Nemesis

But why should comets become more likely to hit us every million years? Here we launch ourselves into deep speculation. It has been suggested that the sun has a sister star, and the two orbit each other with a periodicity of about 26 million years. This hypothetical binary partner, which has never been seen but which has nevertheless been given the dramatic name Nemesis, passes, once per orbital rotation, through the so-called Oort Cloud, the belt of perhaps a trillion comets which orbits the...

 Nemisis

Piet Hut, another Dutch astronomer fifty years younger than Jan Oort, decided to take seriously the possibility that comet showers are periodic. If they are periodic, the theory that they are caused by the random passing-by of alien stars cannot be right. If showers are periodic, they must be explained by a different theory. Piet Hut and his friend Rich Muller found an alternative theory to explain the periodicity in case it turns out to be real. The alternative theory is called Nemesis. Neme...

 The Comet Loss Cone

To understand why comet showers occur, we go back to the Oort Cloud. The theory of comet showers was worked out by Jack Hills, an American physicist now at Los Alamos. He realized that the movements of the comets in the Oort Cloud are not entirely random. Comets in the cloud are generally moving in random directions, but if a comet happens to be moving in an orbit almost exactly toward the Sun, it will not survive for long. A comet in an orbit coming close to the Sun may get boiled away and d...

 Perceiving Infinite Suns in the Night Sky

Elpino. Why then do we not see the other bright bodies which are earths circling around the bright bodies which are suns? For beyond these we can detect no motion whatever; and why do all other mundane bodies (except those known as comets) appear always in the same order and at the same distance? Philotheo. The reason is that we discern only the largest suns, immense bodies. But we do not discern the earths because, being much smaller, they are invisible to us. Similarly it is not impossible...

 Losing the Human Perspective in the Vastness of the Cosmos

When I pause and reflect on our expanding universe, with its galaxies hurtling away from one another, embedded within the ever-stretching, four-dimensional fabric of space and time, sometimes I forget that uncounted people walk this Earth without food or shelter, and that children are disproportionately represented among them. When I pore over the data that establish the mysterious presence of dark matter and dark energy throughout the universe, sometimes I forget that every day—every twe...