51 Pegasi b is a roughly jupiter-sized planet around a star pretty similar to our sun. This planet has been part of astronomy conversations before, as it is a huge planet with a really small orbit; this planet orbits around its star only every 4 days. For comparison, Earth orbits the sun every 365 days.
One way of spotting planets orbiting other stars is via the use of spectral lines. Spectral lines are the different colors of light emitted by an object that tells us what that object is made of. Different elements and molecules will emit light in different colors. This is the same phenomenon that makes phosphorous glow-in-the-dark green. These scientists wanted to use this technique on 51 Peg b, but because this planet is so close to its star, it was proving quite difficult.
It is difficult to see the planet 51 Pegasi b’s spectral lines because they are mixed with the light emitting from the star. The light from the star is far more powerful and thus the light being emitted from the planet cannot be seen — imagine trying to see a weak colored christmas bulb with a bright flashlight shining behind it from across the yard.
The unique thing the scientists did in this paper was to filter out the star light so the planet’s light could be visible. The technique they used is called cross-correlation. This technique teases out a specific known signal in the larger, stronger signal. For this tec6hnique to work, the researchers had to determine what the exact signal for the light emission from the planet would look like for different molecules. This was done for a variety of different molecules — the researchers analyzed the what the light from the planet should look like for methane, carbon dioxide, and water. They then used these signals they made and used the cross-correlation technique on the data they had for the light from the planet and its host star combined.
This technique was successful! The researchers found definite spectral lines corresponding to water in the atmosphere of this planet. This means there is a surprising amount of water in the atmosphere of 51 Pegasi b, about 1 molecule in 10,000 molecules in the atmosphere are water. Unfortunately, they did not see detectable amounts of carbon dioxide or methane. Furthermore, as part of this technique, they were able to more precisely determine the mass of the planet because how much light a planet emits corresponds to how much mass the planet has. They confidently found the mass of 51 Pegasi b to be about half the size of Jupiter, 47.6% the mass of Jupiter to be exact.
This means that there is unlikely life on this planet, as methane and carbon dioxide are common signals of life, but water in the atmosphere is a big find! Finding the molecules that makeup atmospheres of other planets proves to be one of the most conclusive ways to detect life in other planets, but it also proves to be one of the hardest things to measure. This paper has provided a new technique that will be utilized in our continuing search for live beyond Earth!