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Exoplanets: The Search for Life
Transcript of Exoplanets: The Search for Life
from the Greek, exo- meaning outside
The Search for Life within the Solar System
Under Europa's frozen crust there may be a liquid ocean which could trap the heat needed to sustain life - it's a more exciting location than Mars at present
2022 - JUICE arrives at Europa
Curiosity is finding more and more of the conditions for life on Mars, and until recently made direct discoveries. Furthermore, it is also falsifying old theories that we have, which is equally valuable.
Other Gas Giant Moons
* Magnetism from gas giant cores could create shielded habitats
* Volcanism on moons like Io could create heat for extreme life
The History Of Exoplanets
Gamma Cephei Ab - detected by Doppler spectroscopy
binary system, with a planet!
The results were rejected by many, but it was confirmed in 2003!
is now the prime method for detecting high-mass exoplanets
Which was the true first exoplanet?
FIRST CONFIRMED EXOPLANET(S)
discovered by pulsar timing in the radio specctrum
Transit methods are the dominant method today, but previously RV was the best.
How do we detect exoplanets?
SDO 171Å full disk image
How do we detect transiting exoplanets?
The process remains the same with all candidates, and is one of the simplest ways to detect planets
If we graph the light output of the parent star, we see a characteristic drop!
We can relate the drop in intensity to the size of the exoplanet
in practice, transits are never perfect - and finding properties is difficult (but not impossible)
Analysis of KOI - 1787
Where to start?
What we know
Star radius = 0.525 solar radii
Eff. Temp = 5605K
Transit depth = 0.006011
Period = 98 days
Inference and Deduction
We need to estimate certain parameters based on trends in data and proportionalities
0.966 solar mass star
*derived from mass-luminosity proportionality and Kepler data
The transit depth converts to 4.44 Earth radii - this could make it a Super-Earth or a low-mass gas giant.
Solving for the orbit
By inputting what we now know (star and planet mass, period) into Kepler's Third Law , we can retrieve the semi-major axis of the planet.
This works out to be around 0.411AU, or 41% of the Earth-Sun distance. This is a planet which is much bigger than us, yet orbits at a similar distance to Mercury
4.44 Earth radii
~12 Earth masses
0.4112 AU sMa
0.61g surface gravity
1 year on KOI-1787 = 98 days
Calculating the Temperature
We can deduce the surface temperature of the planet in question based on the star!
5 degrees celsius
but with an Earth atmosphere
-9 degrees celsius
neglecting greenhouse effect
We'll look back at KOI-1787 later on - to deduce the habitability of the planet
All calculations performed by Thomas Killestein - data gathered from the Kepler spacecraft via Planet Hunters
Radial Velocity Method
the wobble method
Kepler's Third Law
* How habitable do you think it is?
Since the Earth is much bigger than us, the acceleration it experiences is tiny, whereas we are pulled down at 9.81ms-2
Due to the universal law of gravitation, we are pulled towards the Earth with the same force that the Earth is pulled towards us.
However when applied to two bodies with masses that are roughly similar
Since the bodies have similar mass, both are accelerated towards each other with a measurable acceleration
Both bodies orbit a common centre of mass, called the barycentre
The wobble is derived from the star's orbit around the barycentre
Measuring the wobble
Known as Doppler Spectroscopy
Like with a police car siren, the frequency of the wave increases as it approaches, then decreases as it moves away...
In space, the same thing happens with light, and as a star moves away in it's orbit, the light is red-shifted, and blue shifted as it moves away
Frequency vs Time
Amplitude - represents the magnitude of frequency shift
Frequency - period of the planet in question
RED for red-shifted light
BLUE for blue-shifted light
Good at detecting the mass of planets, but not radius!
Based on a calculation of the mass of the star, we can calculate the mass of the planet(s) in the system!
These are the two most effective methods for detecting exoplanets, but other novel methods exist!
Novel Methods for detecting exoplanets
Direct Imaging / Astrometry
Polarimetry and Radio Aurora Detection
Beta Pictoris is one of the most fascinating systems in the night sky. Careful study has provided enormous breakthroughs in both planetary formation and exoplanets
Chance alignment of two stars can reveal planets in it's light curve
The most precise photometry satellite ever - should be able to detect exoplanets with ease!
Direct Imaging is only for special cases!
At the distance of the Kepler field, the Earth would occupy 190 billionths of a degree!
To resolve it, you would need a telescope at least 2km in diameter!
How much do we know about exoplanets?
Radius of the planet
Mass of the planet
Ideally we should use both to obtain the most accurate results, but both have limitations
Exomoons, exocomets and exoasteroids?
Tau Ceti F
My rendering of Tau Ceti F
What do we know about
Tau Ceti F?
* 1.35 AU orbit
* 6.6 Earth Masses
* 233K surface temp
* One of five planets in the system
It is considered one of the most habitable exoplanets at the moment, but is also unconfirmed due to the statistical method used.
This planet was once considered to be habitable, but further research rules out the possibility of it being terrestrial
Part of a quadruple star system
Discovered by citizen science
Most likely a gas giant planet, at least 20 times the mass of the Earth
Planet Hunters is a project that invites members of the public to analyse data - by using the wisdom of the crowd to refine results.
Each new exoplanet we find is more and more interesting than the last - new announcements are made every week!
The Habitability of Exoplanets
- Tends to be based on the requirements for carbon-based life
- Chief Measurement is the ESI or Earth Similarity Index
TAU CETI F
*based solely on radius, the property I have the greatest certainty about
Alien life may not even need the same conditions as us. Basing our predictions of habitability on the conditions humans need is flawed
Sagan coined the term 'carbon chauvinism' to represent the belief that life need exist using carbon-based chemistry (our own)
Carbon and silicon analogues
Same number of hydrogens, same tetrahedral shape, different central molecule - silicon-based life?
Chiral molecules have mirror
cannot be superimposed.
Different handed molecules may react differently, and so body chemistry could be based on a different form of a molecule we know
Considerations of surface gravity
- Planets with a higher density will have a higher surface gravity
- Therefore any life will need to be adapted to handle the additional support requirements
Very light creatures?
Crawling and sliding?
We don't even need to leave Earth to find life that could survive on exoplanets!
Why should we care about exoplanets?
Other bodies can cause perturbations in the data that cannot simply be the result of statistical noise.
These have been identified as exocomets and exomoons
The Search For Life
Checked against the official analysis, this figure has a 1% error!
- Actual value = 0.418 AU
Plotting the Habitable Zone
If we modify the equations we used to calculate the temperature of KOI-1787, we can work out the upper and lower limits for liquid water for a similar planet - this is purely indicative!
Liquid water * 0 to 100 degrees celsius
* 273 to 373K
KOI-1787 certainly has some chance of being habitable! Even though it lies on the edge of the habitable zone, there is some chance that life could exist under the surface, or extremophillic life on the surface.
The exoplanet is on the outer edge, which means it is almost too cold to support life. However, with some greenhouse effect, it may hold enough heat for liquid water to exist!
GAS GIANT MOONS?
What do we do if there is more than one planet in the system?
The well-defined sinusoidal graph we get from a single body which perturbs the radial velocity suddenly becomes a mix of many different waves...
Fourier Transform Doppler Spectroscopy
Using some advanced mathematics, it is possible to isolate the individual components of an exoplanetary radial velocity curve using the Fourier transform.
Mixed Waveform, could represent an exoplanetary system somewhere.
This is all the components of the waveform
sin(x) + sin(0.5x) + 3sin(0.8x) + cos(x)+ 0.2 + 1.1cos(0.6x+5)+0.1
This method allows us to measure the period and the radial velocity shift of the planet(s) in question
This method is at the cutting-edge of astrophysical research!
What would the RV curve look like for KOI-1787?
Chandra X-Ray image
This discovery yielded
3 planets at once!
The model excludes other atmospheric effects present - just the albedo and absorption
Colonising the Galaxy
Are we alone in the universe?
100 billion stars in the Milky Way Galaxy
It has been estimated that 20% of stars have planets around them
Therefore, there are at least 20 billion planets in our galaxy
From a probabilistic standpoint, even if the chance of developing life is tiny, there will still be some planets which do.
In our galaxy, there is a good chance of at least some planets having life!
In our universe, there are at least 1 septillion stars, or:
Even with a near-zero probability, it is near-certain that life will develop somewhere in the universe.
With exoplanet techniques, we can look for life in the universe, without leaving our neighbourhood.
These numbers are the lower end of estimates made!
Approximately equivalent to the number of grains of sand on Earth
For the advancement of science!
Exoplanets are part of the puzzle that is the universe, and understanding exoplanets brings us one step closer in answering the fundamental questions of how our solar system formed.
Example Transit Curve from Faulkes Telescope
Habitability of Parent Star
Planets that exist outside the Solar System
* could become possible with very-large scale interferometry
For noisier signals, it is possible (and easier) to use a Monte Carlo approach to determine the components by searching a sample space of exoplanets by optimisation
The Rossiter-McLaughlin Effect
Early last year, 768 confirmed exoplanets were released by NASA and Kepler, this firmly swings the balance in favour of transit methods.
KOI-1787 is not one of them sadly
Applying a basic ideal gas model
RMS Thermal velocity of gas (oxygen)
Reasonably valid for a sufficiently thick, non-interacting atmosphere
In real life, a system this complex would not exist and be difficult to probe.
Initially this exoplanet was missed in the data gathered, but a second pass revealed a faint point source
There could be other (undiscovered) methods to discover exoplanets!
Light from a rotating source will appear red on the edge moving away and blue on the edge coming towards us
If there is a transiting exoplanet, it will block a proportion of the red/blueshifted light when it passes over the limb of the sun
Blocking the star's redshifted component will make the star appear slightly blue-shifted, and in sequence red-shifted when it passes over the blue-shifted component
From the distortions in the transit we can infer both the direction and inclination of the orbit!
By finding the inclination of the orbit we can evaluate the sin(i) in radial velocity mass measurements, finding the accurate mass of a planet!
The problem with radial velocity
The plane of the orbit of the planet in most cases is not perfectly aligned with the plane of observation, so the radial velocity shift observed is not correct.
In research, the mass is often expressed as mass*sin(i), where i refers to the inclination of the orbit (to be found)
Amateur Observation of Exoplanets
Given a modest setup it is perfectly possible to image an exoplanetary transit and obtain a light curve.
Around magnitude 8, with a transit drop of around 0.03 magnitudes
Thomas Killestein - 2015
Joshua N. Winn - The Rossiter-McLaughlin effect for exoplanets
Hanno Rein's Open Exoplanet Catalogue
Czech Astronomical Society - Exoplanet Transit Database
Conveniently near the Dumbbell Nebula!
WASP-3 Data from Faulkes
Gaudi and Winn (2007)
There is around a 1 in 90 nonillion chance that a particle could exceed this velocity naturally.
This is not a clear detection of the exoplanetary transit, only the end shape is barely visible.