Oceanic planet likely full of primitive life
The crew of an alien spacecraft entering the Solar System, maybe looking for planets bearing alien life, would be struck by three things about planet three, the third planet out from the Sun. Firstly it is radiating a cacophony of obviously artificial radio emissions. However, perhaps fortunately, the explorers would not be able to interpret them.
The second thing is that planet three is largely covered by water. That is unique in the Solar System.
Finally, the atmosphere consists of about 20% oxygen. This gas is highly reactive and would rapidly disappear from the atmosphere by reacting with iron in rocks and water, and with many other materials, especially carbon, unless continually topped up.
The only process we know of by which this can happen is photosynthesis, taking place in plants. In other words, oxygen is an indicator of life. There are many other chemicals that should not be there in the atmosphere of a lifeless planet. This provides an effective way to search for life on planets orbiting other stars. In fact, the James Webb Space Telescope recently detected evidence of life on an extra-solar planet, that is, a planet orbiting a star other than the Sun.
Not many years ago it was believed impossible that we would be able to search for planets orbiting other stars. It would be like trying to spot a firefly sitting on the edge of a very powerful searchlight. It turns out that thanks to modern telescope instrumentation, it is possible to search for the minute dimmings of a distant star when one of its planets passes in front of it. Using this method, even advanced amateur astronomers have successfully detected these planets. So far, thousands of extra-solar planets have been found.
Actually, we can go much further, thanks to the science of spectroscopy. Centuries ago, Isaac Newton passed sunlight through a prism and broke it down into a spectrum of colours. This is what happens in raindrops, giving us rainbows. With more sensitive instruments, known as spectrometers, we can spread the spectrum out further, and see that the rainbow of colours is crossed by groups of black lines. This where elements in the solar atmosphere are absorbing specific wavelengths of light coming up from the photosphere, the part of the Sun radiating the light and heat. They are imprinting their signatures on the light. If an airless planet passes in front of a star, we just get a dimming of the starlight. However, if that planet has an atmosphere, some of the starlight will pass through it on its way to us. In doing so it will pick up the signatures of chemicals and elements in that atmosphere.
In addition, those signatures will only be there when the planet is passing in front of the star, making them easy to separate from the signatures of elements in that star's atmosphere. This is what happened for a planet now named, not very poetically, K2-18b. This planet, around nine times the mass of our world, orbits in the habitable zone of a star lying some 120 light years away. That is, its light takes 120 years to get here.
In this case the intriguing substance being reported is not oxygen, but a chemical called dimethyl sulphide. On Earth this is produced by microbes and phytoplankton (microscopic plants) in the ocean. As far as we know there is no non-biological origin for this chemical.
It is proposed that K2-18b is an ocean world with an atmosphere rich in hydrogen, which automatically rules out the presence of oxygen, since it would rapidly combine with hydrogen to form water. It is likely K2-18b is a young world, with oceans teeming with primitive, microscopic life.
Perhaps, fortunately, it is too far away for us to mess with any time soon.
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Venus now lies very low in the east before dawn. After dark Jupiter shines yellowish-white in the west and red Mars is high in the southwest. The Moon will reach First Quarter on 4th May.
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory
