Basic exoplanet information
Up until about 20 years ago the only planets we knew about were those around our own sun. These were, in order of distance from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. (Pluto has since been removed from the list of our planets and is now called a dwarf planet).
Over the last 20 years astronomers using telescopes both on the ground and in space have discovered over 4000 planets around others stars in our Milky Way galaxy. Planets around other stars are called ‘exoplanets’.
Although a few hundred have been discovered by ground-based survey teams the majority were discovered by the Kepler satellite orbiting the earth. Kepler ceased operating earlier this year as it finally ran out of fuel to control its position. Another satellite called TESS (Transiting Exoplanet Survey Satellite) was launched in 2018 and is now actively finding new exoplanets.
Planets are too small and do not shine by their own light so they are very difficult to see, even with the most powerful telescopes, so how can we tell if a star has a planet orbiting around it?
There are several methods, but the most common one – as used by the Kepler and TESS satellites – is to look for the change in brightness of the star as the planet passes in front of it. This is called the Transit method. A small telescope on the Kepler satellite was used to measure the brightness of about 200,000 stars in a small section of the sky. It looked at this section of the sky every few minutes. Those which showed changes were singled out for further study. TESS has four small telescopes and is looking at the entire sky over a period of two years..
As the planet crosses the face of the star the brightness of the star as we see it drops by a small amount. For a big planet and a small star the drop can be as much as 4% but if the star is big and the planet small then the drop in brightness is tiny. At present large ground-based telescopes such as those on Mauna Kea in Hawaii can reliably detect changes of as little as 0.01% – that’s 1 part in 10,000.
Above is an actual light curve of a planet transiting a star in the southern hemisphere called WASP-4 (the name WASP stands for Wide Area Search for Planets). This light curve was taken with an amateur class 36cm diameter telescope. The Y-axis shows the brightness of the star (the scale does NOT begin at zero), the X-axis is time and represents about 4½ hours, with the transit lasting for about 2 hours 10 minutes. The vertical lines show when the planet was expected to cross the face of the star. The points show measurements of the brightness of the star taken at 3 minute intervals. In this case the drop in brightness was about 2.7%
The main disadvantage of the transit method is that it can only find planets close to their parent star and so the periods (their year) is usually less than 30 days and more often just a couple of days. It takes a big telescope to spot the drop in light from a planet a long way out from the star and also you have to keep watching the star for several years in order to be sure of getting at least three drops in the light.
Of course this method only works if the planet’s orbit is along our line of sight and that eliminates many of stars we see in the sky.
A second method to detect exoplanets relies on the fact that as the gravity of the star pulls on the planet so does the gravity of the planet pull on the star. This means that the star wobbles a bit as the planet goes around the star. Again, this works best for stars close to their parent star.
This much more difficult to measure than using the transit method and it needs big telescopes and very sensitive spectrometers. A spectrometer looks at the colour of the star which will change slightly as the star wobbles. It is the speed of the star’s wobbles which causes the colours to shift very slightly. A very sensitive spectrometer on a big telescope (think $millions) can detect changes in the speed of the star as little as a few metres per second (walking pace is a bit less than 1 metre/ second for most people).
And then there is the third method – direct imaging. Now, this is really hard!
Imagine a fisherman with one of those very bright lanterns they use for night fishing and he holds a lit cigarette right next to it. It would be difficult to see the light of the cigarette in the glare of the bright lantern – especially when you are on the shore and the fisherman is 10 km out to sea. That’s about the scale of the problem of directly seeing a planet with even a very big telescope.
But there is a way with some specialised equipment. Suppose you could block out the light of the lantern – then you might just be able to see the light of the cigarette. This is what astronomers do with special telescopes to be able to directly see a planet in its orbit around a star. They use an instrument called a coronagraph which blocks out the light from the star when the scope is focused on the star.
Direct imaging is better suited to finding planets further out from the star.
This is a photograph of the star bright star Fomalhaut. The inset picture shows the positions of a planet just inside a dusty ring surrounding the star. The light from the star has been blocked to the maximum degree possible. The same photo was taken two years apart and the way the planet has moved in that time is clearly seen.
In the image below four planets can be seen surrounding the star HR8799
Planets are thought to be formed from the leftovers of the cloud of dust and gas from which a star forms.
In the news recently (September 11) we can see reports that water vapour has been found in the atmosphere of a planet that was discovered by the Kepler satellite – K2-18b. This planet lies in what is called the habitable zone. This is the region out from the star at which the planet’s surface temperature allows liquid water to form on the surface. It is this type of planet that it is thought most likely to harbour some form of life. See https://www.bbc.com/news/science-environment-49648746
As yet we have not found any life forms on any other planet in our Solar System or in any exoplanet. Nor have we found any planets on which we are very confident that life would exist. There’s no earth 2.0 yet. The study of the conditions for life on exoplanets is called Astrobiology. See www.astrobiology.com