Alien-friendly zone gets extended

The chances of finding life around an M dwarf star have just been increased, as researchers suggest that the habitable zone is much larger than originally thought.

Artist’s impression of a planet orbiting an M dwarf star. (Image: Lynette Cook/NASA)

The habitable zone is the region at a particular distance from a star where a delicate dance between a planet’s atmosphere and the radiation received from the star can permit the existence of liquid water. This zone needs to be not too hot, not too cold, but just right which sometimes earns it the nickname the Goldilocks Zone.

But just how large is the area where it is “just right”? Much larger than we originally thought, at least around M dwarf stars – a type of red dwarf – according to astronomers Manoj Joshi and Robert Haberle.

M dwarf stars are more plentiful than solar type stars, which means there is a good chance of finding a life bearing planet around an M dwarf star. They are much cooler than the Sun and they emit light at much longer wavelengths, which could result in the habitable zone being extended by up to 30 per cent. At longer wavelengths snow and ice reflect less light, and this reflectivity is known as albedo.

Light that would be reflected by ice and snow on a planet orbiting a Sun like star is now absorbed when the source is an M dwarf star. If the albedo is low, then less of the essential heat needed for life is being reflected away from the planet. Thus the outer edge of the habitable zone, which was previously “too cold”, now becomes “just right.”

The outer edge of the habitable zone is typically defined as the point where carbon dioxide starts to condense. Decreasing the albedo results in this point being pushed outwards. These new revelations have no impact on the inner edge of the habitable zone, which is the point where the oceans evaporate, as any ice and snow cover would be too small to influence the albedo significantly.

Joshi and Haberle looked at the stars Gliese 436 and GJ 1214 in order to calculate what the albedo would be if there was snow and ice present on terrestrial planets around these stars. If a planet was half covered in snow or ice, and the other half was land or ocean, then the albedo is reduced significantly. Reducing the albedo will warm the planet, which in turn can cause some of the snow and ice to melt in what is known as a feedback effect. However, this temperature is likely to stabilise after some time.

A typical concern of potential habitable planets around M dwarfs is that they are orbiting so close to their relatively cool parent star that they become tidally locked. This means that one side of the planet always faces the star and the other side is in constant darkness. The extension of the habitable zone by up to 30 per cent doesn’t negate this effect however. “The tidal lock radius is quite a way outside the outer edge of the habitable zone,” Joshi tells Skymania News. “I don’t think that planets on the outer edge of the HZ would be significantly less likely to be tidally locked.”

However this is not all bad news because if the atmosphere was thick enough then it is possible for heat to be transported around the planet to the cool side, giving better odds for the existence of life.

Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

3 thoughts on “Alien-friendly zone gets extended

  • 10/27/2011 at 4:11 am

    Stephen Hawking spoke of “God” in a metaphoric­al sense, such as in A Brief History of Time: “If we discover a complete theory, it would be the ultimate triumph of human reason — for then we should know the mind of God.” In the same book he suggested the existence of God was unnecessar­y to explain the origin of the universe. His 2010 book The Grand Design and interviews with the Telegraph and the Channel 4 documentar­y Genius of Britain, clarify that he does “not believe in a personal God”.

    Hawking writes, “The question is: is the way the universe began chosen by God for reasons we can’t understand­, or was it determined by a law of science? I believe the second.” He adds, “Because there is a law such as gravity, the Universe can and will create itself from nothing.”

    Carl Sagan pointed out how, in Hindu cosmology, the universe undergoes an infinite number of deaths and rebirths, and its timescales are in the same ballpark as those of modern cosmology. Here is a quote from Cosmos:

    “There is the deep and appealing notion that the universe is but a dream of the god who, after a hundred Brahma years, dissolves himself into a dreamless sleep. The universe dissolves with him – until, after another Brahma century, he stirs, recomposes himself and begins again to dream the cosmic dream.

    Meanwhile, elsewhere, there are an infinite number of universes, each with its own god dreaming the cosmic dream. These great ideas are tempered by another, perhaps greater.

    It is said that men may not be the dreams of gods, but rather that the gods are the dreams of men.”

    In the book, Cosmos by Carl Sagan, he hypothesiz­ed that, that the universe evolved to the point, that it could look upon itself. We are the end result of the Big Bang, 13.7 Billion years ago, and that the universe itself evolved to become sentient though us. WE are the universe looking at ourselves.

    Who are we? We find that we live on an insignific­ant planet of a humdrum star lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people. – Carl Sagan

  • 10/29/2011 at 12:33 am

    Maybe some day we will get a visit from et after all Dout it will be in my life time i would love the chance to see man kinds first et meeting if it not happenedf already and no body told us but if the crab keeps as it is it will have its very own eaarth in about 3 billion years then life shortly after that at the rate its growing. I mite be wrong with the crab but i red some wer out ther ther is a new earth growing. If any body can tell me wer it is i wood b thank full.

  • 11/20/2011 at 6:39 am

    Good news.I think the greatest discovery in human history is not that far away!

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