With an already massive catalog of different types of celestial objects and more discoveries coming rapidly, it is natural to ask, how are these celestial objects discovered? Today, we’ll explore the methods that astronomers use to search for new objects to provide important insights into the mysteries of the Universe.
Some celestial objects, such as asteroids, comets, stars, and galaxies, are usually found using direct optical observations. This involves taking an image of the object and then identifying the new object within that image. We don’t know when we can find a new object in advance, so large-scale sky surveys are carried out to take photos of different patches of the sky in case the images contain new objects. As most of the low-hanging fruit has already been discovered and cataloged, new objects are often very faint or distant. Therefore, large telescopes and advanced data processing and object identification software are often necessary.
For example, ATLAS, which stands for the Asteroid Terrestrial-impact Last Alert System, is a full-sky survey that scans the sky multiple times daily. This high frequency of searching is because it aims to find potential Earth-impacting asteroids and to provide a sufficient warning time as early as possible. While it doesn’t need a gigantic telescope (with an aperture of only 0.5m), it has found many asteroids that have flown close to the Earth, as well as many other asteroids in different places of the Solar System, thanks to the advanced detection software that it is equipped with.
Some objects, such as stars and galaxies, appear stationary in respect to most other objects in the sky. If you take two images of the same patch of the sky (accounting for the Earth’s rotation), you’re will find the same object in the same place. For this reason, it’s easier to find stars and galaxies than other objects; thus, most of those within reach of existing telescopes are already cataloged.
But there are also objects that move in the celestial sphere — and these are often Solar System objects. One of the ways to distinguish planets from stars is to see if it moves — if it’s a planet, it moves from day to day. If it’s a star, it doesn’t. That’s because planets move around the Sun. The same goes for asteroids and comets — and they are what sky surveys can still discover today.
Basically, multiple images are taken of the same patch of the sky, separated by an arbitrary period of time. Then, the images are compared. Some objects are stationary across multiple pictures, while some are moving. These moving objects are the Solar System objects, such as asteroids and comets. By observing how they move in the images, orbit determination can be carried out to pinpoint their orbit around the Sun, which is used to determine, for example, whether the asteroid will impact the Earth or not.
However, even if we have powerful optical equipment to detect very faint objects, there are still things that emit so little light that it’s easier to find them with indirect evidence instead of direct observation. This kind of evidence relies on observing patterns in the changes in other objects or phenomena. And the objects found this way include exoplanets (planets orbiting stars other than the Sun) and black holes.
For example, within their orbits, some exoplanets pass right in front of the star, as we see from the Earth. While we can’t see a dark spot on the star in a telescope (it’s too small!), we can detect a small dip in the brightness of the star, away from the normal variations. If these small dips are periodic, the most likely reason, other than a stellar companion, is an exoplanet. This method is known as the transit method, which is the most successful method of finding exoplanets so far. There are other ways to find exoplanets indirectly as well, such as the radial velocity method, but we’re not going to get to these in depth here, instead exploring them in another article.
Black holes are another example of objects found indirectly; as they do not emit light (instead absorbing it due to gravity), they cannot be directly seen by any observer. Thus, one needs to rely on other evidence to find them. For example, if, from astrometry data or Doppler shift data, a star appears to be orbiting empty space, that is likely a sign of a black hole. Moreover, light bends around a massive object (due to gravitational lensing), resulting in a distorted image. Therefore, if other objects (e.g., galaxies) are seen distorted in an image, this could also indicate a massive black hole.
In this article, we’ve explored two ways new celestial objects are discovered. We have direct optical observations, which might initially seem uninteresting, given that it’s very intuitive. But the exciting part is that objects don’t need to be stationary in the celestial sphere — they can move around as well, indicating that these objects are the asteroids, comets, and planets that orbit the Sun. Another way to do that is to gather indirect evidence for these objects- things that could be too faint to observe yet produce a significant effect on how other visible objects behave. If you would like to improve this article, please leave your opinions in the comments below.