A shot in the dark
In ‘Mysteries of the Universe’ (January, p49) you suggest that “Maybe the Universe is filled with dark stars and dark planets and dark life!” I suggest this is extremely unlikely. The one thing that we can detect is the gravity dark matter has. If there was a planet in the Solar System made of dark matter, why has its effect on the orbits of the other planets not been detected? Our space probes often use the ‘slingshot effect’ of a planet’s gravity on their journey. Have any of them ever had to take into account the gravity of a dark planet?
On the other hand, perhaps the reason we haven’t observed the speculated ninth planet is because it’s made of dark matter. Since both dark matter and atomic matter possess gravity, both types of celestial bodies must be attracted to each other. What happens when they collide? Has anyone ever observed this? Barry Cash, Bristol
I wasn’t saying there are dark planets in our Solar System – although, interestingly, Neptune was the ‘dark matter’ of its day, detected by its gravitational effect on Uranus before it was actually seen. For extrasolar planets, we infer their existence generally from their gravitational tug on their parent
star. We don’t see them directly. We can deduce only their mass. We have no idea what kind of mass that is. It could be ordinary matter (which is the most likely possibility), but dark matter can’t be ruled out.
Marcus Chown, astrophysicist