Did you know that our own sun is in fact a dwarf star? It is a G type, or yellow dwarf. Next come the smaller K types, or orange dwarfs, followed by even smaller M types, or red dwarfs. While we know life has - at least once - arisen in a yellow dwarf system, could life arise on orange or red dwarf system? While the answer may be yest to both, some think that orange dwarfs may actually make the best place to find life.
Types of Dwarf Stars
- Yellow dwarfs (which are actually white to yellow in color), have surface temperatures of 5,000 - 6,000 K. The average size is a little larger than our sun (110% the size of the sun).
- Orange dwarfs (orange to red in color), have surface temperatures of 3,500 - 5,000 K. Their average size is about 90% that of the sun and are 40 % as luminous as the sun
- Red dwarfs (red in color), have surface temperatures below 3,500 K. The average size is about 40% that of the sun and are 4% as luminous as the sun. Red dwarfs are by far the most common star, accounting for over 75% of all the stars in our galaxy.
As you can tell, as the size decreases, the temperature decreases as well as the luminosity. The width of the Habitable Zone (HZ), thus, decreases as well and moves closer to the star (see illustration below) . But HZs still exist. And red dwarfs are extremely long lived - up to 10 trillion years - giving life plenty of time to arise. However, there are other considerations that need to be factored in. First, some tectonic activity is needed to help control the amount of C02 as well as for mixing chemicals - life needs to arise on a chemically dynamic planet. However too much tectonic activity can wipe life out. Planets orbiting in a red dwarf's HZ may experience extreme tidal forces. Another problem with red dwarfs is that being so close to the star there can be problems with radiation bursts from the star - red dwarfs tend to be rather cranky stars that frequently flare up, releasing dangerous bursts of radiation. Since the average HZ for a red dwarf is only 0.1 to 0.2 AU away, the HZ for a red dwarf may actually not be very hospitable. By the way, the HZ for yellow dwarfs, like our sun, is 0.8 AU to 2 AUs.
The average HZ for orange dwarfs is 0.3 to 1 AU away. Its HZ is wider, allowing for planets to be further from the star and thus experience less tidal forces. Also, their flare activity is only slightly more than yellow dwarfs. Another consideration is that orange dwarfs are longer lived than our sun - they have almost twice the life span: almost 20 billion years compare to the 10 billion or so years for our sun. Their light and heat output is much more stable than the sun, fluctuating less over its life and thus making more of its long lifespan useful to life. Orange dwarfs may just give life more chance to originate and thrive than yellow dwarfs like our sun. While not as common as red dwarfs (the most common star type), orange dwarfs are 3 - 4 times more common than yellow dwarfs. We definitely should not overlook orange dwarfs when searching for extraterrestrial life.
Illustration of HZs for Dwarf Stars: dM = Red Dwarfs (M class stars), dK = Orange Dwarfs (K class stars), and dG = Yellow Dwarfs (G class stars).Red Dwarf Support
Red dwarfs do have their supporters: the "Living with a Red Dwarf" Program, established at Villanova University by E. F. Guinan and S. G. Engle. Guinan and Engle, in their presentation at the 8th Pacific Rim Conference on Stellar Astrophysics in 2008, bring up an interesting consideration: the dangerous flares "are strongly dependent on rotation, and thus age, and diminish as the stars lose angular momentum and spin-down over time via magnetic braking" (Guinan 1). What this means is that the intensity and frequency of a red dwarfs solar flares may diminish over time. Since red dwarfs are extremely long lived, that does crack open just a bit further the door of opportunity for life to find a way to arise and thrive.
One interesting result of a planet orbiting in an orange or red dwarf's HZ is that it will most likely be phased locked because of the tidal forces; like Mercury is to our Sun, or the Moon is to the Earth, the planet's rotational period and orbital period will be the same, resulting in the same side of the planet always facing the star. This can create a small zone along the terminator line (where, on a phase locked planet, day and night perpetually meet) where life may be able to better survive the radiation bursts.
For an interesting related post, see the Color of Life post which tackles the question of what color would plants be on planets circling different types of stars.
Guinan, E. F. and S. G. Engle. "'Living with a Red Dwarf' Program." Summary Paper. Living with a Red Dwarf. Villanova University. n.d. Web. 18 June 2009. <http://www.astronomy.villanova.edu/lward/prcsa2008_LWARD_new.pdf>
Shiga, David. "Orange stars are just right for life." Space. New Scientist. 06 May 2009. Web. 18 June 2009. <http://www.newscientist.com/article/dn17084-orange-stars-are-just-right-for-life.html>.