August 19, 2017 | Henry Richter

Habitability Requires the Right Kind of Star

by Dr Henry Richter

This is a continuation of my discussion on the requirements to allow life to exist on a planet elsewhere in the universe. “Facing Reality about Life on Other Planets” (8/11/17) listed 20 features and characteristics, all of which would be required, for life to exist and flourish. That discussion began with the first characteristic: “a location in a ‘safe’ place in the galaxy, not close to any active regions, overactive stars, or black holes.” This chapter addresses the needed characteristics of the host star to which the planet under discussion is associated. It leads to a discussion of the planet’s orbital requirements as well.

Sunrise a few hours before a total eclipse in Baja, July 11, 1991, by David Coppedge

Star Types

It’s pretty obvious that to have a planet with life, there needs to be a star. We know there are all sorts of stars in the universe. Stars are classified by their spectra – the light they give off, and the portions of light that are absorbed by the elements in the star’s atmosphere, and their temperature. The star is the source of heat for the planets in their system, and therefore for a planet to have a livable temperature, the star’s characteristics and the planet’s position with relation to the star needs to be just right. The temperature of the planet-candidate is very crucial to life. Since life as we know it is water–based, the star’s temperature affects the water greatly. The livable temperature has to be in a rather narrow range – look at all the hysteria about global warming and the effect of just a few degrees’ variation.

The star needs to be within a fairly narrow range of: size, heat output, stability, and spectral composition. Stars are classified into seven main types. In order of decreasing temperature (and luminosity): O, B, A, F, G, K, and M. The temperatures are:

  • O – over 25,000 K
  • B – 11,000- 25,000 K
  • A – 7,500-11,000 K
  • F – 6,500 -7,500 K
  • G – 5,000-6,000K
  • K – 3,500-5000 K
  • M – under 3,400 K

So you see there is really a wide variety of star types, most of which are unsuitable to heat a planet to a livable temperature. Habitable zones are just not possible with most. Our G2 yellow dwarf is really the only possibility. And, how many of these are there? It is estimated that in the billion galaxies with a billion stars each, that there are about 1022 stars in the known universe. That’s 1 followed by 22 zeros. That is more stars in the universe than there are grains of sand on earth – deserts, shorelines, and elsewhere. Estimates are that only one in 10,000 is a G2 yellow dwarf, but that is still a lot of stars that can host planets on which life conditions are possible. Present estimates are that there are maybe 512 G2 stars within 100 light years of us, and these are the ones most easily examined for planets. But more about that in a later writing.

Image credit: Illustra Media

Our sun with a surface temperature of about 5,500 degrees Celsius and the radiating size it is puts out the right amount of heat to produce a habitable zone for a reasonable orbit, which is where our earth travels. The sun emits radiation over the whole electromagnetic spectrum of gamma rays, x-rays, ultraviolet, visible, infrared, microwave, and radio waves. The shorter wavelengths (gamma, x-ray, and ultraviolet) are ionizing in nature and are destructive to chemical and life structures. As we will see later, these do not reach the earth’s surface in any quantity. Visible light is what we see with and which is used in photosynthesis. It also provides some heat, as does the non-visible infrared rays.

The mass of the sun causes gravity which keeps the earth in its orbit around the sun. Let’s consider the earth’s orbit, at about 93,000,000 miles around the sun. It is at that distance that the proper heat balance is achieved giving us a livable climate. The orbit is nearly circular which is very important to maintain a uniform temperature. If the planets orbit deviated much from circular, when closer to the sun,  it would get warmer, and when further away, cooler. That would result in major climate changes each orbit which would not be good for living things. Calculations show that if the earth were 5% closer to the sun, the surface temperature would reach several hundred degrees. If it were 5% further away the earth would be perpetually frozen. So each feature of the existence of the earth as it flies through space turns out to be crucially important to the existence of life.

As the earth spins on its axis, it creates a night-to-day cycle over its surface. The length of each day and night needs to be such that the cycle could be tolerated by living things. Twenty four hours seems ideal for activity and then rest. Imagine if each day and night were one hour long due to a fast spin, which would make a brutal cycle. If each day and night were even 100 hours long, that would require long integration times between activity and rest. Some planets have a rotation that is locked to the orbital period so one side always faces the sun (like one side of the moon always faces the earth). That would be an impossible situation with one side always hot, and the other at the frigid temperature of space. So, again we have the optimum situation for our planet.

The axis of the earth is inclined about 23 degrees from the orbital plane. This is another important fact. As the earth moves around the sun over the course of a year, half of the time the northern region is inclined to the sun and the other half year inclined away, making seasons occur. One thing this temperature difference does is to create some circulation of the atmosphere and the oceans. I do not imagine that seasons are crucial to life, but they are sure convenient for agriculture and living an interesting life, so they seem to be for our welfare. We are fortunate that much of our environment is not just for our existence and survival, but to make our living easier.

One other feature of the solar system is the existence of gas giant planets beyond our earth orbit: an unusual arrangement for the exoplanets found around other stars, many of which have gas giants closer to their stars than Mercury is to the sun. In our solar system, the outer gas giants, particularly Jupiter, serve a function of attracting and absorbing much of the debris and junk flying around which would be injurious to life if they impacted the earth. Who knows of all that goes on around us that protects us from things unknown?

In the next article, we will examine other requirements for habitability.


Dr Henry Richter, a contributor to Creation-Evolution Headlines, was a key player at NASA/JPL in the early days of the American space program. With a PhD in Chemistry, Physics and Electrical Engineering from Caltech), Dr Richter brings a perspective about science with the wisdom of years of personal involvement. His book America’s Leap Into Space: My Time at JPL and the First Explorer Satellites (2015), chronicles the beginnings of the space program based on his own records and careful research into rare NASA documents, providing unequaled glimpses into events and personnel in the early days of rocketry that only an insider can give. His next book, Spacecraft Earth: A Guide for Passengers, is due out later in 2017. For more about Dr Richter, see his Author Profile.

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