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A Course on Astronomy and Astrophysics, Iucaa Transcript A COURSE ON ASTRONOMY AND ASTROPHYSICS, IUCAA MODULE 7 SOLAR SYSTEM ASTRONOMY Chapter 7.6 Extra-solar planets Sujan Sengupta, IIA [00:00:10] Hello, everybody. I'm going to tell you about the worlds beyond our own world that is our solar system. So these are planets which are outside the solar system, the planets that are orbiting the other stars very similar to our own star the Sun. Now the question is, why do we need to study for other worlds, why we need to know about extrasolar planets or simply which are called as exoplanet; because a scientific study of this subject leads to a possible answer of the eternal questions or the eternal query of mankind; that is there anybody out there or are we alone? Section 7.6.1 Introduction [00:01:04] Now study of this extrasolar planets or the planets outside the solar system; it enable us to understand on the planet formation mechanism whether; we know very well how the solar system has been created some 4.5 billion years ago, but we do not know whether that is the general case. So this is a nice area to understand whether our present knowledge on the formation of planet, that is the solar planet is a particular case or it is a general case. [00:01:48] Is the solar system unique or is it common in our Galaxy? Our solar system has a very specific configuration. That is all the small rocky planets are very near to the star, that is the Sun and all the gaseous giant planets are outside. They are called the gas planet or gas giants. So the question is that is all the extrasolar planet or planetary systems are similar to the solar system, that is there also the giant planets are outside outer planets and the rocky planets are the inside planets. Then the next come are there planets similar to the Earth? [00:02:35] The planet Earth, you know, it has a very special characteristic. It has rocky surface. It has a thick atmosphere. It has a very massive satellite natural satellite or Moon. It has a magnetic field. It has several geological special characteristic. So, is there a Transcript anywhere similar planet like Earth? There is no similar planet like Earth in our solar system. But what about the planetary system outside the solar system around the other stars? Can we find out another Earth because that will answer the question whether life is possible in another planet or not. So naturally then another related question or more fundamental question arrived. [00:03:26] What is the origin of life in our planet? Has life originated in the planet itself or it has been imported from outside? Is it very common in our Galaxy or in our Universe or is it a special case for just one planet? That is our Earth. [00:03:50] Now there are a large number. In our own Galaxy there are 200 billion of stars. So this has given the imagination of human being that there could be intelligent life everywhere. There could be life everywhere in the Galaxy. But if that is the case then where are they? Why don't we get any signature? Why don't we get any activities observed in the Solar neighborhood that tells that everywhere there are planet intelligent life or even primitive life. So that is a question asked by Enrico Fermi the famous physicist. Where is everybody? So what is the problem? [00:04:35] Now we know one planet very well. That is Earth and for thousands of years, we know that this Earth or our world that is not enough and we have a we are a member of a family of planet that is called the solar system planet. So for more than 80 years, we knew that there are nine planets, but in 2006 at International astronomical Union, the definition of solar system was reexamined and it was decided that Pluto is not a planet. It is not very similar to the other planets in the solar system. So it is now considered as a dwarf planet. So in the solar system, we have only eight planets but this definition the change of the status of Pluto was driven by a large number of exoplanet outside the solar system [00:05:42] The first planet that has been discovered by Professor Michel Mayor of Geneva Observatory and this is around a star is a very similar to our Sun is called 'Pegasi 51' and the planet is designated that 'Pegasi 51 b'. So here is the location of the planet, you can't see the planet here. Now, once this planet was the first planet was confirmed around this solar type of system; so there was a search huge search initiated to find out other planets; and everybody all astronomers using all type of telescope throughout the world started hunting of planets and hundred of planets have been discovered each week each month each year the number of exoplanets started increasing. [00:06:46] So this search was done basic mainly by the ground-based telescope that ranges from 1 meter diameter to even 6 meter to 10 meter diameter telescope, but because of the signal or because of the limitation in the ground based technology, the search was limited only within 300 light-years from the Sun. That is within the area of 300 light-year in the Transcript Sun. But then in 2009, a space telescope named after Johannes Kepler was launched and that is called Kepler telescope. [00:07:30] So NASA launched this Kepler telescope and after this Kepler Telescope was launched the European Space Organization also launched another Space Telescope called CoRoT with the same purpose that is to detect planets outside the solar system. Now this space telescope that has extended the search area by 10 times and with Kepler we can search for exoplanets upto 3,000 light years, but in a narrow region of our Galaxy. [00:08:08] Now once before the Kepler was launched or before the exoplanet was detected we knew about the solar planets, we knew about the temperature. We know that Venus is the hottest planet in the solar system. Although Mercury was the nearest planet but since Mercury didn't have an doesn't have any atmosphere and because of the presence of a thick atmosphere, Venus is the hottest planet, which is a very similar size to earth, whereas Mercury is a subterrestrial size. It is much smaller than the planet Earth. Similarly, Earth has a temperature which is very appropriate for the water to stay in the liquid form. Then Jupiter is a giant planet several times larger than our planet Earth, is a gas planet and then Saturn, Neptune and Uranus and Neptune and the coolest planet in our solar system is Uranus. Although Neptune is the farthest but this is all because of the atmospheric effect. Now once Kepler was launched and we found several kinds of planets our it was the properties of these planets the temperature the size all went beyond our imagination. Now, we find planet which are much larger than almost double of Jupiter whose temperature is much higher even 2500 degree to 3000 degrees Celsius where iron even melts and we get different size of planets. The planets which are in the transition state of gas planets and rocky planets, which are smaller than Neptune but bigger than Earth, so all kind of planet we have discovered which was beyond our imagination even few years before. Now if you consider the size the Kepler has given such type of size that we define the Jovian planet or the Jupiter of Jupiter size of planet, which whose radius ranges from 6 to 20 times that of the Earth radius then come the Neptunian which ranges from 2 to 6 Earth radius. [00:10:38] Then come a particular size of a planet which doesn't exist in our solar system and these are called mini Neptunes. So probably this planets they have a rocky core but it has an extended atmosphere. So these are in the transition of a rocky planet and a gaseous planet. Such type of planet or the mini Neptunian doesn't exist in our solar system. Then there are super Earth which are larger than the Earth in size, but they are very similar to Earth. So they are radius ranges from 1.25 to almost double of the Earth radius and their mass is only few times that of Earth mass. These are rocky planets, and these are called super Earth and then comes the Earth and then come the sub Earth sub-terrestrial like our Mercury. So you get a various range of planets by temperature means how hot they are and by their size. Now if you classify all the solar planets by size and by the temperature you can get only seven type of objects in our solar system including the natural satellites that those are the moons of the various planets, you know, the largest moon in the solar system is Ganymede, which is a natural satellite of Jupiter. Our Moon is the fifth in its size. Now Transcript the other planets are there the only planet in the temperature range where water may water exist in liquid state is only Earth and some time Mars but when we included we discover all this planets by Kepler by CoRoT by the ground-based several ground-based planets, we found out 17 kind of planets in our Galaxy itself.
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