a Lens’. Gravitational microlensing happens when light from a distant star is bent and focused by gravity as a planet passes between the star and Earth. The phenomenon of gravitational lensing was discovered by Einstein in this theory of General Relativity. These lensing events cannot be predicted and as such observations need to cover a large part of the sky over a large period of time. Alongside them being very quick, fleeting events, this makes method quite inefficient at planet detection.

• Astrometry: Minuscule Movements

Finally finishing up in last position with 2 planets we have Astrometry, who NASA nicknames ‘Miniscule Movements’. This is very similar to the first, Radial Velocity but in this case the wobble of the star in space (due to the gravitational pull of the orbiting planet) is actually observed through direct imaging. A series of pictures are taken of the sky and then the distance between the stars is compared in each shot, if a star have moved in the relation to the others it can be grounds for having an orbiting exoplanet.

The Strange New Worlds

Exoplanets are categorised into broad planet types by radius (compared to solar system objects) and temperature. As such the most common types of exoplanets are ‘Jupiter-type’, ‘Neptune-type’ and ‘Super-Earth’ which is a planet typically 1.5 to 2 times the size of the Earth. Jupiter types tend to be hot and as such ‘Hot-Jupiters’ make up most of the known exoplanets as they are big and bright (due to their heat) which makes them easiest to detect.

Now open this link alongside the post for a bit of visual fun as we talk: https://exoplanets.nasa.gov/alien-worlds/strange-new-worlds/

[Images NASA]

Here we have a couple of the weirdest planets out there. Worlds where it rains glass sideways, egg shaped worlds close to being torn apart, worlds as light has styrofoam, worlds frozen at minus 370 degrees Fahrenheit, worlds with flowing lava rivers, worlds living in eternal darkness, the list goes on…  The information about the size, shape and environment of the planets is all gathered from methods such as doppler and transit , though the images themselves are artists concepts for now. An excellent description of each of these wonderfully wacky worlds is given by NASA so I won’t try to compete, taken a little look around.

What the Future Holds

In August this year came the discovery of Proxima Centauri-b an Earth-sized planet orbiting our closest star, Proxima Centauri only 4.22 light-years away. Although the planet appears to be in the orbit of the star’s habitable zone astronomers are still unsure whether the planet is a rocky dense object or more gaseous. The planet’s surface temperature is also unknown and to answers these questions and more such whether liquid water could exist on the surface we need to be able to conduct atmospheric analysis. Therefore we need to use the transit method – but the tricky part is, although all planets orbit, not all planets transit. Transit is an Earth-specific term which means the planet passes in front of its host star along our line of sight. By this I mean the Earth, planet and the star are all roughly lined up… but if the planet orbits at a slightly different angle from our point of view on Earth we may not be able to see it on its passing journey and hence can’t use the transit method to learn more. This we what we need to find out to learn more about Proxima Centauri-b.

Though if the findings are positive, when/if we as a species master interstellar travel this will certainly be the first candidate to be visited. Far far in the future, if one is very optimistic about the advancement of the human race and their investment in space exploration perhaps planet hopping will become the equivalent of island hopping. NASA also likes to indulge in this fantasy with the idea of the ‘Exoplanet Travel Bureau’ which I find very entertaining. Take a look at some of the artwork (which I find very quaint and retro for such a futuristic idea).

So there we have it a brief history of exoplanetary science, methods of detection and little peak into the weird and the wacky. Watch this space, exoplanetary science  really is just getting started…