How can we search for exoplanets? With the global warming and sea pollution
constantly growing trends, the future of Earth might be at serious risk. In addiction,
the overpopulation of some regions might cause a global food crisis in nearly 30 years.
These are just a few reasons why some brilliant minds such as Jeff Bezos and Elon Musk have
started to plan a colonization of other planets like Mars, launching several rockets of their
futuristic companies Spacex and Blue origin. Unfortunately, the possibility of setting life on
Mars is far away from reality and the atmospheric conditions might be an enormous and insuperable
problem. However the two famous enterpreneurs are not alone in trying to find a new home for the
entire humanity: a full community of physicists, engineers and computer scientists is searching
for a new accommodation for human life. What are exoplanets? How can we reveal their
presence? Is it possible to find a place similar to our beautiful Earth? These and other great
questions will be discussed in this video. Stick with me and I’ll tell you everything in a moment.
It’s surely interesting to wonder about a possible future destination for everyone, but in order to
understand how this can be done we need to take a step back and try to look at our planet: Earth.
We all agree, except for some sceptics, that it has a spheric shape and that it
has two main rotating motions: one around the sun, which causes the existence of
four different seasons, and another around his axis, which is the source of day and night.
But why we use to call Earth as a planet? What is a planet?
The first revolution around the sun takes more time than the rotation around the axis,
but this is not something that Earth shares with other “colleagues” of the solar system:
for example, on Venus a year lasts less than a day. So what do all planets have in common?
They all complete a close motion around the Sun and they have a stable shape typical
of a certain mathematical equilibrium called hydrostatic equilibrium. Practically speaking,
this is the reason why all planets are spherical. Addictionally, to gain the definition of planet
a body must have a clean orbit without rocks or other astronomical detritus. As a matter of fact,
in 2006 Pluto was declassified to the rank of dwarf planet because Eris was discovered,
a big satellite that had nearly 27% of Pluto’s mass.
Finally, a planet must have less than thirteen times the mass of Jupiter,
the biggest one in the solar system. In parallel, an exoplanet is an astronomical
object orbiting around a star different from the sun, but with the same features we already listed.
Is it easy to detect the presence of an exoplanet? Definitely not. They’re not simple to find and
only a bit more than four thousand have been discovered after 1992, when the first two
planets were seen orbiting around a pulsar. Even if this number seems really big, it’s totally
incomparable with the amount of stars discovered, which is almost 500000 only in the milky way.
Stars are way easier to be discovered and catalogued as they shine on their own. On
the contrary, planets happen just to radiate some tenuous infrared rays,
that are surely not capable of reaching our detectors on earth. As a matter of fact,
just less than 30 exoplanets have been detected by this technique called “direct imaging”.
Given the fact that it’s not possible to measure their emission, how can we “see” a planet?
Through indirect methods. Physicists love to study little changes in the radiation of the major star
that are capable of revealing the presence of one or more planets.
Let’s investigate the main techniques that are commonly used in astronomy
for searching exoplanets. The first one deals with radial velocity.
A planet has certainly a consistent amount of mass that weakly attracts the star,
which has some slight displacements due to its presence. More precisely, both the star and the
exoplanet are orbiting around the center of mass, a virtual point on the line that connects the two
bodies, which is located near the heaviest one. This movements of the star can be well detected
through a very common technique: redshift. Have you ever heard anything about this?
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Redshift is probably one of the most utilized techniques in astronomical observations. It is
based on the “doppler effect”. Don’t be too scared by this strange name, it’s not so difficult to
understand. This effect describes why the alarm of an ambulance seems more and more acute when it’s
getting closer, and fainter while it’s going away. When a sound source is moving towards the observer
its frequency gets bigger, whereas while it’s going away the frequency progressively decreases.
Since sound is physically described as a wave, electromagnetic radiation
can undergo the same effect. According to that, when a star is getting closer
its light will appear more blue (blue-shifted). On the other hand, light will shift towards red
when the star is moving away from earth. Actually, it is not light that shifts in a
direction or another, but the spectrum lines of absorption of the star. What are they? Shortly,
the light of every emitting body like a star have some missing lines in the spectrum,
which is like a catalogue of the different colours contained in the emission.
The absence of some wavelengths is caused by the absorption that happens in the star atmosphere,
which is composed by different types of atoms. Each atom has its own wavelength of absorption
and when the star moves radially these missing lines shift towards red or blue.
Thanks to this effect scientists have been able to get a good amount of exoplanets, nearly
456. By using the radial velocity technique, physicists can calculate the mass of the planet.
The second method is known as transit photometry. Even if it seems really complicate,
it’s very intuitive and easily comprehensible. The whole procedure revolves around this hypothesis:
the system of star and planet must be perpendicular to the observer.
Given this assumption, when the planet gets between the star and the Earth a steady drop
in the light curve (the diagram that shows the intensity of light in time) can be observed.
Therefore, the probability of finding a system orthogonal to Earth is quite small.
Consequently, just a few 232 planets have been observed with this method,
nearly 5% of all the discovered planets. Although it’s not the most used one,
transit photometry is able to give some precious data. As opposed to the previous one,
this procedure gives information about the radius of the exoplanet.
Going on with a list of the most important techniques for exoplanet detection,
astrometry is certainly a well-known one. It can be seen as a different variation of the radial
velocity technique. The principle is practically the same, astronomers search for little wobbles
in the position of a star. Therefore, they don’t look for redshift in the star spectrum,
but just focus on the slight changes of the position in the sky. However, this method is
very difficult to apply, as the detectors must be really precise in order to see all the little
variations of the coordinates of the star. A fourth method is called gravitational
microlensing and it’s probably the one that needs more explanations.
In 1915 Albert Einstein came up with General Relativity, one of the most amazing theories
in the history of Physics. It states that everything exists in a precise space and time,
also known as spacetime continuum. This last one can be altered or “distorted” by the presence of
a massive object as a star or a black hole. What does it mean? Imagine the space-time
continuum as a big cloth sheet. When a sphere with a mass (the Sun) is put on the centre, the sheet
curves. If another lighter object (the Earth) tries to go on a straight line through the sheet,
it starts to move circularly around the first one as a result of the curvature of the sheet.
In some cases, the mass of the object is so strong that the same effect can be observed with light.
Try to replace the Earth of the last example with a visible ray of light,
in the proximity of the heavier object, which can be a massive red-giant star or even a black hole,
the beam starts to deviate its trajectory due to the deformation of spacetime continuum. As
a consequence of that, an external observer that detects these photons, might think that
the light source is in a different position from the original one. This prediction made by
Einstein’s theory was validated by an observation made by Arthur Eddington in 1919, the first
proof of the accuracy of General Relativity. However, something very strange and puzzling
might happen if you look at some images of the deep space produced by the Hubble telescope,
one of the biggest detectors in space. If you pay attention you’ll find the same objects in
different positions. How is that possible? Is Hubble broken? Or there are some unknown
mysterious effects that cause this repetition of identical images? Definitely not, Hubble works
extremely well and we are perfectly capable of describing this phenomenon. As a matter of fact,
if we are trying to see a distant star and a massive object as a black hole happens
to be between Earth and the observed object, the light of the star will split in two or more beams
due to the deformation of space time continuum. It’s exactly the same effect illustrated before
with an addictive hypothesis about the alignment of the star and the massive object. The two
images of the same star will be very far from each other if the black hole has a big mass.
But how this can be linked to exoplanets? Well, it’s very simple. On smaller scales,
the same effect happens with exoplanets. Their gravitational force has a little influence on
the trajectory of starlight. Therefore, the mass of the exoplanet is not sufficient to separate two
different images detectable by our devices. Fortunately, physicists are able to use
maths to describe the interaction of these two indistinguishable pictures.
By observing the “composed image” of the star for a period of time, scientists can say if the small
changes in intensity and shape can be caused by an exoplanet gravitational microlensing.
Although it has some difficult theories laying at the very foundation, this method is really useful
and has brought to the discovery of 19 exoplanets. We have seen the most important methods for
detecting this type of object, but how can we know if a planet might be appropriate for a future
colonization? It might be really disappointing, but it’s almost certain that we are very far from
colonizing another planet, because the distance of the candidates is so enormous that everyone
would die in the journey. As a consequence of that, scientists are more interested
in finding planets that might host life already, even if in the form of plants or bacteria.
In order to select some planets that can have experienced the birth of living organisms,
physicists have coined the term of “planetary habitability”.
A long list of requirements is needed to be a habitable planet.
Firstly, the star around which the body orbits must have a low temperature in order to live
for a long time that allows the evolution of organisms (as a matter of fact hotter
stars live less than colder ones). Therefore, it must be enough hot in order to warm the planet,
it’s surely quite impossible to develop life at extremely low temperatures.
Moreover the star must be really stable as wide changes in luminosity are harmful to organisms.
Secondly, the planet must be made of rock and should have a proper distance from the star.
Furthermore, another important feature of a star that might host a life-bearing planet
is metallicity. This last one is the measure of heavier elements also known as metals. It has been
proved that if a star has a high metallicity is easier to find habitable exoplanets.
To sum up, different methods have been developed in order to find new places in which we can live,
nevertheless reaching these distant locations is not already possible and we don’t have the
certainty that we can surely survive in these futuristic destinations.
Even if more advanced techniques are ready to be used to better catalogue habitable planets,
don’t start packing your bags, Earth will continue to be our home for a very long time
and we should probably try to took care of it while physicist search for a new welcoming home.
"This video ends here! Thanks for watching everyone! Did you find this topic exciting?
Are you amazed by the existence of exoplanets? What is your favourite method of detection?
Is there anything more you want to hear about habitability or space colonization? Let me know in
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