In our search to learn more about our universe, we have employed many mechanisms, be it satellites, probes or telescopes. While some celestial objects have been discovered through X-rays and gamma-rays, there are some that are very faint and far to be detected in visible light. Thankfully, infrared (IR) has come to our rescue. Unlike X-ray and gamma-ray astronomies which deal with the death of stars, infrared astronomy is concerned with their birth.
The great astronomer William Herschel, who discovered Uranus, did a simple and beautiful experiment in 1800, in which white light was sent through a prism and temperatures measured for different colours of the ensuing spectrum. He reasoned that the unexpected excess in temperature in the control region next to red is due to light which cannot be seen. This light was eventually named infrared rays. While this was the first representation of non-visible light, the whole of electromagnetic spectrum was eventually discovered. The others being ultraviolet, radio, X-rays and gamma-rays. Any body with a temperature is a source of IR rays. The wavelength of IR extends from the red edge of the visible spectrum at 700 nanometres to 1,000,000 nanometres.
The study of infrared, depending upon the instruments, is divided into three regions: near-infrared, mid-infrared and far-infrared. While most big optical telescopes in sites with less moisture can be used for studies in the near IR region, instruments above atmosphere are needed for other regions of IR. IR studies have and are still contributing significantly to the exciting field of exoplanets. By early 1900s, IR rays had been detected from moon and several other objects in the solar system.
The important IR satellite missions so far are the Infrared Astronomical Satellite (IRAS), Infrared Space Observatory (ISO), Spitzer Space Telescope, Herschel Space Observatory, Stratospheric Observatory for Infrared Astronomy (SOFIA) etc. Significant results keep coming from both Spitzer and SOFIA, which are expected to function for at least 10 more years.
Let’s take a look at some of the many discoveries in IR astronomy:
Birth of stars
The IR rays give important information about the birth of stars since they are suited to study protostars and star formation regions, which are at lower temperatures. Large number of stars which are too cool to emit visible light or are hidden behind obscuring dust have been detected by this method. Brown dwarfs are an odd set of objects that are neither planets nor stars. The best hope for finding brown dwarfs is infrared telescopes, which can detect the heat from these objects. IR studies seem to indicate that there is one brown dwarf star for every six stars in our galaxy.
Also, SOFIA is a modified Boeing 747 jetliner with a 2.5 metre-IR telescope, which can cruise near the edges of the atmosphere. In October 2016, it observed the collapse of a few interstellar clouds on their way to becoming new stars. Detecting such infall in proto-stars, which happens very fast, is very difficult to observe, but is critical to confirm our overall understanding of star formation.
Peering into galaxies
The expansion of the universe was discovered by the study of the redshift of external galaxies by American astronomer Edwin Hubble in the last century. However, expansion stretches light further and the wavelengths are shifted down into the infrared. As a result of this Doppler effect, at large redshifts, visible light from distant sources is shifted into the infrared part of the spectrum. Therefore, IR studies give us a lot of information about the very young and distant galaxies.
Our universe is about 14 billion years old. But recently in March 2016, Hubble and Spitzer spotted a galaxy born only 400 million years after the Big Bang. This galaxy is small, about 25 times smaller than the Milky Way. Apart from the distant ones, many galaxies (more than 20,000) have been detected only in the infrared spectrum. Many of these are star-burst galaxies with formation of enormous numbers of new stars, and are thus extremely bright in the infrared spectrum.
Also, interstellar matter radiates strongly in the infrared spectrum. Due to all these reasons, IR pictures reveal the structure of our galaxy in a much better way. The centre of our galaxy is one of the brightest infrared sources in the sky and IR studies have showed us the rapid rotation of stars and gases near the centre, thus pointing to the existence of a super massive black hole.
Planetary studies
IR studies have also shed light on some of the vital aspects of our solar system like IR detection from the moon (as early as 1856), the composition of Venusian atmosphere, possible internal source of energy in Jupiter, the methane atmosphere of Titan moon, etc. An enormous ring around Saturn that had remained hidden earlier was detected thanks to infrared. What’s more, infrared also made it possible for Herschel Observatory to detect water in comets. Unlike most of the other celestial objects of the asteroid belt in our solar system, the dwarf planet of Ceres contains a significant amount of ice, which leads one to believe that a part of water on Earth has come from comets and asteroids.
Exoplanets decoded
The exciting field of exoplanets really began by the observation of IR-emitting dust round stars. After this initial fillip from IR astronomy, several thousands of these exoplanets have been found by ground-based telescopes and Kepler Spacecraft. These detections are indirect in the sense that they look for a regular dip in the light level given out by the system.
However, Spitzer recently became the first telescope to directly detect light from such planets outside of our solar system; it captured the warm infrared glows of two previously detected ‘hot Jupiter’ planets directly. These planets are gas giants that zip closely around their parent stars and shine brightly in infrared wavelengths.
Since the star-planet contrast is more favourable in IR, as the planet emits its own light, it is easier to directly detect such planets. Further, since molecules in the atmospheres of exoplanets have the largest number of spectral features in IR wavelengths, the temperatures, winds, and atmospheric compositions on these distant planets can also be obtained. Last year, Spitzer confirmed a very close (21 light years away) rocky planet by finding its density. More details on the closest exoplanet, the one in Proxima Centauri, will also be available from IR studies soon.
Finally, because the majority of the stars in the galaxy are low-mass and predominantly IR-emitters, if aliens exist and ever do visit us, they will probably have infrared vision! Our own eyes evolved to make maximum use of the Sun’s light, which peaks in the visible. But the eyes of such aliens would have evolved to use their home star’s infrared light. Looks like infrared can unlock many more secrets of our universe and more.