Found: watery super Earth
USING observations made with the Hubble Space Telescope, astronomers have discovered water vapour in the atmosphere of a “super earth” —a planet larger and more massive than ours, but far smaller than giant planets such as Jupiter and Saturn.
The planet, about eight times the size of ours, is called K2-18b. The star it orbits is a red dwarf star, so is much smaller and less luminous than our Sun.
In most cases, we cannot directly image planets orbiting other stars, so it may seem surprising that we can deduce constituents of their atmospheres.
The way in which this is done is related to the discovery method of the planet.
The designation “K2” indicates that it was detected using the Kepler spacecraft, which was especially designed to search for planetary transits. These are events in which a planet moves in front of its parent star as seen from here.
The planet reveals itself by blocking a small part of the star’s light, and the amount of dimming allows astronomers to deduce the planet’s size.
The temporary dimmings occur about every 33 days, so that is the orbital period of the planet around the star. For this particular study, eight transits were observed — allowing astronomers to build up a good deal of data.
Wobbles in the motion of the star caused by the planet’s gravity allow us to find the planet’s mass. These have also revealed the presence of another planet in the system that does not transit the star.
Let’s get back to the planet’s atmosphere. When a planet passes in front of its star, some of the star’s light passes through any atmosphere that the planet may have. By observing the way in which part of the starlight is absorbed by the atmosphere, it is possible to make certain deductions about the atmospheric constituents.
Specifically, when astronomers measure the way in which light is affected as it passes through a material, they note particular, exact wavelengths of light that are absorbed. These wavelengths form a ‘signature’ of the substances in the object. Astronomers call this an absorption spectrum, and the effect is part of the wonderful science of spectroscopy.
This is also the method that astronomers use to determine the makeup of the stars themselves. The Sun’s spectrum, for example, contains almost uncountable ‘missing’ wavelengths of light due to absorption of the Sun’s own light within the Sun itself.
There is still some uncertainty about the atmosphere of K2-18b. There is no doubt that water vapour has been detected, but there are various possibilities as to the amount. As so often happens, more than one scenario could fit the data.
There could be very little water, perhaps as part of an atmosphere containing mainly hydrogen and nitrogen, or one containing mostly hydrogen. At the other extreme, there may be a great deal of water in a hydrogen atmosphere.
As always, the presence of water on another world makes us think of the possibility of life being there. We’re a long way from a detection like that, but it does at least remind us that there are planets that have some things in common with ours. There are plenty of them!
However, they are hardly on our doorstep. Stars other than our Sun are at distances from us that our brains can’t even pictures. K2-18b is over 100 light years away, meaning that its light has taken more than 100 years to get here. The dimmings that astronomers are now observing actually happened around the turn of the last century.