How to find exoplanet? Find new planets.
Staring at the sky in awe
is one of the few quint essential human experiences. Gazing into the vast
emptiness of space wondering where our place in the universe stands. For
millennia the mystery of these celestial bodies captivated the human mind,
seeding religions and mythical stories.
We couldn’t ignore
the moon above us, influencing our tides, or the sun, our benevolent provider,
burning in the blue sky. At night, individual stars melded together into a
boundless blanket of glittering light, until we began to notice patterns in
their chaos, guiding us as we explored our world. These star maps helped us
make a monumental discovery, some stars in the sky moved differently to others.
Wandering through the
night sky, planets like ours orbiting our sun. Mercury, Venus, Mars, Jupiter
and Saturn, all visible to the naked eye were discovered this way by ancient
baby lonian astronomers over 4 thousand years ago. Later Galileo improved the
telescope, allowing him to observe even fainter points in the sky, invisible to
the naked eye. He discovered Jupiter's moons.
Ganymede, Call is to, Io and Europa and Saturn’s moons soon
followed mostly discovered by Cassini. Uranus & Neptune had been
observed numerous times through telescopes, but astronomers continually mistook
them them for stationary stars or comets. It was only when astronomers noticed
the circular orbit of Uranus that it was accepted as a planet.
Then as it’s orbit was further examined astronomers noticed
deviations, that could only be caused by a nearby planet, and thus Neptune, our
solar systems final planet, was discovered *on screen joke - Sorry Pluto* It
tooks thousands of years to just discover the planets in our solar system, but
in the last 30 years we have discovered over 3717planets orbiting stars so far
away that we will never be able to visit them with current technology.
Perhaps the most fascinating planetary system we have
discovered is that of Trappist-1, an ultra-cool dwarf star located 40
lightyears away from our Sun, which has not only 1 temperate terrestrial
planet, but 7. Each of the planets are rocky and between the size of Earth and
Mars.
The star is small and cold compared to ours, but the planets
orbit so closely to the star, closer than mercury orbits the Sun, that they all
have the possibility of containing liquid water. This is a lot of information
about planets that we can’t even see directly. So how do present day
astronomers find these, relatively speaking, tiny objects floating in space?
The first planets
discovered orbiting this star were discovered by a Belgian team using the
Trappist-south telescope located high the Chilean mountains. The team had their
telescope trained on the dwarf star waiting for a dimming of it’s light, a tell
tale sign of a planet in orbit passing in front of the star, and thus blocking
some of it’s light. But how have we discovered so much about this system,
simply by a dimming of light in its star?
If we calculate the distance to the star, using the parallax
method, we can discover a lot more about the planets passing in front of it.
You can see how the parallax method works right now by holding your thumb in
front of you with one eye open.
Now close that eye
and open the other. Did you notice it appeared to move relative to the
background, this is called parallax and we can use it to measure the distance to
a star, but instead of closing our eyes and staring at the sky, we take
measurements of the planets position from opposite sides of our orbit around
the sun.
We know the distance between these two points, and we can
measure the change in the stars position relative to distant stars behind it,
giving us the parallax angle, thus we have all the information we need to
calculate the distance to the star using some simply trigonometry. Now that we
know the distance to the star, we can calculate its mass and size by observing
its apparent brightness and temperature observed from earth.
We have models for how stars form, with massive stars being
hotter and brighter than smaller stars, and so by measuring these values we can
get a lot of information about the star. That’s how we know Trappist-1 is an
ultra-coold warf star. Now that we know more about the star, we discover what
we are really interested in.
The planets size,
mass, and whether it resides in the goldilocks zone where humans could survive.
To do that we need to know how far away the planet is from the star. We do this
using Kepler’s Third Law which looks like this. It tells us the orbital period
for planet, that’s how long it takes to orbit it’s star. Here G is the Newton’s
Gravitational Constant, capital M is the Star’s mass, lowercase m is the
planet’s mass, and a is the orbital radius.
We can ignore the planet’s mass for now, as it’s miniscule
compared to the star’s. If we do this for Earth, it introduces an error of just
0.001%. The period is measured by seeing the time between transits, and so we
have all the variables we need to calculate the orbital radius.
Now that we know how far away the planet is, figuring out
it’s diameter from the transit dimming is simple. The next bit of information
gathering requires a bit of a perspective shift. We are used to the idea that
planets orbit a star, but it would be more accurate to say that the planet and
star orbit the centre of mass of the star-planet system, which is not at the
centre of the star.
This means the planet’s mass is actually impacting the orbit
of the star around this point. The larger the mass of the planet the larger the
deviation. We can measure the magnitude of this deviation by detecting the
doppler shift in the stars light frequency.
As the star moves towards us the light becomes slightly more
blue, and when the star moves away from us it becomes slightly more red. Using
the law of conservation of momentum we can easily calculate the mass of the
planet. We know the velocity of the planet from orbital radius and period
measurements, the mass of the Star from our distance and luminosity measurements,
and the velocity of the sun is measured from these doppler shifts in the light
frequency.
Leaving us with all the information we need to calculate the
mass of the planet. Using these tools Astronomers have inferred quite a lot of
information about these planets. The 7 planets orbit so closely to their star that
a year ranges from just 1.5 days for the innermost planet to 20 days for the
outermost planet.
They are so close
packed that the planets will at times appear in each others sky, larger than
our moon appears in ours.
Maybe the coolest part of the entire thing, is the planetary
system is named after the beer the scientist used to toast their discovery. But
it isn’t all good news, because they orbit so close, many of the planets will
be tidally locked, like our moon is to the earth, meaning one side of the
planet will constantly face the sun.
Which may restrict life to the terminator, the narrow band
of perpetual twilight. On top of this, orbiting this closely to a dwarf star
exposes it to radiation from solar flares, which can strip it’s atmosphere away,
especially if it is not protected by a strong magnetic field like earth.
For now, we don’t have a lot of information on these
planets, beyond their position, period and mass. But we’ll know more soon, as
the James Webb Telescope will be able to give us more information about the
atmosphere on these planets by observing the light that shines through their
atmosphere during transit. By examining the spectrum of light coming through
the atmosphere we can discover what gases are in its atmosphere.
Here we have the spectroscopy analysis of Venus, Earth and
Mars. All have the fingerprints of carbon dioxide in their atmosphere, while
only Earth has the traits of an atmosphere rich in ozone and water. If we see
results like these coming from any of the Trappist-1 planets, a world wide
toast with Belgian beers may be in order.
Learning how astronomers discover our universe is
fascinating, but it’s unlikely you will ever have the tools to discover your own
worlds, but you can explore the world you stand on and its night time sky using
fairly inexpensive equipment to produce incredible photos like this. You can
learn how to do this using this course on Skill share, which will teach you how
to find the best times of month plan your shoot, how to find dark areas near
your location, how to identify and plan the stars you will photograph and how
to set up your equipment.
This is probably my favourite course I have taken on Skill share
to date, and I bet you will like it too. These days you can teach yourself
pretty much any skill online and Skill share is a fantastic place to do it.
With professional and understandable classes, that follow a clear learning
curve, you can dive in and start learning how to do the work you love
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