Table of Contents
BEST IELTS Academic Reading Test 467
IELTS ACADEMIC READING TEST 467 – PASSAGE – 2
IELTS ACADEMIC READING TEST – 467
READING PASSAGE – 2
EXTRASOLAR PLANETS
Searching for extrasolar planets can be quite exciting and the questions, potentials and possibilities they provide are unlimited. But finding extrasolar planets is a very tricky and difficult process. While nearly 2000 extrasolar planets have been discovered, astronomers must use several methods not only to detect them, but also to confirm their existence.
As it stands, planets, when compared to their host stars are extremely faint in terms of brightness. At visible wavelengths, planets are usually one millionth the brightness of their companion stars. Planets are extremely difficult to detect because of this. Furthermore, the parent star produces a glare which washes out the brightness of the planet. This makes it even more difficult and astronomers must block the light from the parent star in order to reduce the glare, but at the same time make sure the light from the planet is detectable. Attempting this is a major technical challenge.
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So far, all extrasolar planets that have been directly imaged are both large in size and widely separated from their parent star. In terms of size, these planets are exceedingly more massive than Jupiter. In terms of distance, many are farther away from their parent star than Pluto is from our own Sun. Another factor which has helped in directly imaging these planets is that most of them are very hot. Because they are so hot, they emit intense infrared radiation.
Thus, the imaging done has been on the infrared scale as this would make the planet brightest at this wavelength. Astronomers also know that during the gas accretion phase of planet formation, in which large planets form from gases thrown off by its parent star, the contrast in brightness in terms of infrared radiation is highest in the newly forming planet than the star itself. Hence, most surveys for new planets include this approach.
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While specialised direct imaging instruments are being used, vast majority of extrasolar planets have only been detected by using indirect methods. There are several methods of indirect detection that astronomers find useful. One method is the transit method. This is considered and used in case a planet should cross or transit in front of its parent star’s disk. As the planet does this, the observed brightness of the star drops slightly. The amount of brightness of the star that is reduced depends on the size of the planet.
The one drawback of this method is that this has produced a large number of false positives. This makes the need for confirmation from another method necessary. The transit method does reveal the radius of the planet and it also has the benefit of allowing planet’s atmosphere to be analysed through spectroscopy. Since this method requires that part of the planet’s orbit intersect the line of sight between the host star and the Earth, the probability of detecting a randomly oriented planet to transit the star is negligible.
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Another method is the Doppler Method. When a planet orbits its star, the star itself moves in its own small orbit around the system’s centre of mass. By studying the star’s radial velocity, which is the star’s speed towards or away from Earth, astronomers can detect the displacements in the star’s spectral lines due to the Doppler Effect. This method is very useful because it can be applied to stars with a variety of different traits. The only disadvantage of this method is that the true mass of the planet cannot be determined.
Transit timing variation is another method also used at times. This is used when multiple planets are present. Each planet will slightly disrupt the other’s orbits. Any small variation in the times of transit for one planet can indicate the presence of others. This method is useful for determining and confirming the existence of other planets in a system. Transit duration variation is used when a planet is orbiting multiple stars or when the planet has moons. Here, the transit time can vary greatly per transit. While this method has not discovered new planets, it has been used to successfully confirm many planets that are found with systems of two or more stars.
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At times, the gravitational field of a star can act as a lens; this is referred to as microlensing or gravitational microlensing. What happens is that the light of a distant background star is magnified. Any planets orbiting the lensing star can cause a detectable disruption in the magnification as it would appear over time. Microlensing is the most sensitive method of detecting any planets that lie within 1-10 astronomical units (1 AU 93,000,000 miles) away from stars similar to our Sun.
Another method is astrometry. Scientists precisely measure a star’s position and then observe the changes in that position over time. The motion of a star due to the gravitational influence of a planet is generally observable. So far, this method is not very reliable because the gravitational influence of the star is negligible because the motion produced is extremely small. This method has detected some possible extrasolar planets, but is not useful at all to investigate the properties of a planet found in another method.
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Questions 15-19
Choose the correct letter, A, B, C or D.
15. One reason why it is difficult to spot a planet orbiting its host star is that
A. the host star is often less bright than neighbouring stars
B. it lies outside the visible wavelength
C. it is significantly smaller in mass than the star
D. glare of the host star eclipses the brightness of the planet
16. Which of the following is not a factor associated with extrasolar planets that have been directly imaged?
A. They are much larger than Jupiter in terms of size.
B. They are in close proximity to their parent star.
C. They are extremely hot and emit infrared radiation.
D. Most are farther away from their stars than Pluto is from the sun.
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17. The transit method works on the principle that
A. the observed brightness of a planet drops when it crosses its parent star’s disk
B. the brightness of a planet increases when it crosses the disk of its parent star
C. the observed brightness of the parent star decreases based on the size of a planet crossing its path
D. the method produces false positives
18. A star’s radial velocity is important for using the Doppler Method because
A. it detects a star’s speed as it moves towards or away from the Earth
B. displacements in a star’s spectral lines can be detected should a planet orbit it
C. a star will orbit around the star system’s centre of mass
D. displacements in a star’s visual lines caused by a planet’s orbits cannot be detected
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19. What is the basis for microlensing?
A. The gravitational field of a star acts as a lens.
B. Light from a star in the background is intensified which a planet can disrupt.
C. Light from a distant background star is magnified by the star in question.
D. It has a greater sensitivity for detecting planets that are 1-10 astronomical units from a star.
Questions 20-28
Look at the following descriptions (Questions 20-28) and the list of terms in the box below.
Match each description with the correct term A-F. Choose the appropriate letter A-F for questions 20-28.
List of Terms
A. Transit Method
B. Doppler Method
C. Transit Time Variation
D. Transit Duration Variation
E. Gravitational Microlensing
F. Astrometry
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20. Suitable for planetary systems with more than one star
21. Cannot determine planetary mass
22. Increases intensity of light coming from a distant star
23. Indicates presence of other planets in the system 24. Could be a false indicator
25. No planet discovered so far with this method
26. Very difficult to find a new planet using this method
27. Depends on accurate measurement of stellar positions
28. Suitable mainly for planets that are relatively less distant from their star
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ANSWERS
15. D
16. B
17. C
18. B
19. B
20. D
21. B
22. E
23. C
24. A
25. D
26. A
27. F
28. E
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