Astrobiology, Physics

Astrobiology 102: How to find the aliens?

Despite the fact that my last post might have made it seen that chances of finding life are pretty minute, people are still looking! And no, I don’t mean the people who are sure aliens have already visited us (Although if they did manage to find concrete evidence, that would be pretty awesome!) Organisations like NASA, other governmental organisations and private companies have all had a stab at looking for life! 

Painting of the Kepler telescope against a dark purple textured background with planetary systems
Artists’ rendition of the Kepler Space Telescope
via JPL

Finding places for life to exist

Before we can find life, we have to find the places for life to exist, which is where telescope-based astronomy comes into play! Of the thousands of exoplanets we have unearthed, most of them were found by the Kepler space telescope! Named after Johannes Kepler, a german astronomer who was the first to correctly explain planetary motion, the Kepler Space telescope was in operation for 9 years, using a 1.4m curved mirror and a photometer to measure the brightness of the ~150,000 main sequence stars in its field of view, in order to detect any dimming that could be caused by a transit. The spacecraft actually has a heliocentric orbit, instead of orbiting the Earth, so it actually follows the Earth around (ish!), and it spent it’s 9 year long mission gazing at a view in the night sky that is approximately the same size as your fist held at arm’s length!

Image showing a planet crossing a star and the resulting graph of the light curve (drop in brightness) it creates
The Transit Method
Via EarthSky.org

Kepler worked by looking for transits: the slight reduction in a star’s brightness when a planet/large body passes in front of it. This is the most common method, partly due to Kepler’s success. And this tiny dip in light can actually tell us rather a lot about the planet! We can figure out the size and distance of the planet from its star by measuring how long it takes to transit (therefore giving an indication of the orbit length and thus distance from star), and how much light it blocks out (bigger planets will block out more light than smaller planets at the same distance).

But we can also figure out, occasionally, what the atmosphere of the planet contains: This is done by measuring the wavelengths of light that passes through the exoplanet’s atmosphere, as different elements/compounds absorb specific wavelengths, so we can tell what’s there by what is missing! When we stumble across a multi-planetary system more like our own- which is very exciting- detecting the transits can be more challenging, but it is still doable, and actually carries an exciting possibility: finding out the mass from the transit! Usually, the mass, and therefore the density, is impossible to discern from a transit, but the gravitational influence of many planets can very slightly alter the orbital periods, depending on their masses! 

Gif of a transit, seen in visible light, then split into red, green, blue wavelengths
Detecting Exoplanet Atmospheres
via University of Sweden

When the transit method is not possible (eg. we are not face on to the system) or we want to find out the mass of a system with one planet, then we have to use a different method: Radial Velocity (aka the wobble method):

Gif of the wobble method showing the star moving as the planet orbits (birds eye view)
Over-exaggerated effect of gravitational forces
via Wikipedia

Newton’s third law says that for every action, there is an equal and opposite reaction. So when a star’s gravity pulls on a planet to keep it in orbit, the planet pulls back, causing the star to move back and forth. The effect is negligible, but detectable. We can detect it by measuring the changes in a stars wavelength as it moves towards and away from the Earth, like in the gif below, due to the change in wavelength as a result of doppler shift (the effect that makes ambulance sirens sound different dependent on its direction). A benefit of this method is that we can infer the mass of the planet more easily, so combines well with the transit method to give us a complete view of the new planet!

Diagram showing doppler effect of the wobble method. As star moves away from us, light is red shifted, as it wobbles towards us it is blue shifted
Wobble method via EarthSky.org

Other imaging methods, which are more rare, include direct imaging and microlensing, two budding fields with varying amounts of promise!

  • Direct Imaging involves blocking out most of the star’s light using a coronagraph and detecting the faint light that the planet reflects. It allows us to investigate the planets orbit and atmosphere, as well as the bonus that we can actually see it, making it more tangible to understand. Unfortunately, we have to be face on to the planets orbit, and it has to be large enough, and far enough away from its star for us to see it, while also being in a fairly close star system!
  • Microlensing uses the consequences of gravitational lensing (which can cause light to bend around large gravitational masses eg. stars) that can make distant stars in the background of closer ones to appear to become brighter due to the lensing having a focussing effect. During this brightness, there may be a blip of intensity as the planet also passes in front.

Our Eyes in the Sky

As I said above, most exoplanets have been discovered using the transit method, mostly by the Kepler Space telescope. Unfortunately, this is now not in service, but has been replaced by TESS- the Transiting Exoplanet Survey Satellite that launched on a Falcon 9 rocket last year. It is an accelerated mission compared to Kepler, covering more sky in just 2 years by targeting brighter stars. 

Image result for TESS telescope
TESS, via Universe Today

As TESS’ main role is to discover and identify planets, so it is up to other space telescopes (like Spitzer- which works in the infra-red, and Hubble- which observes across infra-red, visible & ultraviolet wavelengths), as well as earth-based observatories to confirm and find out more about them! One of these methods is spectroscopy: Inferring what elements are in an atmosphere by splitting the light that passes through it with a prism. This is crucial for deciding whether or not life as we know it could exist on the planet: knowing its distance from its star is important for identifying temperature, and what elements/compounds could exist as a liquid/gas there, but it is another thing to actually prove what is there! The reason it is so crucial is to detect biomarkers- indicators of life. One of the main biomarkers is Oxygen, as if it is not replenished (eg, by photosynthesis), it will simply combine with other elements, no longer existing as a pure element. So if we discovered significant amounts of oxygen, it could indicate the presence of life!

And if you want to help find some exoplanets, but aren’t an exoplanet astronomer/physicist by profession, you still can! Zooniverse is a really cool platform for citizen science, and allows you to help researchers around the world to analyse data, and you get to learn about all the awesome science, and scientific methods, as you go along. And a bonus: if you do discover something cool in the data, your name/username may appear in a scientific paper! Currently, there is one exoplanet project with data from K2 (Kepler), and my personal favourite- Planet Hunters (TESS) is on a break but set to come back with more data! I would definitely recommend taking a look, there are loads of different projects from many fields!

Visiting far off worlds

Obviously, if we want to find proof of life on exoplanets, we will probably need to visit them! At the moment, extrasolar human spaceflight is pretty far off (sadly we don’t have access to warp drives or hyper speed!), but it would be possible to send spacecraft out into the galaxy, and perhaps reach our first exoplanet in a (relatively) short amount of time! I think that Breakthrough Starshot is the initiative that is closest to being feasible! The brainchild of Russian billionaire Yuri Milner and some high profile scientists like Stephen Hawking, Breakthrough Starshot is a project that involves sending about a thousand nanocrafts that, propelled by earth-based lasers, will travel at up to 20% the speed of light to reach the Alpha Centauri system- more specifically flyby the vaguely earth-sized planet Proxima B- and beam back images and data, well within a human lifetime!

concept art of the brealthrough starshot light sail
Breakthrough Starshot: Powered by Lasers
via Breakthrough Initiatives

However, there are quite a few issues with the project: the lasers would require lots of energy in order to have a combined output of 100 gigawatts! To put that into context, you would need 40,000 wind turbines to power the array- that’s quadruple the number of Wind Turbines in the whole of the UK! This makes you begin to wonder if it is really worth all that energy, especially as it is unlikely of the energy would come from carbon zero resources. What do you think? As well as the insane energy requirements, there is also a whole host of engineering problems to overcome- you can see the list on their website

wind farm

Until we get past these difficulties, our search for life remains bound to our solar system and looking over at distant star systems to discover awesome planets! But, when we do venture out into the galaxy, or spot a strange signal from afar, I hope I’m around to see it!

Do you think we should invest in interstellar travel? Or should we stick to engineering ever larger telescopes? Are you liking the Astrobiology Series? Let me know in the comments!

☆☆☆

If you can’t trust an atom… trust in science!

☆it’s like magic, but it’s true whether you believe it or not!☆

See you next time!

☆☆☆


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