I2AO Part 9: Extrasolar Planets
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PART 9: EXTRASOLAR PLANETS by Lesa Moore
Extrasolar planets (sometimes called exoplanets) are planets that orbit stars other than the Sun. The first discovery of an extrasolar planet orbiting a main-sequence star occurred in 1995. The discovery was made by Michel Mayor and Didier Queloz. The planet was 51 Pegasi b. The 2019 Nobel Prize in Physics was awarded to its discoverers. Many extrasolar planets have been found since. This section will cover detection methods, the reasons for bias in the discoveries, and a count of known extrasolar planets..
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There are five main methods that may be used for detecting extrasolar planets. For an update on how many planets have been detected by each method, refer to the NASA web page here: https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/?intent=021
- Radial velocity
When a planet orbits a star, the star is not "fixed" in position with the planet orbiting around it. The planet and the star both orbit their common centre of gravity. This means that the star is sometimes moving towards, and sometimes moving away from, the observer. By recording the star's spectrum using many observations over time, small Doppler shifts in the spectrum reveal the movement of the star. For a single-planet system, this is relatively easy to interpret. In multiple-planet systems, the analysis becomes more complicated, but is achieved through Fourier analysis.
In 2000, an iodine cell was introduced to calibrate the spectra taken with the Anglo-Australian Telescope in the Anglo-Australian Planet Search (AAPS). The iodine converts to gas when the cell is heated, and the cell is then placed in the light path from the star. This superimposes numerous reference absorption lines on the star’s spectrum, allowing the spectrum to be calibrated to very high precision.
The unknown factor when interpreting the data is the inclination of the orbit of the planet to the observer's line of sight. The orbit may cause the planet to transit in front of the star, or the plane of the orbit may be near-perpendicular to the line-of-sight.
Figure 1, below, shows the planet receding and the star approaching. The star's spectrum is blue-shifted. In Figure 2, the planet is approaching, and the star is receding, red-shifting the star's spectrum. Figure 3 shows the iodine cell.
Figure 1 - Radial velocity method - star approaching
Image Credit: NASA
Figure 2 - Radial velocity method - star receding
Image Credit: NASA
Figure 3 - The Iodine Cell
Image Credit: The Anglo-Australian Planet Search
- Transit
If a planet's orbit brings it directly in front of the star along the line of sight of the observer, the star's light will dim as the planet transits. The light curve allows interpretation of single and multiple-planet systems.
By taking a spectrum of the star alone, and a spectrum when a single planet is in transit, it is possible to obtain the spectrum of the planet's atmosphere by subtracting the stellar spectrum from the combined spectrum. Examples: In 2014, HAT-P-11b, a Neptune-sized exoplanet, was found to have water vapor in its atmosphere. In 2025, dimethyl sulfide (DMS) was detected in the atmosphere of the exoplanet K2-18b.
In the case of a transiting planet, the orbital inclination is limited by the diameter of the star (i.e., if too inclined, the planet won't block any starlight). The transit provides useful information about the relative sizes (diameters) of the planet and the star.
The Kepler Space Telescope operated for nine years (2009 to 2018) and ended up observing some 500,000 stars in its extended mission (K2) to look for changes in brightness indicative of transiting extrasolar planets. The TESS mission (Transiting Exoplanet Survey Satellite) continues the search. Analysis and continued observations of potential targets mean that the count of discoveries from each mission continue to rise and already number in the thousands.
Figure 4, below, illustrates the dip in light due to one transiting planet. Figure 5 shows the multiple and compound dips produced in a multiple-planet system. Light curves are displayed at bottom left of images.
Figure 4 - Transit of a single planet
Image Credit: NASA
Figure 5 - Transits of multiple planets
Image Credit: NASA
- Direct imaging
By using a coronagraph to block out the normally overwhelming light from the star, it is possible to get direct images of planets in some systems. However, the images do not look like Voyager photographs of Jupiter. The planets appear merely as bright dots in an image.
Figure 6, below, shows how the starlight overwhelms the light from the planet (left of star). In Figure 7, the coronagraph completely covers the star, allowing the planet to be seen. Figure 8 is a gif sequence showing the motion of planets detected around the star, HR 8799, by the Keck telescope.
Figure 6 - Overwhelming starlight
Image Credit: NASA
Figure 7 - Star is occulted by coronagraph to reveal planet
Image Credit: NASA
Figure 8 - Four exoplanets of the HR 8799 system: The planets were imaged by the W. M. Keck Observatory over the course of seven years. Motion is interpolated from annual observations.
Image Credit: This file is licensed under the Creative Commons Attribution 4.0 International license.
Attribution: Jason Wang (Caltech)/Christian Marois (NRC Herzberg)
- Gravitational microlensing
If a star-planet system passes in front of a more-distant star, this may produce a gravitational microlensing event. The gravity of the system magnifies the light from the more-distant star by bending light rays in much the same way as a magnifying glass does. If one star passes in front of another, this results in a smooth rise and fall in the light curve. The appearance of a small kink in the measured light curve indicates the presence of a planet.
Figures 9 and 10, below, represent light rays by lines that diverge from the source, and then converge due to the gravity of the foreground system. A single lensing object would produce a single peak. The small spike in the light curves for both figures is due to the planet. Light curves are displayed at bottom right of images.
Figure 9 - Gravitational lensing double-peak
Image Credit: NASA
Figure 10 - Gravitational lensing when foreground system has passed by
Image Credit: NASA
- Astrometry
In systems where observations are made from the pole of the orbit of a planet or planets, the movement of the star discussed in the radial velocity section above is lateral to the line of sight. With high-precision astrometry, i.e., measuring exactly where the star is, it is possible to detect the changes of position of the star due to the orbiting planet(s).
Figures 11 and 12, below, show the small shift in position of a star due to an orbiting planet. If seen from the pole of the planet's orbit, the star will make complete circles. If viewed from other angles, the star will trace out an ellipse.
Figure 11 - Sample position of star, relative to other stars in the field.
Image Credit: NASA
Figure 12 - The target star has changed position.
Image Credit: NASA
Planet properties: To confirm an extrasolar planet discovery, ideally, the target system will be observed for one complete orbit of the planet. However, even in our own Solar System, Neptune takes more than 165 years to orbit. Naturally, it is easier to find planets that have short-period orbits of one to two years. Also, two detection methods rely on movement of the star, i.e. radial velocity and astrometry. The more massive a planet is (e.g., comparable to Jupiter) and the closer the planet is to the star (e.g., closer than Mercury is to the Sun), the easier it will be to detect the planet. Many of the earlier discoveries revealed a class of planets known as "hot Jupiters" that have rapid orbits very close to the host stars. As observations have now been made over decades, and more-sensitive instruments have become available, detections of Earth-sized planets in longer-period orbits have become possible.
Star properties: If the host star is a dwarf, smaller and less massive than the Sun, its motion as it orbits the common centre of gravity will be more pronounced than for a very massive star. This means there is more chance of a radial-velocity (Doppler) detection. Dwarf stars offer more chance of a transit detection because the dip in the light curve will be deeper (a greater percentage of the starlight is blocked). There is also a greater opportunity for direct imaging because there is less contrast between the star and the planet, and even less contrast if observed in infrared.
According to the NASA Exoplanet Archive, as of May 2025, there are more than 5900 confirmed extrasolar planets. The data may be visualised in various charts available here: https://exoplanetarchive.ipac.caltech.edu/exoplanetplots/. Two snapshots have been extracted below, but this field is expanding constantly with ongoing discoveries and new detection techniques.
Figure 13, below, is a chart showing the cumulative total of confirmed extrasolar planets by year. Figure 14 plots planet mass against orbital period (more massive at the top, longer periods to the right).
Figure 13 - Cumulative Count vs Year of Discovery
Image Credit: https://exoplanetarchive.ipac.caltech.edu/exoplanetplots/, accessed 22 May 2025.
Figure 14 - Planet Mass vs Orbital Period
Image Credit: https://exoplanetarchive.ipac.caltech.edu/exoplanetplots/, accessed 22 May 2025.
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Author: Lesa Moore, 22nd May 2025