Raise Your Glasses to the Skies!

by Nicholas Mee on July 3, 2017

Photograph taken during the 2004 transit of Venus.

Science fiction writers have long assumed that there are planetary systems around most stars. The stars are so distant that confirming this belief was impossible until relatively recently. But in the last two decades the number of confirmed exoplanets, as they are known, has steadily increased. The most fruitful method for finding them is to search for eclipsing binaries. When a planet crosses the face of a star as viewed from Earth, there is a slight dip in the star’s brightness. This is just what happens with Algol and other eclipsing binary stars, but planets are typically far smaller than stars, so they block a much smaller proportion of the star’s light. Fortunately, modern detectors, which are essentially sensitive versions of the CCD chips in ordinary digital cameras, can measure very small changes in brightness. If Jupiter passed in front of the Sun, as viewed from outside the solar system, the gas giant would block about 1% of the light from the Sun. This could easily be detected using a CCD camera. Rocky planets such as the Earth and Venus are much smaller and so would be harder to detect. The photograph above shows the 2004 transit of Venus, as viewed from Dorset. Venus blocks about 0.01% of the Sun’s light.

The Kepler Space Telescope

In 2009 NASA launched the Kepler space observatory, named after the great German astronomer Johannes Kepler who first accurately described the orbits of the planets around the Sun. The aim of the mission was to discover planets around distant stars using the eclipsing binary method. Only by chance would a planetary system around another star be oriented in just the right way for the planets to pass in front of the star as viewed from Earth. Nevertheless, by keeping a watchful eye on enough stars we are sure to find some. Kepler continuously monitors the brightness of 145,000 stars and forwards the data to astronomers who perform computerized searches for periodic variations in their brightness.

Artist’s impression of NASA’s space observatory Kepler. Credit: NASA.

Kepler has so far discovered over 2300 planets in about 1000 star systems whose existence has been confirmed in follow-up observations. In addition to these there are many other candidates awaiting confirmation. From these results it is clear that our galaxy contains many billions of rocky planets, some of which must bear a striking resemblance to our own.

Raise Your Glasses to the Skies!

One of the most interesting planetary systems that we know of is the TRAPPIST-1 system discovered by astronomers at the University of Liege using the same eclipsing technique. The Belgian project known as the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) uses a 60cm robotic telescope at the La Silla Observatory in Chile. Further planets in the same system were discovered in follow-up observations by NASA’s Spitzer Space Telescope and the European Southern Observatory’s Very Large Telescope at Paranal in Chile. The convoluted acronym TRAPPIST was devised to commemorate Trappist beer, the favourite ale of the Belgian observers, which is still brewed today by the strict Trappist order of Cistercian monks. The discovery of the TRAPPIST-1 system was celebrated with a glass or two.

An artist’s impression of the seven planets of the TRAPPIST-1 system. Credit: NASA/R. Hurt/T. Pyle

 TRAPPIST-1 is one of our stellar neighbours at just 39 light years distance. It is an ultra-cool red dwarf whose mass is just one twelfth of the mass of the Sun. The red dwarf is little bigger than Jupiter in size, although it contains almost 100 times Jupiter’s mass. Seven planets are known to orbit TRAPPIST-1 and they are labelled ‘b’ to ‘h’ in the order that they were discovered. The orbits of all the planets would lie well within Mercury’s orbit around the Sun, so the TRAPPIST-1 system is like the solar system in miniature. Planet b orbits the star in just one and a half days, while planet h orbits in 18.8 days. Five of the planets (b, c, e, f, g) are similar in size to the Earth. The other two (d, h) are intermediate in size between Mars and Earth. The view at night from each of the planets would be spectacular, as they are much closer to each other than the planets in the solar system, so the planets in their skies would appear much larger than the Moon does in ours. The red dwarf TRAPPIST-1 is a very small star that emits a tiny fraction of the Sun’s radiation. This means that although the planets are so close to the star, the orbits of three of them (e, f, g) are thought to lie in the habitable zone of the system. In this region the temperature would be similar to that on Earth where water is liquid and the existence of life is possible.

The view from TRAPPIST-1f by Tim Pyle. Credits: NASA/JPL Caltech.

Does this mean that we might have neighbours in the TRAPPIST-1 system? Probably not. Although red dwarf stars are much fainter than the Sun, every so often they erupt and emit intense stellar flares from their surface. It is believed that any planet close enough to the red dwarf to engender life would regularly be sterilized by these flares.

Further Information

The official website of the Kepler Space Telescope is at:
https://www.nasa.gov/mission_pages/kepler/main/index.html

The website of the TRAPPIST project is at:
http://www.trappist.one/


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