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Astronomy Exoplanets

Methods used to detect exoplanets

Introduction

Exoplanets or also known as extrasolar planets, are planets that revolve around stars outside of our solar system. They are detected using various methods, including radial velocity, transit photometry, and direct imaging. The study of exoplanets is a rapidly growing field and has provided new insights into the diversity of planetary systems and the potential for life elsewhere in the universe.

Exoplanets compared to Earth(Credit: Caltech Division of Geological and Planetary Sciences)

Types of Exoplanets

There are several types of exoplanets based on their characteristics, such as mass, size, composition, and distance from their host star. Some examples include:

  • Hot Jupiters are massive exoplanets very close to their host star, resulting in extremely high temperatures.
  • Super-Earths: exoplanets with masses between Earth and Neptune, which are thought to have a higher likelihood of being rocky.
  • Mini-Neptunes: these are exoplanets smaller than Jupiter but larger than Earth, with thick hydrogen-helium atmospheres.
  • Terrestrial planets: exoplanets that are similar in size and composition to Earth.
  • Rogue planets are exoplanets that are not orbiting a star but floating alone in the galaxy.

There is also a classification based on their distance from their host star:

  • Inner planets: exoplanets that orbit very close to their host star.
  • Outer planets: exoplanets that orbit far away from their host star.

It is worth noting that this classification is not strict, and some exoplanets can have more than one category characteristic.

Variety is a major theme in exoplanet discoveries over the past quarter century, as shown in this illustration. Most have been discovered by the “transit” method – watching for the tiniest of shadows as a planet crosses the face of its star. Credit: NASA/JPL-Caltech

Why do we need to find exoplanets?

There are several reasons why scientists are interested in finding exoplanets:

  • Understanding planetary formation and evolution: By studying exoplanets, scientists can learn more about how planets form and evolve, which can provide insights into the processes that occur in our solar system.
  • Search for life: The discovery of exoplanets in the “habitable zone” of their host star, where temperatures are just right for liquid water to exist, has increased the potential to discover life beyond Earth.
  • Characterizing other planetary systems: The study of exoplanets can provide information about the diversity of other planetary systems and the likelihood of finding other planets similar to Earth.
  • Advancement in technology: The search for exoplanets has pushed current technology limits and led to the development of new instruments and techniques for observing the universe.
  • Understanding the universe: The discovery of exoplanets is a part of understanding the universe and our place in it. It is a way to explore the possibilities of what exists in the vastness of space and how ordinary conditions led to the emergence of life on Earth.
  • It also has implications in astrobiology and astrochemistry. It has a huge range of potential applications in future space exploration, including searching for habitable worlds and identifying planets that may be suitable for future colonization.
During 9.6 years in orbit, the Kepler space telescope observed more than half a million stars and discovered more than 2,600 planets. Credit: NASA

What are the methods used to find exoplanets?

There are several methods used to detect exoplanets, including:

  • Radial velocity method: This method measures the wobbling motion of the host star caused by the gravitational pull of an orbiting planet. It can detect exoplanets that are massive and close to their host star.
  • Transit photometry: This method looks for a slight dip in the brightness of a star caused by an exoplanet passing in front of it. This method is best for detecting exoplanets that are close to their host star and aligned to pass in front of the star as viewed from Earth.
  • Direct imaging: This method takes pictures of exoplanets directly, typically by blocking out the host star’s light. This method is best for detecting exoplanets far from their host star and having a significant separation from the host star.
  • Microlensing: This method detect exoplanets based on the gravitational lensing effect; it happens when a star or a planet passes in front of its host star. This method is best for detecting exoplanets far from their host star and having a slight separation from the host star.
  • Pulsar timing: This method detect exoplanets based on the timing of the pulsations of a pulsar, a highly magnetized neutron star. This method is best for detecting exoplanets far from their host star and having a slight separation from the host star.

Each method has its advantages as well as limitations, and scientists often use a combination of methods to confirm the existence of an exoplanet and to determine its characteristics.

Radial velocity method

The radial velocity method, also known as Doppler method, is used to detect exoplanets by measuring the wobbling motion of the host star caused by the gravitational pull of a revolving planet. A star’s radial velocity is the star’s motion along the line of sight, which changes as the star moves towards or away from the observer.

The radial velocity of a star is calculated by observing the Doppler shift of the star’s spectral lines, which are caused by the star’s motion towards or away from the observer. The Doppler shift is a shift in the wavelength of light caused by motion. As a star moves towards the observer, the wavelength of light from the star becomes shorter, and as it moves away, the wavelength becomes longer. By measuring the Doppler shift of the star’s spectral lines, scientists can determine the star’s radial velocity.

When a planet revolves around a star, it causes the star to wobble slightly due to the planet’s gravitational pull. This wobbling motion causes the star’s radial velocity to change periodically, and the period of this change is equal to the orbital period of the planet. By measuring the radial velocity of a star over time and looking for periodic changes, scientists can detect the presence of an exoplanet.

This method is sensitive to massive planets close to their host star, and it’s more likely to detect exoplanets that are closer to their host star and have a shorter orbital period.

However, it is ineffective at detecting smaller or smaller exoplanets or farther away from their host star.

It is also worth noting that other situations, such as the presence of other planets, binary stars, or other stars in the vicinity, can also cause similar radial velocity changes, making it difficult to confirm the presence of an exoplanet using this method alone.

Once a planet is found by the radial velocity method, it is followed by the following:

  • Follow-up observations: Follow-up observations using other techniques, such as direct imaging, transit photometry, or astrometry, can be used to confirm the existence of an exoplanet and to determine its characteristics.
  • Spectroscopy: Spectroscopy can be used to study the atmosphere of an exoplanet, looking for signs of water vapour, methane, or other gases that could indicate the presence of life.
  • Timing: By measuring the timing of the transits or the radial velocity changes with high precision, scientists can detect the presence of other planets in the system or confirm the planet’s mass.
  • Combining different methods: The use of different methods, such as radial velocity and transit or radial velocity and direct imaging, allows for confirming the presence of a planet and also checking for consistency between the different measurements and excluding false positives, such as background eclipsing binaries.

It is worth noting that while these methods can provide strong evidence for the existence of an exoplanet, it is challenging to confirm the presence of an exoplanet with 100% certainty.

It is also essential to note that it is becoming increasingly common to use machine learning techniques to confirm exoplanets and to identify new exoplanets candidates in large datasets of observations.

Diagram detailing the Radial Velocity (aka. Doppler Shift) method. Credit: Las Cumbres Observatory

Transit photometry

The transit photometry method, also known as the transit method, is used to detect exoplanets by looking for a slight dip in the brightness of a star caused by an exoplanet passing in front of it.

As an exoplanet orbits its parent star, it will pass in front of the star as viewed from Earth at regular intervals. When this happens, the exoplanet will block a small amount of light from the star, causing a slight dip in the star’s brightness. By measuring the brightness of a star over time and looking for periodic dips, scientists can detect the presence of an exoplanet.

The depth of the transit, or the amount of light blocked by the exoplanet, depends on the size of the exoplanet relative to the size of the star. By measuring the transit depth, scientists can determine the exoplanet’s size.

The duration of the transit, or the amount of time that the exoplanet is in front of the star, depends on the distance of the exoplanet from the star and the size of the star. By measuring the duration of the transit, scientists can determine the distance of the exoplanet from the star.

The timing of the transits, or the period at which the exoplanet passes in front of the star, depends on the orbital period of the exoplanet. By measuring the timing of the transits, scientists can determine the orbital period of the exoplanet.

This method is best for detecting exoplanets that are close to their host star and aligned to pass in front of the star as viewed from Earth. It is particularly effective at detecting exoplanets that are small in size or that are farther away from their host star.

It is worth noting that this method requires a high precision photometry, and it is more likely to detect exoplanets closer to their host star and have a shorter orbital period. Also, it is essential to note that other factors, such as the presence of other stars in the vicinity or the presence of spots on the surface of the host star, can also cause similar dips in brightness, making it difficult to confirm the presence of exoplanet using this method alone.

Once a planet is found by the transit photometry method, it is followed by:

After finding a planet through the transit photometry method, scientists typically use a combination of techniques to study the planet further and confirm its existence. Some of the things scientists typically do include:

  • Radial velocity measurements: Scientists use the radial velocity method to detect the host star’s wobbling caused by the exoplanet’s gravitational pull. This can be used to confirm the exoplanet’s existence and determine its mass.
  • Follow-up observations: Scientists use follow-up observations to confirm the exoplanet’s existence and determine its characteristics. These observations can include additional transit photometry, direct imaging, or spectroscopy.
  • Spectroscopy: Scientists use spectroscopy to study the atmosphere of the exoplanet and to look for signs of water vapour, methane, or other gases that could indicate the presence of life.
  • Timing: By measuring the timing of the transits or the radial velocity changes with high precision, scientists can detect the presence of other planets in the system or confirm the planet’s mass.
  • Combining different methods: By combining the results of different methods, scientists can confirm the existence of the exoplanet and determine its characteristics with greater accuracy.
  • Characterize the planet: Using the information gathered by different methods, scientists can estimate the radius and mass of the exoplanet and use that information to estimate its density and infer information about its composition and structure.
  • Study the host star: Scientists also study the host star of the exoplanet to learn more about its properties, such as age, composition, and activity, which can provide information about the conditions in the planetary system.

It is worth noting that the study of exoplanets is a rapidly growing field, and new techniques and technologies are constantly being developed that can be used to study exoplanets in greater detail.

Light Curve of a Planet Transiting Its Star. Credit: NASA Ames

Direct imaging

The direct imaging method is used to detect exoplanets by taking pictures of them directly, typically by blocking out the host star’s light. This method is best for detecting exoplanets far from their host star and having a significant separation from the host star.

Direct imaging of exoplanets is a difficult task because they are very faint compared to their host stars and are also very close to their host stars. Therefore, it is not easy to separate the exoplanet’s light from the host star’s light. To overcome this challenge, scientists use various techniques to block out the host star’s light and enhance the contrast between the host star and the exoplanet.

One of the most common techniques in direct imaging is coronagraphy, which uses an opaque mask placed in front of the host star to block out its light. Another technique is adaptive optics, which uses a deformable mirror to correct the distortions caused by the Earth’s atmosphere and to enhance the contrast between the host star and the exoplanet.

Another technique is called Differential Imaging; it consists of taking two images of the same field, one with the star and the other without, and then subtracting them to obtain the image of the planet.

Scientists also use polarimetry, a method that measures the polarization of the light from the exoplanet, to help distinguish it from the host star’s light.

It is worth noting that this technique is still in development and unable to detect exoplanets around distant stars; the current technology is limited to detecting exoplanets that are relatively close to Earth and around relatively bright stars.

It is also essential to note that even with these techniques, detecting exoplanets using the direct imaging method is still challenging. It typically requires a large amount of observing time and advanced instruments.

Once a planet is found by the Direct imaging method, it is followed by the following:

After finding a planet through the direct imaging method, scientists typically use a combination of techniques to study the planet further and confirm its existence. Some of the things scientists typically do include:

  • Radial velocity measurements: Scientists use the radial velocity method to detect the host star’s wobbling caused by the exoplanet’s gravitational pull. This can be used to confirm the exoplanet’s existence and determine its mass.
  • Follow-up observations: Scientists use follow-up observations to confirm the exoplanet’s existence and determine its characteristics. These observations can include additional direct imaging, transit photometry, or spectroscopy.
  • Spectroscopy: Scientists use spectroscopy to study the atmosphere of the exoplanet and to look for signs of water vapour, methane, or other gases that could indicate the presence of life.
  • Combining different methods: By combining the results of different methods, scientists can confirm the existence of the exoplanet and determine its characteristics with greater accuracy.
  • Characterize the planet: Using the information gathered by different methods, scientists can estimate the temperature, radius, and mass of the exoplanet and use that information to estimate its density and infer information about its composition and structure.
  • Study the host star: Scientists also study the host star of the exoplanet to learn more about its properties, such as age, composition, and activity, which can provide information about the conditions in the planetary system.
  • Study the orbit and environment: Scientists can use direct imaging to study the orbital and physical properties of the planet, such as its semi-major axis, eccentricity and inclination, and study the environment around the planet, such as the presence of disks or other companions.

It is worth noting that the study of exoplanets is a rapidly growing field, and new techniques and technologies are constantly being developed that can be used to study exoplanets in greater detail. Also, it is essential to note that the direct imaging method is a very challenging one, and the planets detected with this method are usually the brightest and the closest to their host stars, making the confirmation and characterization of these planets more difficult than for the planets detected by other methods.

When most of the bright light of HR 8799 is blocked by a coronagraph, it is possible to detect four planets. Credit: Keck telescope

Microlensing

The microlensing method is used to detect exoplanets by measuring the gravitational lensing effect caused by the exoplanet.

Gravitational lensing happens when the gravity of a massive object, such as a star or a planet, bends and amplifies the light from a background star. This causes the background star to appear brighter for a short period.

When a foreground star with a planetary companion passes in front of a background star, the exoplanet’s gravity can act as a lens, causing a slight deviation in the light curve of the background star. This deviation can be used to detect the presence of the exoplanet.

This method is best for detecting exoplanets far from their host star and having a slight separation from the host star. It is also good at detecting exoplanets that are on wide orbits and can detect exoplanets of any mass or size.

It is worth noting that this technique is only sensitive to exoplanets in the proper alignment with the background star and the observer. The probability of this alignment is low. It is also challenging to identify the exoplanet’s host star, making it difficult to study the properties of the exoplanet and its host star.

After finding a planet through the microlensing method, scientists typically use a combination of techniques to study the planet further and confirm its existence. This can include follow-up observations, such as radial velocity measurements, transit photometry, direct imaging, and spectroscopy, to study the exoplanet’s atmosphere and to look for signs of life.

Extrasolar Planet Detected by Gravitational Microlensing. Credit: NASA, ESA, and K. Sahu (STScI)

Pulsar timing

The pulsar timing method is used to detect exoplanets by measuring the timing of the pulsations of a pulsar, a greatly magnetized neutron star.

pulsar is a very fast-spinning neutron star that emits regular pulses of electromagnetic waves(radio waves). The timing of these pulses is exact, and any deviation from the expected arrival time can indicate the presence of an orbiting exoplanet.

When an exoplanet orbits a pulsar, its gravity causes the pulsar to wobble slightly. This wobbling motion causes the timing of the pulsar’s pulses to change periodically, and the period of this change is equal to the orbital period of the exoplanet. By measuring the timing of a pulsar’s pulses over time and looking for periodic changes, scientists can detect the presence of an exoplanet.

This method is best for detecting exoplanets far from their host star and having a slight separation from the host star. It is also good at detecting exoplanets that are on wide orbits and can detect exoplanets of any mass or size.

It is worth noting that this technique is only sensitive to exoplanets that orbit pulsars, which are relatively rare. Also, it is essential to note that it requires high-precision timing measurements, and it is challenging to confirm the presence of an exoplanet using this method alone.

After finding a planet through the pulsar timing method, scientists typically use various techniques to study the planet further and confirm its existence.

This can include follow-up observations, such as radial velocity measurements, transit photometry, direct imaging, and spectroscopy, to study the exoplanet’s atmosphere and to look for signs of life.

Pulsars are highly regular in timing, emitting radiation pulses at precise intervals. However, when a planet orbits a pulsar, it causes the pulsar’s timing to be slightly perturbed due to the gravitational pull of the planet. By measuring the timing of a pulsar’s pulses with high precision, scientists can detect these perturbations and infer the presence of exoplanets.

This method is best for detecting exoplanets far from their host star and having a slight separation from the host star. The method is also good at detecting exoplanets of any mass, and it can detect exoplanets on wide orbits.

This visualization shows how stars appear in the six main methods for finding exoplanets. Credit: ScienceNews.org

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