Orphan Worlds

In the sky this week…

  • Saturn lies very low in the west after sunset, and is getting hard to see.
  • Jupiter rises around 10 p.m. and Mars around 3 a.m.
  • The Moon will reach First Quarter on September 4.

Ken Tapping, August 31, 2011

Imagine 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 sits at about a dozen.

Stars and planets result from the collapse of clouds in 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 of which are known and are being actively studied.

This artist’s conception illustrates a Jupiter-like planet alone in the dark of space, floating freely without a parent star. Credit: NASA/JPL-Caltech

This artist’s conception illustrates a Jupiter-like planet alone in the dark of space, floating freely without a parent star. Credit: NASA/JPL-Caltech

Jupiter, the largest planet in the Solar System, is too small to have become a dwarf star, but it is still radiating heat from its formation. Even today, 4.5 billion years after it formed, Jupiter radiates more heat than it receives from the Sun. Incidentally, that interesting planet is currently visible in the eastern sky late in the evening. 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 on 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. Secondly, orphans are 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 –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!

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mail: ken.tapping@nrc-cnrc.gc.ca

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