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magine a world with no sunrises, where the sun has not risen for millions or billions of years, or perhaps ever. Imagine a world without a sun. Our ideas about how stars and planetary systems form suggest such planets should exist. With no nearby star to illuminate them, such planets are extremely hard to spot. However, they are now being found. So far the score is sits at about a dozen.
Stars and planets are the result of the collapse of clouds of cold cosmic gas and dust. As the lumps formed by the material grow, their cores become hotter and more compressed. If the density and temperature get high enough, nuclear fusion starts and we have a star. If the lump does not become high enough for nuclear fusion to begin, the object, which has become very hot during its formation, gradually cools off. The larger lumps are known as “brown dwarfs”. These radiate strongly in the infrared part of the spectrum, Many examples are known and they are being actively studied.
Jupiter, the largest planet in the Solar System, is too small to have become even a dwarf star, but it is still radiating heat from its formation. Even today, 4.5 billion years after it formed, it radiates more heat than it receives from the Sun. Incidentally, that interesting world is visible in the eastern sky late in the evening at the moment. Look for a bright, star-like object, resembling an aircraft landing light. Get out the binoculars or telescope and have a look. You will also see its four largest moons: Io, Europa, Ganymede and Callisto, on either side of the planet, like beads on a wire.
There are two main ways we get orphan planets. How the orphan turns out depends upon its origin. Firstly, if a collapsing cloud of gas and dust produces a shower of stars, plus a collection of lumps that were not large enough, mixed in with the stars would be numbers of orphan planets, formed many light years from any star, far from any source of light and heat. The second way would see the orphans being formed as part of a new planetary system, orbiting their new sun along with other planets. However, in the unstable youth of the system, some planets had close encounters with others, resulting in some falling to new positions closer to the star and others being catapulted out into space.
Our Earth inherited a lot of hydrogen and helium from its birth cloud. However, due to our planet being warmed by the Sun, these were lost. A planet born as an orphan would have been cold enough to retain them. If our Earth had been thrown out of the Solar System soon after it was born, how would it compare with an Earth-like world that was born an orphan?
Today, our ejected Earth would be colder than minus 220 C, coated with frozen gases, mainly nitrogen and an atmosphere of neon and a little helium. Paradoxically, the planet that was born an orphan would be a lot better off. It would have retained its youthful hydrogen and helium atmosphere, which would impose enough of a greenhouse effect for the heat from the core to have temperatures on the surface possibly high enough for liquid water to be present. Imagine life on such a world!
Saturn lies very low in the west after sunset, and is getting hard to see. Jupiter rises around 10 p.m., Mars comes up around 3 a.m. The Moon will reach first quarter on September 4.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.