Astronomical bodies: Exoplanets meet and greet

Proxima Centauri b

Proxima Centauri b is an earth-sized exoplanet orbiting our closest neighbouring star, Proxima Centauri, and was discovered on the 23rd August 2016. Proxima Centauri itself is a small, dim, red dwarf star located only 4.2 light years away, which in terms of cosmic distances, is nothing but a step next door.

Compared to our own sun, it is extraordinarily dim, with a luminosity of only 0.17% of our own Sun, however to compensate the planet orbits at a much closer distance of 0.048AU, where 1AU is the distance between the Earth and the Sun.

By comparison, Mercury orbits at an average distance of around 0.38AU (this is a rough value as Mercury has some interesting orbital characteristics due to effects caused by general relativity).

Due to its recent discovery and the limitations of our current detection methods, we only know the minimum mass and radius of the exoplanet, and therefore our estimations of the surface gravity is only a lower limit, and not exact. Its mass is at least 27% larger than Earth, with its radius a minimum of 10% larger, resulting in a minimum surface gravity of 10.3ms-2, just under 5% larger than Earth’s surface gravity.

It is also thought that dependent on the exact values of the planet's mass and radius, water could make up for 0.05%-50% of the planet's mass, indicating some potential for habitability. Again for comparison the Earth’s mass is 0.02% water.

HD189733 b

The excitingly-named HD189733 b is a hot Jupiter - a classification of exoplanet that describes those which are around the size of Jupiter, but much closer, sometimes with orbits as short as a few days, and along with HD209458 they were the first two exoplanets to be directly spectroscopically observed.

In other words, using NASA’s Spitzer Space Telescope, scientists were able to split the light into a whole spectrum of wavelengths, and by plotting the intensity of light for each wavelength, they can detect dips and patterns in the intensity which indicates the presence or absence of various molecules. This technique is known as infra-red spectroscopy.

This particular hot Jupiter orbits its parent star very closely, and as a result completes a single orbit in just 2.2 days. The planet itself is tidally locked with its parent star, much like how our own moon is tidally locked with the Earth. As a result of this, the planet has one side constantly facing the star, and another constantly facing into space, in other words the planet is locked in permanent day (or night, depending on what side you’re on).

Due to its proximity to its parent star, the temperature range is quite high and using the same NASA telescope as before, a thermal map of the planet was also obtained showing a temperature range of around 700oC – 939oC with high wind speeds of around 8,700km/h distributing the heat throughout the exoplanet.

The planet itself is 16.2% more massive than Jupiter, and 13.8% larger in radius. Defining its surface gravity is a bit tricky, seeing as it doesn’t really have a surface, however taking the gravity simply at the edge of its atmosphere so we can contain all of its mass yields a gravitational acceleration of 21.2ms-2, over twice that of the Earth’s pull.

In 2008 researchers were able to use two different methods to detect the colour of the exoplanet, and evidence suggests that it has a deep blue colour, likely due to raining molten glass, caused by the extreme temperature and pressure present.

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