Astronomy: Huge sodium cloud as possible indication of first exomoon
Although thousands of exoplanets are known, no moon has yet been found outside the solar system. This may now have changed.
Artist's impression of the volcanically highly active possible exomoon at WASP-49 b
(Image: NASA/JPL-Caltech)
A huge cloud of sodium around an exoplanet 635 light years away could be the result of a volcanically extremely active moon. If this is confirmed, it would be the first known exomoon. The evidence was compiled by a research group led by planetologist Apurva Oza from the California Institute of Technology, after the cloud itself was discovered in 2017. Among other things, because it seems to originate from a source that produces 100 tons of sodium per second and the exoplanet emits it, they speak of "very convincing" evidence. The movement of the sodium cloud itself also speaks for the existence of a moon, as it is sometimes traveling faster than the exoplanet. In addition, it keeps getting bigger, as if it were being refilled.
A second Io?
The observations reminded the group of Jupiter's moon Io, the most volcanically active celestial body in the solar system. It is repeatedly compressed by the gravitational pull of its planet, ejecting sulphur dioxide, sodium, potassium and other gases into space. The resulting clouds can reach an extent that is much larger than Jupiter itself. The team suspects something similar for the exoplanet WASP-49 b. Like its star –, – consists mainly of hydrogen and helium and only small amounts of sodium. Both therefore contain too little of this to explain the huge cloud. But even if the exoplanet could produce enough of it, there would be no process by which it could eject the sodium into space. The team therefore suspects that this is the trace of a volcanically active exomoon.
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Among other things, they were able to determine that the cloud is located far above the atmosphere of the exoplanet. It also moves in a way that no part of the atmosphere could. Computer models had shown that a moon that takes eight hours to complete an orbit could explain the movements and activity. Because it is likely to be deformed by the gravity of its planet in the same way as we know it from Io and thus lose a lot of mass, it could be heading for a violent end and be torn apart at some point. However, this is still just an assumption; confirmation that traces of an exomoon can be seen is still pending. The work is presented in the journal Astrophysical Journal Letters.
The huge sodium cloud is not the first indication of an exomoon, but it is a very different one from the previous ones. These have involved the dimming of stars that were attributed to possible moons. However, this interpretation has already been contradicted and other explanations are likely. With regard to this method, two astronomers have also determined that they could only discover exomoons in this way with the current technology, which would all be real oddities compared to the more than 300 moons in the solar system. Apurva Oza's group has now taken a completely different approach, so this restriction does not apply to them.
(mho)
