How can we be alone?

The latest estimate is that there are around 6 billion Earth-like planets in our galaxy alone. However, when we really dig into the issue regarding what makes a planet suitable for life as we know it, this large number could be a considerable understatement.

Firstly we know about places where liquid water and warmth are available for living things, but otherwise they are very un-Earthlike, such as Europa, one of the moons of Jupiter, where tidal forces warm an ocean hidden under a roof of ice. For the moment let's just stick to the Earth-like planets. The starting point in identifying an Earth-like planet is that it is the right size, it has an atmosphere, and its surface temperature is high enough to support a water ocean.

There also needs to be a water cycle, where water evaporates from the ocean and returns to it as rain. If there are landmasses, they will be irrigated and material will be eroded from the land and taken into the sea as nutrients for living creatures.

However, there is a range of conditions under which this may happen.

Firstly, the planet should be in the “Goldilocks Zone”, where the planet receives enough warmth and light from its star to ensure a high enough surface temperature and to drive a water cycle. This is where the situation becomes more complicated. Planets, including ours, exist in a thermal equilibrium. Heat from our star warms our world. As the temperature rises, the Earth radiates increasing amounts of infrared, sending heat off into space. Eventually the input and output are equal and the planet's temperature stabilises.

Intriguingly though, if we do this calculation for the Earth, we find our planet should be frozen solid, with a mean temperature more or less equal to the Moon's, around -50C. This obviously isn't the case, and the explanation is the greenhouse effect. Gases such as water vapour, carbon dioxide and methane are “greenhouse gases,” which means they impeded the ability of a planet to re-radiate heat into space. The result is that in order to meet a balance of input and output, the planet has to be hotter.

Planets with lots of greenhouse gases can be further from their stars and still have comfortable temperatures. Planets with atmospheres low in greenhouse gases must be closer. The atmospheres of young planets are rich in greenhouse gases.

Over the 4.5 billion years since the Earth formed, the Sun has brightened steadily, but on Earth living things removed them and replaced them with oxygen, which is not a greenhouse gas, keeping our environment stable and our planet inhabitable. In the 1970's, James Lovelock proposed the Gaia

Hypothesis (Gaia is the Earth goddess), in which he proposed that once life is established, it has a certain power to keep its environment comfortable.

There are two other factors. Firstly there are clouds. Water evaporated from the oceans by solar heat forms clouds, which can reflect solar energy back into space, providing a stabilizing influence. Of course, more energy in the atmosphere can drive more severe weather.

Secondly there is dust. Every day, warm air heated by contact with warm ground rises, carrying dust with it. This can act as an insulator, keeping in heat, or as a reflector, sending it back out, depending on the grain size and the amount.

In addition to being the right distance from their stars, we need our planets to have an atmosphere and a signature of water vapour. If we see oxygen, which needs living things to produce and maintain it, we can be pretty sure there are living things. Maybe fortunately, the distances between stars ensure it will be a long time before we can interfere with our alien brethren or they with us.

--Jupiter

and Saturn rise in the southeast around midnight, with Mars following in the early hours. Venus lies low in the sunrise glow. The Moon will reach Last Quarter on the 12th.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory in Penticton.