Role of Methane in Astronomy

Methane plays an important role in the atmospheres of many cool stars and exoplanets. Methane spectra have been proved very useful for astronomers to study different atmospheric characteristics of exoplanets and stars. Methane is one of the most important greenhouse gas in Earth's atmosphere. Methane plays an important role in combustion. Using infrared CH4 spectra, the hydrocarbon combustion processes can be observed. While in astrophysical atmospheres, methane has many vital roles. The CH4 absorption ranging from 1.0 to 2.5 μm is a characteristic property of T-type brown dwarfs. Thus T-type brown dwarfs can be identified directly through the "methane imaging". Similarly, absorption at around 3.3 μm is used to identify L-type dwarfs. Similar absorptions were also found in many large planets and even in the atmosphere of Titan (the second-largest moon of Saturn). However, along with methane absorption, water vapour absorption and carbon monoxide absorption are also needed for better results. 

Till today, successful methane spectral observation is made on only 6 exoplanets:

1. HD 189733 b - A Jupiter sized, blue coloured exoplanet located about 65 light-years from the solar system.

2. HD 209458 b - Also known as Osiris, located about 160 light-years away from the solar system. 

3. HR 8799 b - A Jupiter sized planet located about 130 light-years in constellation Pegasus.

4. XO 1 b - Named as Negoiu, located about 535 light-years away from the solar system. 

5. 51 Eridani b - A 200 million years old, Jupiter like planet located about 95 light-years away.

6. HD 102195 b - Originally named as Lete, a Jupiter sized planet, with an orbital period of just 99 hours and located in constellation Virgo. 

The instruments used earlier were having low resolving power, hence they cannot identify individual molecular species on the exoplanets. However, with modern spectroscopical techniques like Doppler tomography, astronomers are able to identify various molecular species. Today, we can confirm the presence of water vapour, carbon monoxide, TiO, CH4 and many neutral and ionized atoms. Today's results are much more accurate than those made from previous instruments. The above observations of 6 exoplanets clearly stress the use of methane spectra for hot, Jupiter like atmospheres. This technique is also useful for atmospheres of brown dwarfs with elevated temperatures. 

Methane is an important constituent of cool carbon stars. T-dwarfs are therefore also referred to as 'methane dwarfs'. The newly discovered Y-class of dwarfs also showed an excess of methane. Various equilibrium models have proved that methane is the main carbon-containing species in the atmosphere at temperatures below 1500 K. However, the reason for the abundance of CH4 in many large exoplanets is still controversial. At room temperature, the methane spectrum is very complex. Hence the spectrum is useful and accurate only for temperatures above 1500 K.

In 2012, a high-resolution transmission molecular absorption database (HITRAN) was designed to work at temperatures of Earth's atmosphere. Then in 2014, SN Yurchenko, Jeremy Bailey, Morgan Hollis and 2 other researchers presented new data of methane spectra which contained about 9.8 billion transitions, which would compete up to a temperature of 1500 K. Its range was from far infrared to 0.9 μm. Based on the opacity of spectral lines, they concluded that as the opacity increases, the significance of methane on thermal evolution and pressure-temperature profile also increases on brown dwarfs and exoplanets.

However, a small portion of opacity is also resulted due to other molecules like water vapour. Hence the scope of this model is not perfectly accurate. Here the researchers assumed that those effects on opacity would be relatively small. To create an even more accurate data model of the opacity of methane in hot atmospheres, researchers would need more transitions. 

The above discussion was centred on dwarfs and exoplanets. But, methane also plays an important role in the atmosphere of the planets in our solar system. As we all know that the solar system was formed through the gravitational collapse of a gas nebula. The centre of the nebula collapsed much faster than the outer regions of the nebula. This resulted in the birth of the Sun at the centre and planets in the outer regions. The temperature near the Sun was high enough to evaporate methane. The evaporated methane was later carried by solar winds to the outer regions. This resulted in methane-rich outer gaseous planets and methane poor inner terrestrial planets. 

The atmosphere of Mercury does not support the creation and existence of methane. Venus has a small proportion of methane in its atmosphere. Earlier astronomers predicted the formation of organic compounds on Venus. By detecting methane and other organic molecules, the hypothesis was finally proved. Earth's atmosphere has a good amount of methane. As life forms get emerging, the amount of methane also gets increasing. Also, after the pre-industrial era, the concentration of CH4 has been increased. As stated above, it is a prominent greenhouse gas and hence plays a vital role in global warming. There are no chemical reactions in Earth's atmosphere that creates methane. It is added to the atmosphere by microorganisms and various human activities. 

Methane is the most abundant gas in the troposphere of Jupiter after helium and hydrogen. Methane cannot condense at the temperature found in Jupiter's atmosphere. Its atmosphere has almost perfect conditions for the stable existence of methane. Saturn's atmosphere also has similar chemistry of the formation and existence of methane. Methane plays an important role in the geography and chemistry of Titan. Titan's surface and atmosphere showed an abundance of methane. The presence of such a large amount of organic compound might support microorganisms to grow on Titan. However, other atmospheric factors make it nearly impossible for life to grow on Titan.

We can conclude from the above facts and research that methane plays a very major role in astronomy. Methane not only impacts the solar system but also plays its part in extraterrestrial bodies like exoplanets and brown dwarfs. The development of new technologies and methods of 'methane imaging' might help us understand the geography of exoplanets even more deeply.