Saturday, August 7, 2021

How we detect exoplanets?

Exoplanets are typically close to a star and since a star is much more luminous than the planet, direct detection of exoplanets is extremely difficult. This means that most confirmed exoplanets have been detected indirectly using some methods. There are a number of different methods for detecting exoplanets. Let us have a look at some of these methods.

Transit method

This method involves simply observing stars and then analyzing the dips in their brightness that repeat periodically. This method may sound straightforward, but there are many complications in it.

Sometimes the transit of exoplanet around its host star may get confused with other astronomical events like grazing eclipses from a stellar companion or transits of sub-stellar objects with radii similar to that of a giant planet or distant binary star systems whose angular separation is small enough that it blends with the target. Such errors are very common and quite difficult to identify.

In most cases, some follow-up radial velocity observations is performed to establish if there is a companion or not and, if it is a companion, to determine the mass and, hence, whether it is a planet or not.
Radial velocity method

Credit : ESA

In planetary systems, the planets and the host star all orbit the system’s common center of mass. Since the star is the most massive object in the system, the center of mass normally lies close to the center of host star. Directly observing this motion is very difficult. However, using high-precision spectrograph, the Doppler effect can be used to determine the radial velocity of these stars.

In a conventional spectrograph, the wavelengths of the star’s lines are found by comparing their positions along the spectrum with those of emission lines produced by an artificial and local source and whose wavelengths are known accurately from laboratory studies.

As the star orbits the common centre of mass, its spectral lines will shift slightly as it moves towards and away from the observer. This Doppler shift in the spectral lines can then be used to determine the radial velocity of the star, and, if this shows periodicity, then it can be used to then infer that something must be in orbit about this star.

However this method only determines the line-of-sight velocity of the host star, which means that the actual inclination of the orbital plane of the planet, relative to the Earth, is not known. Consequently, the actual orbital velocity of the star could be greater than the measured radial velocity, and hence, the actual mass of the planet could be greater than that determined from the host star’s radial velocity.

In addition to the mass of the planet, the radial velocity of the star can be used to determine the period of the orbit and, hence, the distance of the planet from the star. The radial velocity curve can also be used to determine the eccentricity of the planet’s orbit.

Gravitational microlensing method
Gravitational_micro_rev.jpg: created by NASAderivative work: Malyszkz, Public domain, via Wikimedia Commons

The detection of exoplanets via gravitational microlensing is closely related to the transit detection method because both approaches require regular, very precise photometry of many stars over periods of years. Microlensing though, produces a brief apparent one-off increase in the star’s brightness and the physical mechanism involved is quite different from that for a transit.

According to Einstein’s theory of general relativity, space curves in the presence of mass means that light can be deflected or lensed by a massive body. When the lens is extremely massive, such as a galaxy cluster, its mass can act to produce multiple, distorted images of even more distant galaxies.

However when a star in our own galaxy acts to lens the light from a more distant star that, from our perspective, passes behind the lens star. What we observe, in this case, is an apparent increase in brightness that can last for tens of days.

If the lens star has a companion planet and it happens to be in the right position, it can act as an additional lens and can produce an additional change in brightness, that is of a much shorter duration than the overall event.

One thing to note here is that this method is sensitive to the planet-to-star mass ratio and to the angular separation between the planet and its host star.

Direct imaging method
"File:VLT Snaps An Exotic Exoplanet First.jpg" by ESO/Schmidt et al. is licensed under CC BY 4.0

Unlike the Doppler and transit approaches to discovering exoplanets with their inbuilt biases towards discovering hot Jupiters, finding exoplanets by direct observation is biased towards detecting exoplanets that are a long way out from their host stars.

The reason for this bias is obvious – the light from the much brighter star swamps the light from the exoplanet unless they are well separated from each other. Directly imaging extrasolar planets is very difficult, especially as the planet is typically close to a host star that is much brighter than the planet.

There are some other methods too but using these methods very few or almost non exoplanet has been found till now. Therefore these methods are not discussed here.

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