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Here is an olde article of mine from 2001. Still relevant I hope. Enjoy.
Summary: We keep hearing of a new planet or moon that might hold life. Or of projects to monitor radio signals, or even send them a message. Finding life somewhere beyond Earth’s atmosphere would be wonderful. About five hundred planets have now been discovered. Many moons too. Is there life out there? Here is a summary of what has been found.
As recently as last October we heard of yet another new planet that might have life (“Gliese 581 g”). About 500 planets have been discovered now, most of these within the last twenty years. The quest goes on to find life “out there” on some planet or moon. Radio waves have been monitored, even messages sent.
What we seek, ideally, is another Earth. A rock-like world just under 10,000 miles in diameter, with a breathable atmosphere and water. Earth is actually about 7,900 miles in diameter, with an atmosphere roughly 78% nitrogen and 21% Oxygen. (All my figures will be rounded off here for convenience). The planet is located in the Sun’s “Goldilocks Zone”: the best distance away for a surface temperature that is “just right”. For Earth that’s a mean temperature of 14C ( and a minimum of minus 89C, maximum of 58C). Well, that’s the ideal we are looking for. Starting from Earth, and moving roughly outwards into space, let us see what planets and moons may hold life.
Our nearest neighbours: The Moon, Venus, Mars and Mercury
First, The Moon. With a diameter of 2,160 miles it has only a trace atmosphere (near-vacuum). The mean surface temperature is 123C by day, Minus 233 by night. So, almost no chance of life. “Signatures” of ice have been found in some “shielded” craters but... Best chance of life is in underground caverns.
Nearest planet Venus: about the same size as Earth. Located at the inner edge of our Goldilocks zone. Mid twentieth century scientists and artists depicted lush jungles or a planet-wide ocean. However, in the 1960s Russian and other probes found it was a true “Hell”. The surface temperature is over 460C (about the boiling point of lead). Much of the ground is covered by volcanic lava. The atmospheric pressure is 92 times that on Earth (densest atmosphere in the solar system). Only chance of life is either high in the carbon-dioxide and sulphuric acid clouds (!) or very deep underground.
To mention Mercury: nearest planet to the Sun. This planet is rather similar to the Moon but much hotter. Enough said. (Venus is even hotter on the surface than Mercury).
Mars, 4th planet from the Sun, is a much better prospect. At least you can stand on the surface there in a decent spacesuit without being fried, frozen or blown away. Its surface temperatures range from about 0C to minus 130C. There are traces of water about, notably at its polar ice caps. Solar radiation is a negative factor, even at that distance from the sun (about 125 million miles). With a diameter of 4225 miles, Mars only retains a thin atmosphere. Outside chance of bacterial life here, maybe underground.
The moons of Jupiter and Saturn
Io, the 3rd largest moon of gas giant Jupiter (5th planet) is our next port of call. Its diameter is a healthy 2264 miles. Io is noted for its volcanicity, especially plumes going 186 miles up. Its surface temperature is minus 73C to minus 143C. Thin sulphur dioxide atmosphere. There might be some life hugging those volcanoes. Io is often mentioned as maybe having life, anyway.
Next, Europa. This one is quite famous now. Smallest of the 4 “Gallilean Moons” , but possibly covered at least in part by a 62 mile thick water ocean, topped by ice sheets. Forget the surface: hydrothermal vents could be proving sufficient heat for life to flourish. This is one of the most exciting finds so far. Remember, Europa.
To gas giant Saturn now (6th planet) with its iconic rings. Next stop Enceladus, one of its smaller moons (diameter 310 miles). Bottom line here is that the “Cassini” probe flew through a plume of water vapour which came from Enceladus. Hence it is thought there may be lots of liquid water beneath the surface, all heated by tidal forces. There is plenty of carbon too...
Final stop within the solar system is Titan, Saturn’s largest moon. Titan resembles a young Primordial Earth, having a thick cloudy atmosphere of Nitrogen, Methane etc. It has lakes or seas of methane, but water is rock solid. If life can live in methane, instead of water, then bingo! If!
To the stars
That’s 6 out of about 500 planets covered. All but 2 of the rest are out there, orbiting other Stars. Most stars are red dwarves (spectral type M), but some are slightly bigger than the sun and white or yellow (F type), others are like the Sun (G type), and more are orange (K type) and smaller. F, G and K types look best (size and heat-wise) for having habitable planets. Some M types feature too however.
“Locally” our best candidate stars are: Alpha Centauri system (3 stars – G, K and M types) 4 Light Years away; Epsilon Eridani (K) 10 LY (2 proposed large planets); 61 Cygni A and B (both K) 11LY; Epsilon Indi A (K) 12LY, Tau Ceti (G) 12LY; Gliese 676 (M) 15LY (which has 4 big planets) and finally Gliese 581 (M) 20LY (6 planets known).
Epsilon Eridani and Tau Ceti have long been targeted as likely locations for “New Earth”. However, Tau Ceti has been “relegated” down the list as it lacks the metals required for Earthlike planets. Last September planet Gliese 581g was found: an earth sized world in its star’s Goldilocks zone. Only drawback is that the planet is tidally locked so only one side faces the sun (so no days or nights as such). Still, the best find so far amongst the stars.
So, I hope, like me you are ready for the next big discovery. At least one more rocky “terrestrial” planet or moon, revolving around a suitable star or (in the case of a moon) gas giant like Jupiter. When you hear of such a discovery, just remember you were “warned” about if first right here.
PS 2\1\2021 – Since I wrote the above, literally thousands of exoplanets have been discovered. Also they now believe that the Jupiter moon Ganymede (the largest satellite in the solar system) has a great ocean beneath its surface, rather like Europa apparently has. But otherwise I think this article covers our cosmic “locality” fairly well.
© Paul Butters 18\1\2001.
