EXOPLANETS

OUT THERE

We live in a thin spherical shell a few kilometers thick around the surface of the Earth . Even if we explore the deepest abysses in the ocean or climb the most impervious peaks of Himalaya , our reign has ridiculous dimensions .Our little world is also populated by ghosts : we think that everything we perceive is real but as we stray a little from the quite place where we live , everything we know changes . If we want to explore the secrets of the universe we must renounce these certainties preparing for the unthinkable . The research of exoplanets represents one of the last frontiers of modern science ….follow me on this wonderful  journey .

THE ORIGINS  

With the term exoplanets or extra-solar planets  we mean all the planets  which orbit around a star different from the Sun .

Their existence had already  been speculated  by Giordano Bruno in the 16th century and  by Christiaan Huygens  and Newton in the following  centuries  . The first to indirectly observe an exoplanet was captain  W.S. Jacob,   who was in charge of the astronomical  observatory  of the India Company in Madras .

In 1855 he took over some anomalies in the orbit of the binary star 70 ophiuchi, which made him hypothesize the presence of a planet. Nevertheless,   that remained a hypothesis difficult to prove for the instruments of the   time,   so  it took almost 150 years ore till the discovery of the first exoplanet  .

On the 5^th of October 1995 a group of researchers of the Haute-Provence Observatory in France announced the discovery of an exoplanet  with a mass similar to that of Jupiter  orbiting around star 51 Pegasi .

The particularity of this planet is the distance from its star, only 7 million kilometers ( it is eight times closer than Mercury to the Sun ), despite having the same mass as Jupiter,  it is larger because of  very high temperatures, since hot gases take up  more space than cold ones but they are less dense .

To date we have discovered 4, 208 exoplanets  in 3, 119 planetary systems and there are more than 200 pending confirmations.

HOW MANY TYPES OF EXOPLANETS ARE THERE?

There are 4 types of exoplanets:

·        Neptune-like:  they are large gaseous worlds  ( such as Jupiter, Saturn, Uranus and Neptune )

·       

        Hot  Jupiters: they orbit very close to their star (it is thought that these planets  formed far from the star and only later they neared it with a “ planetary migration’’), with high surface temperatures  and many of them have a tail just like a shooting star due to the evaporation at  the atmosphere and surface level

·       Super Earths:      larger than the Earth , they are made primarily of rocky and icy materials  and their mass is between 2 or 10 times bigger than the Earth’s

·        

      Earth analogs:  they are similar to the Earth  ( as for size , composition  and distance from the star )

HOW DO WE FIND EXOPLANETS ?

There are several methods to detect an exoplanet  and generally they are divided in direct and indirect methods . The first ones consist in the direct observation of the planet, the second ones consist in the observation of the effects produced by the planet .

Researchers use mainly five methods :

Direct imaging : as the name says,  it is a direct method. Thanks to a coronagraph we block the glare of the star in order to search the infrared radiation  reflected by planets  . We search in the  infrared band because stars emit radiations mainly as visible light,  so if a star is one billion times brighter than a planet in visible radiation, in the infrared band it is 1 million times  brighter (there are huge numbers thought,  but in this way we increase exponentially  the  probabilities  to detect a planet). This method has many limitations, first of all it is almost impossible to detect a planet if the star is much brighter than it, then the planet has to orbit quite far and it has to have big dimensions.                                                                               

Astrometry: it consists of measuring the precise position of the star (even comparing it  to other stars) in order to observe whether its orbit is altered by planets . Actually,  we don’t have to think that,   for example,  the Sun revolves around its center of gravity, it revolves -  like all the other bodies of the star system -  around the center of gravity of the Solar system . Sometimes the center of gravity of the system can be outside the same star. Hence,  especially when the star isn’t  too massive and has a complex planetary system the influence on the orbit becomes evident .

                                                                      

  

Radial velocity or Doppler method:  since the star orbits around the center of gravity of the star system,  it has a certain radial velocity. The presence of other celestial bodies causes a variation of the radial velocity , researchers can notice these variations studying  the electromagnetic spectrum of the star .

                                                                               

Gravitational microlensing:  a planet,  like all the other bodies in the universe,  has a certain mass that bends the space. If some photons emitted by  the star pass near a planet,  its mass will curve their trajectory focusing the rays of light like a lens,  so studying these alteration of the light emitted by the star we can detect the presence of a planet .

                                                                                  

Transit: this is the most effective and promising  method by which we have discovered most of the exoplanets we know.  When an  exoplanet  transits between the Earth and its star, it causes a decrease of its brightness,  so  the star is eclipsed for a small percentage of its surface, detecting this small drop we can take over the  presence of a planet . A limitation of this technique is that the orbital plane of the star system must be inclined toward the Earth  otherwise we couldn’t see the eclipse .

                                                                   

                                                   

WHY DO WE SEARCH FOR EXOPLANETS ?

We search for exoplanets for many reasons. One of the main ones is to find out if there is life besides us in this infinite sea and to find potentially habitable planets   where we might  move once  the resources of the Earth run out. We are mainly interested in planets   located in the habitable or Goldilocks   zone.  The habitable zone is an area where we can find liquid water.  This concept is based on the favorable conditions for life as we know it on the Earth , where liquid water is essential for all known life forms , so planets with liquid  water  are considered between the most favorable to  host extraterrestrial life .

To date there are different candidates, among which Trappist 1 d  that  orbits around a red dwarf with other six exoplanets. Many exoplanets orbit around red dwarves , these type of star is very common in fact it is thought that they make up sixty percent of the stars in the Milky Way .

Unfortunately the real problem is: how can we get so far ?

Call it a day,  I really hope I succeeded in conveying some of my enthusiam for this intriguing scientific topic with my post.  Stay tuned for part two !

GIORDANO C.