MadSci Network: Astronomy
Query:

Re: How far does a planet need to be from a nova to survive it?

Date: Sat Apr 22 12:05:43 2006
Posted By: Peter Thejll, Staff, Solar-Terrestrial Physics,
Area of science: Astronomy
ID: 1145275797.As
Message:

This is a lovely question! It is very hard to answer, of course, as we know very little about any other life forms than those on Earth---nothing, in fact!

As a starting point, let's assume that we want to know what would happen to Earth if the Sun got brighter. Recurrent nova increase their luminosity by a very large amount---many orders of magnitude---and also spew particles and X-rays into space. We take just the simplest question, "What would happen to life on Earth, if the Sun's luminosity rose by a certain amount"?

The central point is that life as we know it depends almost entirely (except those underground bacteria and those worm-like creatures around the hot volcanic vents under the oceans) for sunlight on their sustenance, and any major change in that would spell big trouble.

During the solar cycle the Sun's luminosity changes by a few tenths of a percent, and life survives. During major excursions, such as the Little Ice Age, we estimate that the Sun's luminosity may have changed by 1%, and that, supposedly, caused major climate change in many areas---at the very least some people living in marginal regions (close to the tree line in mountain valleys, etc.) suffered. A few probably actually died, but that was mostly because of social factors (they were poor and sick and had nowhere else to go).

OK, so perhaps a 10% sudden (week to week or so) change in solar luminosity would spell really bad trouble for humanity in many areas and a 20% change would kill lots of people. This is still very little compared to what a nova undergoes, and the point is that it does not matter how far away from the Sun we are as long as life is adapted to current levels of solar luminosity---if life exists on Pluto it is probably accustumed to the small variations in solar irradiance due to the eccentric orbit of Pluto but given the special conditions on Pluto life would probably (I am guessing) be wiped out by any change that was "big" (say, bigger than 100%) compared to anything that occurs "usually."

Therefore, it is likely---if my guesswork is right---that any planet could not sustain life if life was already dependent on the stellar luminosity, if the star was irregular, as a nova is. This leaves the possibility that life exists which is not dependent on a star's luminosity being constant---and now the real guesswork starts!

One could imagine civilisations that were very smart and had worked out how to use fusion or fission as their main energy source, such a civilisation might be able to move into the depths of a planet every time the nova recurred. Then you want to know whether a planet evapoprates duing a nova outburst, right? As far as I know, the inner planets of our solar system, would survive the Sun becoming a red giant (which is somewhat like a nova in terms of expansion of gasses, but not, of course, in terms of searing heat and radiation) but would tend to alter their orbits and spiral into the Sun, perhaps leaving Mars and the outer planets more or less put.

In a recurrent nova system I am therefore pretty sure that planets like Mercury, Venus, Earth and perhaps Mars would be blasted on their surfaces by the radiation, but might actually survive as solid objects, only to be frictionally braked by the strong solar wind and suffer orbital decay into the remnant star, dissappearing after 10 nova cycles or so.

Hope this helps!

[Also note that a nova occurs in a system in which there are two stars, one of the stars being a white dwarf. That means that any planets in the system would have had to survive the first star changing from a regular star into a white dwarf. That process alone might be enough change to sterilize the surface of any of its planets hosting life. Moderator]


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