15 February 2017
Artistic impression of a ‘hot Jupiter’ in the star cluster Messier 67.
Light curve of the star M2-99 over a period of about three months. The spurious contributions from the instrument and the star were removed. The planet dims the star by 0.2 percent every 18 days. When the four transit events are superimposed, the lower phase curve is obtained, which differs very little from a model curve with clear planetary parameters.
DLR (CC-BY 3.0).
In 2009, NASA's Kepler space probe was launched, embarking on a mission to hunt for exoplanets. In 2013, due to the failure of two of its reaction wheels, the mission had to be modified. Mission control managed to change the operational modus and manoeuvre the telescope orbiter into a different position in its orbit around the Sun that enabled the mission to continue. The solar wind was used to stabilise the probe so that, in May 2014, the mission could be revived – this 'second life' was dubbed K2. Researchers from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), together with colleagues from other institutions, have now analysed the K2 data and discovered six new planets around other stars, including two extraordinary companions to a star one and a half times the mass of the Sun.
Although more than 3000 extrasolar planets have been discovered since 1996, none of these planets are alike. Each new discovery reveals the diversity and variations in the origin and development of planetary systems. The planets that have been confirmed and reported in publications in the last six months all have their peculiarities – there is either a brown dwarf orbiting a star in addition to the planet, or the star is a rare type, known, thus far, to harbour but a few planetary systems. The Kepler telescope observed a field of approximately 190,000 Milky Way stars in the Cygnus constellation for almost five years. Hundreds of so-called transit planets have been found in the collected data. From the observer's perspective, these planets pass in front of their star (transit), dimming its light by a small but measurable fraction.
A brown dwarf, a 'failed star', in a long-period orbit?
In the evaluation of the Kepler data, scientists working with Alexis Smith from the DLR Institute of Planetary Research in Berlin-Adlershof have, among others, identified two new companions to star K2-99. K2-99 is a very iron-rich star about 1.6 times the mass of the Sun. K2-99 is on its way to becoming a Red Giant, a star that has reached a late phase of stellar evolution – our Sun will reach this phase in five billion years, once it has fused all of its hydrogen into helium. "This star is being orbited by a Jupiter-like planet," says Smith, "but in contrast to Jupiter, which needs 12 years to complete one orbit around the Sun, K2-99b orbits its star in just 18 days. The interesting thing about K2-99 is that we also see signals from a second object in a long-period orbit of several hundred days –perhaps a brown dwarf." Brown dwarfs are of great interest because they fill the evolutionary gap between planets and stars and are considered to be 'failed stars', which we still do not know much about.
Orbit in the fast lane
But it was not just the star's two companions that caught the attention of the scientists: K2-99 itself is not a normal main sequence star, but rather a subgiant that shines slightly brighter than the Sun. Until now, only a few transit planets have been found orbiting such subgiants. Although the first exoplanets to be found were Jupiter-sized planets in close orbits – 'hot Jupiters', there are still some unanswered questions. Not all combinations of planetary parameters such as high or low density and shorter or longer orbital periods seem to be possible. Hence, for example, the planet K2-60b recently discovered by DLR researcher Philipp Eigmüller and his colleagues in the K2 observations is taking up an empty space among planets with a smaller radius than Jupiter. Moreover, this planet also orbits its star in a mere three days. For comparison, it takes the innermost and 'fastest' planet in the Solar System, Mercury, 88 days to complete one orbit around the Sun. Another exoplanet that was discovered in this analysis, and which received the astronomical designation K2-107b, also shows such a short, three-day orbit. Its radius is almost one-and-a-half times that of Jupiter.
In addition to the four planets described above, the DLR planetary researchers together with other European and US colleagues also discovered a Neptune-sized planet in a 10-day orbit around the star K2-98. In contrast to the ice giant Neptune, however, this planet must be very warm due to the proximity to its star. The scientists have calculated that precisely because of this proximity, K2-98b will be engulfed by its own star in approximately three billion years, when K2-98 has become a Red Giant. Finally, another 'hot Jupiter' was discovered around K2-31, which has a mass 1.8 times that of Jupiter and orbits its star in as little as 1.25 days.
Last modified:21/02/2017 16:32:03