Kepler 438 - a Red Dwarf with extreme stellar eruptions - - The Exoplanet Directory

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November 19, 2015

Kepler-438b is one of the very Earth-like planets. It orbits a 472 light years distantRed Dwarf of about half the solar mass and 0.044-times the luminosity of the Sun. The age of the star is estimated at about 4.4 billion years. The star is also only slightly more metallic than the sun [1].

Comparison of Kepler-438b with the terrestrial planets of the Solar System. Numbers are in Earth radii.

The planet orbits its star at a distance of 0.166 astronomical units or 73 star radii on an almost circular orbit. One orbit takes 35.23 days. At this distance to the star, the planet receives approximately 1.4 times the solar radiation of Earth. It may be a rocky planet with a probability of 69.6% [1].

Comparison of the orbital distance of Kepler-438b with Mercury. Numbers are given in astronomical units (1 AU = Earth's orbital distance).

Kepler-438b was discovered using a new evaluation method. Due to the weakness of the signal and other disturbing influences, confirmation by means of other optical methods was not possible so far. Instead, known spurious signals and false-positive signals were added to the data set in a model simulation in order to be able to judge whether it is a real signal - this is a statistical confirmation. The planet passed the test and actually appears to exist [1].

Artistic representation of Kepler-438b as seen from orbit. The planet is shown as a hot Super-Earth with a dense, sulfuric atmosphere.

The planet also has a high probability that surface conditions correspond to an Earth-like, escalating greenhouse effect or even to the present state of Venus. The climate depends of cause on the exact atmospheric composition and the models used. But it is certainly a hot world [1].

Artistic view of the upper atmosphere of Kepler-438b. The entire planet is covered in smog and sulphuric clouds.

Intense solar storms seen in Kepler-438

An interesting new twist comes from an investigation at the University of Warwick under the direction of Dr. David Armstrong. The data collected during the original campaign was analyzed again and it showed that Kepler-438 generates very violent solar storms. Every few hundred days a solar storm was be registered, and each of these events was more violent than all the known sunbursts of our own Sun, sometimes up to ten times as intense [2].

In each of these eruptions, the star released a lot of energy in the form of ultraviolet radiation as well as X-rays and gamma radiation. An atmosphere like that of Earth can absorb a large portion of this radiation, so that only very little of it penetrates the surface. On the other hand, the coronal mass ejections released during the solar storms are more critical. These are electrically charged particles, which are emitted at high speed and collide with the atmosphere. On impact, gas particles of the upper atmosphere will be removed, resulting in a gradual ablation of the atmosphere.

On Earth's surface, the magnetic field of our planet protects us from the harmful effects of particle storms. Whether Kepler-438b has a magnetic field is not known. However, it is believed that the Earth's magnetic field is generated by the planet's rotating iron core. Kepler-438b, however, is so close to its star that the tidal forces from the star must have already slowed the rotation of the planet. In this case, the magnetic field would also be extinguished and the coronal mass ejection would have removed the atmosphere of the planet. Instead of being "life-friendly", as suggested in some press reports, Kepler-438b would rather be an airless and dry desert [2].

However, some questions still remain. The exact extent of the tidal braking of the planet depends on the time, and therefore the age of the planetary system as a whole as well as the original angular momentum of the planet. There is also the possibility that the planet has not formed on the same orbit on which it was observed. In the case of a formation in cooler areas of the planetary system farther outside, it might have had a larger stock of water and gas, and might have reached its current orbit by gravitational interaction with other bodies of the solar system. In this case, the ablation of its atmosphere would not be complete.

However, the results indicate so far that Kepler-438b is not a life-friendly planet.

1. Torres et al. (2015): Validation of Twelve Small Kepler Transiting Planets in the Habitable Zone.
2. Armstrong et al. (2015): The Host Stars of Keplers Habitable Exoplanets: Superflares, Rotation and Activity.

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