Exoplanet Detection

Exoplanets or extrasolar planets are planets that are outside our solar system. Exoplanets are very difficult to detect using telescopes, as they are hidden by the brightness of the star they orbit. Usually, the effects an exoplanet has on the star they orbit are the key to its detection.

What are the methods for exoplanet detection?

There are various methods for exoplanet detection, including direct imaging, radial velocity/Doppler spectroscopy, transit photometric, astrometry, and pulsar timing.

Direct imaging

The direct imaging technique uses large telescopes equipped with adaptive optics and coronagraphs to image the exoplanets by direct measurement of their luminosity. However, when planets are located far from their orbiting star, thermal emission of the exoplanets is observed.

This method is more suited to warmer and bigger exoplanets with eccentric orbits that are easier to detect. This is due to the fact that the higher the mass of the planet, the higher the thermal energy and hence the higher the temperature. The temperature of the planet is also related to its radius and brightness, which is why this method can identify the size of the detected exoplanets. It can even detect brown dwarfs based on their thermal emission.

Brown dwarfs are celestial objects that are not big enough to sustain nuclear fusion but have high temperatures to emit infrared radiation. They are an intermediate between a star and a planet and usually have a mass of less than 0.075 solar masses. Brown dwarfs have insufficient mass to sustain nuclear fusion but are hot enough to radiate energy, especially at infrared wavelengths.

Radial velocity/Doppler spectroscopy

When the elliptical motion of the star is observed from a distance, the light spectrum of the star changes: when a star is moving towards the observer, its light wavelength is shortened, and it is shifted towards the blue end of the spectrum.

When the light wavelength is lengthened and appears to be shifted to the red end of the spectrum, the star is moving away from the observer. This is called the Doppler effect, which indicates the gravitational effect of an exoplanet on the host star. The electromagnetic spectrum and the exoplanet detecting telescopes can be seen in Figure 1 below. At the red end of the spectrum, the electromagnetic waves have longer wavelengths, whereas, at the blue end of the spectrum, the wavelengths are shorter.

Figure 1. Electromagnetic spectrum. Source: NASA’s James Webb Space Telescope, Flickr (CC BY 2.0).

The radial velocity method, also known as Doppler spectroscopy, detects exoplanets by observing the Doppler shift of a star and its small deviations in its orbit over time. This suggests that an orbiting mass has gravitational effects on this star.

When an exoplanet is detected, its minimum mass can be calculated using the radial velocity method by measuring the amplitude of the sun’s radial velocity over time.

Transit photometry

The exoplanet is detected with the transit photometry technique by observing a star’s brightness over time. When a periodic decrease in brightness is detected, an exoplanet must be in transit in front of the star, causing a decrease in brightness proportional to the relative sizes of the star and the planet. This method is capable of measuring an exoplanet’s radius relative to its orbiting star.

Astrometry

Astrometry is the oldest recorded method of exoplanet detection, which was first discovered in the 17th century. This technique consists of precise measurements of a star’s coordinates in the sky, which are used as a reference point.

Deviations from the reference point are recorded over time. If an exoplanet is presently orbiting a star, a gravitational pull on the star will be present, causing it to shift slightly from its orbit.

The star and the exoplanet both have gravitational effects on each other, causing them to orbit around a mutual centre of mass called the barycentre. Because the star is greater in terms of mass, the barycentre will be closer to the larger body’s radius. It is thus easier to find exoplanets orbiting lower-mass stars or other low-mass objects like brown dwarfs.

Pulsar timing

Pulsar is a neutron star, which is a very dense residue of a supernova. These emit radio waves as they periodically rotate intrinsically. The motion of a pulsar is recorded from the slight changes in its radio pulses’ timing, which can be used to estimate the characteristics of its orbit.

Figure 2. Pulsar and its companion star. Source: NASA Goddard Space Flight Center, Flickr (CC BY 2.0).

This method was first used to study the motion of pulsars but was later found to be very accurate in detecting very small planets. However, this method is not used frequently, as pulsars are relatively rare.

What are the advantages of exoplanet detection?

Detecting exoplanets has various advantages as they can help answer some questions about our origins as humans or whether intelligent life exists beyond the earth. Studying exoplanets: