Astrobiology 102: How to find the Aliens

This post was created as part of my crest project in 2020. You can read the original here or watch it as a youtube video!

Despite the fact that last week might have made it seem like 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 government agencies 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
Artist’s rendition of the Kepler Space Telescope

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 (planets in other solar systems) 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 how planets move, the Kepler Space telescope was in operation for 9 years, using a 1.4m curved mirror and a photometer (which counts the number of photons to measure the intensity of light hitting the mirror) to track 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 in its orbit around the sun, 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 against the sky!

Kepler Mission Star Field
Kepler’s field of view

Kepler worked by looking for transits: the slight reduction in a star’s brightness when a planet passes in front of it. Kepler’s success has made this the most common method.

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 (telling us the length of its orbit), and how much light it blocks out (bigger planets will block out more light than smaller planets at the same distance).

Image showing a planet crossing a star and the resulting graph of the light curve (drop in brightness) it creates

Occasionally we can also figure out what a planet’s atmosphere contains: This is done using spectroscopy: splitting the light that passed through the exoplanet’s atmosphere and looking for characteristic absorptions spectra- black lines where elements in it’s atmosphere have absorbed particular wavelengths: we can tell what’s there by what is missing!

The transit can help identify the planet’s volume, distance from star and possible composition, but discerning its mass is usually impossible. But sometimes we stumble across a multi-planetary system.

If the planets are packed close together, then they interact and cause the orbital period to vary. This gravitational influence depends on their masses. Usually we often have to use other methods- like radial velocity

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 tiny, but detectable.

We can detect it by measuring the changes to the wavelength of the star’s emitted light as it moves towards and away from the Earth, also known as the Doppler effect (the effect that makes ambulance sirens sound different depending 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

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 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, far enough away from its star but close enough to ours to be visible. 
  • Microlensing uses the consequences of gravitational lensing (when light bends around large gravitational masses and is focused onto earth), to find planets by looking for blips of more intense lensing as the planet passes in front. Unfortunately, it only works for systems that are in line with the bright galactic centre (which is only a very few number of stars).

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’ 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!

Spectroscopy can 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- though not confirm- the presence of life!

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 and active exploration – physically visiting the target destination- is pretty far off (as sadly we don’t have access to warp drives or hyper speed!), but it would be possible in the near future 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 the late 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 in just 20 years, with its goal to fly by the vaguely earth-sized planet Proxima B, beaming back images and data 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 that 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, or passively exploring distant star systems from afar using telescopes. 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? 

Dictionary

Active Exploration: Actually visiting the target body with landers or orbiters

Passive Exploration: Observing from afar using telescopes

Spectroscopy: Splitting light with a prism and analysing the spectrum

Exoplanet: A planet outside of the solar system

Solar System/ Planetary System: A star with planets orbiting around it

Photometer: An instrument that measures the intensity of light hitting it

Main Sequence: The majority of a star’s life when it fuses Hydrogen into Helium: eg. our sun

Heliocentric Orbit: Orbiting the sun

Transit: When a planet passes between a star and the observer

Spectra/Spectrum: The arrangement of light according to wavelength/ the whole range of wavelengths of light

Gravity: The force that pulls an object towards a more massive object

Doppler Effect: The change in wave frequency as a result of the wave source’s motion

Infra Red: Wavelengths longer than visible light

Ultra Violet: Wavelengths shorter than visible light

Visible Light: The light that our eyes can detect

Element: One of the 118 substances on the periodic table defined by the number of protons in its nucleus. Cannot be chemically broken down into simpler substances.

Extrasolar: Out of our solar system

Nanocraft: Tiny, very light spacecraft

Speed of Light: 300,000,000m/s (in a vacuum). Nothing can travel faster than this.