Tag Archives: Life

What did early church fathers think about abortion and infanticide?

Unborn baby scheming about early church traditions
Unborn baby scheming about early church traditions

This is from Birds of the Air.


Recently I came across a reading of the Didache. “The what?” you may ask. The Didache is a book written somewhere in the first or second century. For a long time it was up for consideration as Scripture. It was believed to be the Teaching of the Twelve Apostles. Eventually it was agreed that the book was an excellent book, but not inspired Scripture. So I was pleased to be able to download this admirable book containing good teachings from the early Church fathers.

The book seemed to be largely a lot of quotes from Scripture. You’ll learn the basic rules of Christianity — “First, you shall love God who made you; second, love your neighbor as yourself.” You’ll learn that “grave sins” are forbidden, like adultery, murder, fornication, and so on. (They specifically include pederasty in the list.) There are instructions regarding teachers, prophets, Christian assembly, and so on. Lots of the normal, good stuff. But, since this was written sometime prior to 200 AD, I was somewhat surprised at this instruction: “You shall not murder a child by abortion” (Didache, Ch 2).

I got curious about what babies look like when they are just a few weeks old, so I went looking for pictures of them.

This post from Life News has ten excellent pictures of life inside the womb.

Here’s my favorite from 10 weeks:

Unborn Baby - 10 weeks old
Unborn Baby – 10 weeks old

This is a first trimester baby!

I decided to go hunting to see what is developed at this time, and found this list:

  • From this week until birth, the developing organism is called a fetus.
  • The fetus is now the size of a small strawberry.
  • The feet are 2mm long (one tenth of an inch).
  • The neck is beginning to take shape.
  • The body muscles are almost developed. Baby has begun movement.
  • While still too small for you to feel, your little one is wriggling and shifting.
  • The jaws are in place. The mouth cavity and the nose are joined.
  • The ears and nose can now be seen clearly.
  • Fingerprints are already evident in the skin.
  • Nipples and hair follicles begin to form.

The unborn baby is now called a fetus. Though the fetus is constantly moving, you will not be able to actually feel fetal movement for several more weeks. All of the organs, muscles, and nerves are in place and beginning to function. As the hands and feet develop fingers and toes, they have lost their paddle like look. The touch pads on the fingers form and already have fingerprints.

During this week of pregnancy the crown to rump length of the fetus is 0.9 inch to 1.2 inches (22 to 30mm), weight 0.07 ounce (2gm). They are now on the way to forming their testicles or ovaries, getting ready for the next generation. Until the ninth week of fetus development, the fetal reproductive apparatus is the same one for the both sexes. The head is still large and curves into chest.

Each week your uterus grows larger with the baby growing inside it. You may begin to see your waistline growing thicker by this time. A pelvic exam will detect that your uterus has grown from it’s normal, size of your fist, to a little bigger than a grapefruit.


Philosopher Doug Groothuis explains the logic of the pro-life position

I'm Scheming Unborn Baby, and I approve this study
I’m Scheming Unborn Baby, and I approve this message

At Christian Post, an article by Douglas Groothuis. (H/T Mary)

Here’s the most useful bit:

When we separate personhood from humanity, we make personhood an achievement based on the possession of certain qualities. But what are these person-constituting qualities? Some say a basic level of consciousness; others assert viability outside the womb; still others say a sense of self-interest (which probably does not obtain until after birth). All of these criteria would take away humanity from those in comas or other physically compromised situations.4 Humans can lose levels of consciousness through injuries, and even infants are not viable without intense and sustained human support. Moreover, who are we to say just what qualities make for membership in the moral community of persons?5 The stakes are very high in this question. If we are wrong in our identification of what qualities are sufficient for personhood and we allow a person to be killed, we have allowed the wrongful killing of nothing less than a person. Therefore, I argue that personhood should be viewed as a substance or essence that is given at conception. The fetus is not a lifeless mechanism that only becomes what it is after several parts are put together—as is the case with a watch or an automobile. Rather, the fetus is a living human organism, whose future unfolds from within itself according to internal principles. For example, the fertilized ovum contains a complete genetic code that is distinct from that of the mother or father. But this is not a mere inert blueprint (which is separable from the building it describes); this is a living blueprint that becomes what its human nature demands.

Yet even if one is not sure when personhood becomes a reality, one should err on the side of being conservative simply because so much is at stake. That is, if one aborts a fetus who is already a person, one commits a deep moral wrong by wrongfully killing an innocent human life. Just as we do not shoot target practice when we are told there may be children playing behind the targets, we should not abortion fetuses if they may be persons with the right not to be killed. As I have argued, it cannot be disputed that abortion kills a living, human being.

Many argue that outside considerations experienced by the mother should overrule the moral value of the human embryo. If a woman does not want a pregnancy, she may abort. But these quality of life considerations always involve issues of lesser moral weight than that of the conservation and protection of a unique human life (which considers the sanctity or innate and intrinsic value of a human life).6 An unwanted pregnancy is difficult, but the answer is not to kill a human being in order to end that pregnancy.

I think that the real question in the abortion debate right now is whether a living organism with a human nature and a human genetic code that is distinct from its mother and father deserves the right to life, or whether it needs to develop some other capability in order to be worthy of protection from violence.

Consider something from philosopher Francis J. Beckwith.


Some argue that personhood does not arrive until brain waves are detected (40 to 43 days).11Others, such as Mary Anne Warren,12 define a person as a being who can engage in cognitive acts such as sophisticated communication, consciousness, solving complex problems, self-motivated activity and having a self-concept. This would put the arrival of personhood at some time after birth. Still others, such as L. W. Sumner, 13 hold a more moderate position and argue that human personhood does not arrive until the fetus is sentient, the ability to feel and sense as a conscious being. This, according to Sumner, occurs possibly as early as the middle weeks of the second trimester of pregnancy and definitely by the end of that trimester.

Although these criteria differ from each other in important ways, they all have one thing in common: each maintains that if and only if an entity functions in a certain way are we warranted in calling that entity a person. Defenders of these criteria argue that once a human being, whether born or unborn, acquires a certain function or functions–whether it is brain waves, rationality, sentience, etc.– it is then and only then that a person actually exists. Those who defend these personhood criteria typically make a distinction between “being a human” and “being a person.” They argue that although fetuses are members of the species homo sapiens, and in that sense are human, they are not truly persons until they fulfill a particular set of personhood criteria.

Although functional definitions of personhood may tell us some conditions that are sufficient to say that a being is a person, they are not adequate in revealing to us all the conditions that are sufficient for a particular being to be called a person. For example, when a human being is asleep, unconscious, and temporarily comatose, she is not functioning as a person as defined by some personhood criteria. Nevertheless, most people would reject the notion that a human being is not a person while in any of these states. In other words, while personhood criteria, such as the ones presented by Warren can tell us that a being is a person, these criteria are not adequate to declare a being a non-person: The exercise of rational thought tells us that a being is a person; when that person is sleeping, and thus is not exercising rational thought, that lack of exercise of the thought function does not make her a non-person at that time. Consequently, it seems more consistent with our moral intuitions to say that personhood is not something that arises when certain functions are in place, but rather is something that grounds these functions, whether or not they are ever actualized in the life of a human being. Thus, defining personhood strictly in terms of function is inadequate.

If you are pro-life because of your feelings, or because someone told you to be, you ought to know that being pro-life is quite rational and supported by medical evidence. People who are pro-abortion are pro-abortion because they want recreational sex without the complications of having to care for the consequences (babies!) of their own actions. Even if they do not engage in the sex and the abortions themselves, they advocate for abortion rights, and they are guilty of encouraging a culture where 57 million unborn children have died since 1973. We’re long past Stalin numbers with this thing now.

We ought to care about not hurting other people. If grown-up’ selfish pursuit of happy feelings conflicts with another person’s right to life, then maybe we need to take a step back from being happy and start trying to be good instead.

If unborn babies don’t have consciousness or don’t feel pain, may we kill them?

Unborn baby scheming about pro-life apologetics
Unborn baby scheming about pro-life apologetics

Was having a conversation by e-mail yesterday with a pro-abortion atheist, and he gave two reasons why he supported abortion in the first and second trimester. First, he said that unborn babies can’t feel pain, so it’s OK to kill them. Second, he said that unborn babies don’t have consciousness, so it’s OK to kill them. I thought it might be useful to link to something that answers both of these objections.

Frank Beckwith is the author of “Defending Life: A Moral and Legal Case Against Abortion Choice“. He wrote that book for Cambridge University Press, a top academic press. But before Cambridge University Press, Beckwith wrote four easy-to-understand essays for the Christian Research Journal. Part IV is the one that has the response to the two questions raised by my atheist friend.

Part I. The Appeal to Pity

Part II. Arguments from Pity, Tolerance, and Ad Hominem

Part III. Is The Unborn Human Less Than Human?

Part IV. When Does a Human Become a Person?


Some ethicists argue that the unborn becomes fully human sometime after brain development has begun, when it becomes sentient: capable of experiencing sensations such as pain. The reason for choosing sentience as the criterion is that a being that cannot experience anything (i.e., a presentient unborn entity) cannot be harmed. Of course, if this position is correct, then the unborn becomes fully human probably during the second trimester and at least by the third trimester. Therefore, one does not violate anyone’s rights when one aborts a nonsentient unborn entity. [13]

There are several problems with this argument. First, it confuses harm with hurt and the experience of harm with the reality of harm. [14] One can be harmed without experiencing the hurt that sometimes follows from that harm, and which we often mistake for the harm itself. For example, a temporarily comatose person who is suffocated to death “experiences no harm,” but he is nevertheless harmed. Hence, one does not have to experience harm, which is sometimes manifested in hurt, in order to be truly harmed.

Second, if sentience is the criterion of full humanness, then the reversibly comatose, the momentarily unconscious, and the sleeping would all have to be declared nonpersons. Like the presentient unborn, these individuals are all at the moment nonsentient though they have the natural inherent capacity to be sentient. Yet to countenance their executions would be morally reprehensible. Therefore, one cannot countenance the execution of some unborn entities simply because they are not currently sentient.

Someone may reply that while these objections make important points, there is a problem of false analogy in the second objection: the reversibly comatose, the momentarily unconscious, and the sleeping once functioned as sentient beings, though they are now in a temporary state of nonsentience. The presentient unborn, on the other hand, were never sentient. Hence, one is fully human if one was sentient “in the past” and will probably become sentient again in the future, but this cannot be said of the presentient unborn.

There are at least three problems with this response. First, to claim that a person can be sentient, become nonsentient, and then return to sentience is to assume there is some underlying personal unity to this individual that enables us to say that the person who has returned to sentience is the same person who was sentient prior to becoming nonsentient. But this would mean that sentience is not a necessary condition for personhood. (Neither is it a sufficient condition, for that matter, since nonhuman animals are sentient.) Consequently, it does not make sense to say that a person comes into existence when sentience arises, but it does make sense to say that a fully human entity is a person who has the natural inherent capacity to give rise to sentience. A presentient unborn human entity does have this capacity. Therefore, an ordinary unborn human entity is a person, and hence, fully human.

Second, Ray points out that this attempt to exclude many of the unborn from the class of the fully human is “ad hoc and counterintuitive.” He asks us to “consider the treatment of comatose patients. We would not discriminate against one merely for rarely or never having been sentient in the past while another otherwise comparable patient had been sentient….In such cases, potential counts for everything.” [15]

Third, why should sentience “in the past” be the decisive factor in deciding whether an entity is fully human when the presentient human being “is one with a natural, inherent capacity for performing personal acts?” [16] Since we have already seen that one does not have to experience harm in order to be harmed, it seems more consistent with our moral sensibilities to assert that what makes it wrong to kill the reversibly comatose, the sleeping, the momentarily unconscious, and the presentient unborn is that they all possess the natural inherent capacity to perform personal acts. And what makes it morally right to kill plants and to pull the plug on the respirator-dependent brain dead, who were sentient “in the past,” is that their deaths cannot deprive them of their natural inherent capacity to function as persons, since they do not possess such a capacity.

These four essays are a very good introduction to common responses to pro-abortion arguments. I recommend that people get familiar with this, as once you look into it, you will see that the abortion issue can be debated with as much confidence as William Lane Craig defends Christian theism. You will have the same access to scientific evidence and rational arguments on this topic, and so you will have the upper hand. And that’s fun.

The best introductory book on the abortion / right to life issue is “The Case for Life” by pro-life debater Scott Klusendorf. The best comprehensive book is a tie between “The Ethics of Abortion” by Christopher Kaczor, and Frank Beckwith’s “Defending Life: A Moral and Legal Case Against Abortion Choice“.

Four ways that the progress of experimental science conflicts with atheism

Apologetics and the progress of science
Apologetics and the progress of science

When people ask me whether the progress of science is more compatible with theism or atheism, I offer the following four basic pieces of scientific evidence that are more compatible with theism than atheism.

Here are the four pieces of evidence best explained by a Creator/Designer:

  1. the kalam argument from the origin of the universe
  2. the cosmic fine-tuning (habitability) argument
  3. the biological information in the first replicator (origin of life)
  4. the sudden origin of all of the different body plans in the fossil record (Cambrian explosion)

And I point to specific examples of recent discoveries that confirm those four arguments. Here are just a few of them:

  1. An explanation of 3 of the 6 experimental evidences for the Big Bang cosmology (From an article from Caltech)
  2. Examples of cosmic fine-tuning to allow the existence of conscious, embodied life (From the New Scientist)
  3. Evidence that functional protein sequences are beyond the reach of chance, (from Doug Axe’s JMB article)
  4. Evidence showing that Ediacaran fauna are not precursors to the Cambrian fossils, (from the journal Nature)

Atheists will typically reply to the recent scientific discoveries that overturned their speculations like this:

  1. Maybe the Big Bang cosmology will be overturned by the Big Crunch/Bounce so that the universe is eternal and has no cause
  2. Maybe there is a multiverse: an infinite number of unobservable, untestable universes which makes our finely-tuned one more probable
  3. Maybe the origin of life could be the result of chance and natural processes
  4. Maybe we will find a seamless chain of fossils that explain how the Cambrian explosion occurred slowly, over a long period time

Ever heard any of these responses?

Below I list some resources to help you to respond to the four responses of atheists to the experimental data.

1) The Big Crunch/Bounce has been disproved theoretically and experimentally.


Nature 302, 505 – 506 (07 April 1983); doi:10.1038/302505a0

The impossibility of a bouncing universe


*Center for Theoretical Physics, Laboratory for Nuclear Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

†Department of Physics, University of California, Irvine, California 92717, USA

Petrosian1 has recently discussed the possibility that the restoration of symmetry at grand unification in a closed contracting Robertson–Walker universe could slow down and halt the contraction, causing the universe to bounce. He then went on to discuss the possibility that our universe has undergone a series of such bounces. We disagree with this analysis. One of us (M.S.) has already shown2 that if a contracting universe is dominated by radiation, then a bounce is impossible. We will show here two further results: (1) entropy considerations imply that the quantity S (defined in ref. 1 and below), which must decrease by ~1075 to allow the present Universe to bounce, can in fact decrease by no more than a factor of ~2; (2) if the true vacuum state has zero energy density, then a universe which is contracting in its low temperature phase can never complete a phase transition soon enough to cause a bounce.


The universe is not only expanding, but that expansion appears to be speeding up. And as if that discovery alone weren’t strange enough, it implies that most of the energy in the cosmos is contained in empty space — a concept that Albert Einstein considered but discarded as his “biggest blunder.” The new findings have been recognized as 1998’s top scientific breakthrough by Science magazine.

[…]The flood of findings about the universe’s expansion rate is the result of about 10 years of study, said Saul Perlmutter, team leader of the Supernova Cosmology Project at Lawrence Berkeley National Laboratory.

Perlmutter and others found such a yardstick in a particular kind of exploding star known as a Type 1A supernova. Over the course of several years, the astronomers developed a model to predict how bright such a supernova would appear at any given distance. Astronomers recorded dozens of Type 1A supernovae and anxiously matched them up with redshifts to find out how much the universe’s expansion was slowing down.

To their surprise, the redshift readings indicated that the expansion rate for distant supernovae was lower than the expansion rate for closer supernovae, Perlmutter said. On the largest scale imaginable, the universe’s galaxies appear to be flying away from each other faster and faster as time goes on.

“What we have found is that there is a ‘dark force’ that permeates the universe and that has overcome the force of gravity,” said Nicholas Suntzeff of the Cerro Tololo Inter-American Observatory, who is the co-founder of another group called the High-z Supernova Search Team. “This result is so strange and unexpected that it perhaps is only believable because two independent international groups have found the same effect in their data.”

There has only been one creation of the universe, and the universe will never reverse its expansion, so that it could oscillate eternally. That view is popular, perhaps in part because many people watched videos of Carl Sagan speculating about it in public school classrooms, but all it was was idle naturalistic speculation, (Sagan was a naturalist, and held out hope that science would vindicate naturalism), and has been contradicted by good experimental science. You should be familiar with the 3 evidences for the Big Bang (redshift, light element abundances (helium/hydrogen) and the cosmic microwave background radiation. There are others, (radioactive element abundances, second law of thermodynamics, stellar lifecycle), but those are the big three. Point out how the experimental evidence for the Big Bang has piled up, making the problem even worse for the eternal-universe naturalists.

2) The multiverse has not been tested experimentally, it’s pure speculation.


Multiverse thinking or the belief in the existence of parallel universes is more philosophy or science fiction than science. ”Cosmology must seem odd to scientists in other fields”.

George Ellis, a well-known mathematician and cosmologist, who for instance has written a book with Stephen Hawking, is sceptical of the idea that our universe is just another universe among many others.

A few weeks ago, Ellis, professor emeritus of applied mathematics at the University of Cape Town, reviewed Brian Greene’s book The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos (Knopf/Allen Lane, 2011) in the journal Nature. He is not at all convinced that the multiverse hypothesis is credible: ”Greene is not presenting aspects of a known reality; he is telling of unproven theoretical possibilities.”

According to professor Ellis, there is no evidence of multiverses, they cannot be tested and they are not science.

Ellis is not the only multiverse sceptic in this universe. A few months ago, science writer John Horgan wrote a column in Scientific American, expressing his doubt in multiverses.

When you get into a debate, you must never ever let the other side get away with asserting something they have no evidence for. Call them on it – point out that they have no evidence, and then hammer them with evidence for your point. Pile up cases of fine-tuning on top of each other and continuously point out that they have no experimental evidence for their speculations. Point out that more evidence we get, the more cases of fine-tuning we find, and the tougher the problem gets for naturalists. There is no evidence for a multiverse, but there is evidence for fine-tuning. TONS OF IT.

3) Naturalistic theories for the origin of life have two problems: can’t make the amino acids in an oxydized atmosphere and can’t make protein and DNA sequences by chance in the time available.

Building blocks:

The oxidation state of Hadean magmas and implications for early Earth’s atmosphere

Dustin Trail, E. Bruce Watson & Nicholas D. Tailby

Nature 480, 79–82 (01 December 2011) doi:10.1038/nature10655

[…]These results suggest that outgassing of Earth’s interior later than ~200?Myr into the history of Solar System formation would not have resulted in a reducing atmosphere.

Functional protein sequences:

J Mol Biol. 2004 Aug 27;341(5):1295-315.

Estimating the prevalence of protein sequences adopting functional enzyme folds.

Axe DD.

The Babraham Institute, Structural Biology Unit, Babraham Research Campus, Cambridge CB2 4AT, UK. doug.axe@bbsrc.ac.uk

Proteins employ a wide variety of folds to perform their biological functions. How are these folds first acquired? An important step toward answering this is to obtain an estimate of the overall prevalence of sequences adopting functional folds.

[…]Starting with a weakly functional sequence carrying this signature, clusters of ten side-chains within the fold are replaced randomly, within the boundaries of the signature, and tested for function. The prevalence of low-level function in four such experiments indicates that roughly one in 10(64) signature-consistent sequences forms a working domain. Combined with the estimated prevalence of plausible hydropathic patterns (for any fold) and of relevant folds for particular functions, this implies the overall prevalence of sequences performing a specific function by any domain-sized fold may be as low as 1 in 10(77), adding to the body of evidence that functional folds require highly extraordinary sequences.

So atheists are in double jeopardy here. They don’t have a way to build the Scrabble letters needed for life, and they don’t have a way to form the Scrabble letters into meaningful words and sentences. Point out that the more research we do, the tougher the problem gets to solve for naturalists, and the more it looks like an effect of intelligence. Write out the calculations for them.

4) The best candidate to explain the sudden origin of the Cambrian era fossils was the Ediacaran fauna, but those are now recognized as not being precursors to the Cambrian fossils.

Science Daily reports on a paper from the peer-reviewed journal Science:

Evidence of the single-celled ancestors of animals, dating from the interval in Earth’s history just before multicellular animals appeared, has been discovered in 570 million-year-old rocks from South China by researchers from the University of Bristol, the Swedish Museum of Natural History, the Paul Scherrer Institut and the Chinese Academy of Geological Sciences.

[…]This X-ray microscopy revealed that the fossils had features that multicellular embryos do not, and this led the researchers to the conclusion that the fossils were neither animals nor embryos but rather the reproductive spore bodies of single-celled ancestors of animals.

Professor Philip Donoghue said: “We were very surprised by our results — we’ve been convinced for so long that these fossils represented the embryos of the earliest animals — much of what has been written about the fossils for the last ten years is flat wrong. Our colleagues are not going to like the result.”

Professor Stefan Bengtson said: “These fossils force us to rethink our ideas of how animals learned to make large bodies out of cells.”

The trend is that there is no evolutionary explanation for the body plans that emerged in the Cambrian era. If you want to make the claim that “evolution did it”, then you have to produce the data today. Not speculations about the future. The data we have today says no to naturalism. The only way to affirm naturalistic explanations for the evidence we have is by faith. But rational people know that we need to minimize our leaps of faith, and go with the simplest and most reasonable explanation – an intelligence is the best explanation responsible for rapid generation of biological information.


I do think it’s important for Christians to focus more on scientific apologetics and to focus their academic careers in scientific fields. So often I look at Christian blogs, and I see way too much G. K. Chesterton, Francis Chan and other untestable, ineffective jibber-jabber. We need to bring the hard science, and stop making excuses about not being able to understand it because it’s too hard. It’s not too hard. Everyone can understand Lee Strobel’s “The Case for a Creator“. That’s more than enough for the average Christian on science apologetics. We all have to do our best to learn what works. You don’t want to be anti-science and pro-speculation like atheists are. I recommend reading Uncommon Descent and Evolution News every day for a start.

How tidal effects improve the habitability of a planet

Circumstellar Habitable Zone
Circumstellar Habitable Zone

Science Daily reports on a new factor that affects planetary habitability: tides. Specifically, tides can affect the surface temperature of a planet, which has to be within a certain range in order to support liquid water – a requirement for life of any conceivable kind.


Tides can render the so-called “habitable zone” around low-mass stars uninhabitable. This is the main result of a recently published study by a team of astronomers led by René Heller of the Astrophysical Institute Potsdam.

[…]Until now, the two main drivers thought to determine a planet’s temperature were the distance to the central star and the composition of the planet’s atmosphere. By studying the tides caused by low-mass stars on their potential earth-like companions, Heller and his colleagues have concluded that tidal effects modify the traditional concept of the habitable zone.

Heller deduced this from three different effects. Firstly, tides can cause the axis of a planet`s rotation to become perpendicular to its orbit in just a few million years. In comparison, Earth’s axis of rotation is inclined by 23.5 degrees — an effect which causes our seasons. Owing to this effect, there would be no seasonal variation on such Earth-like planets in the habitable zone of low-mass stars. These planets would have huge temperature differences between their poles, which would be in perpetual deep freeze, and their hot equators which in the long run would evaporate any atmosphere. This temperature difference would cause extreme winds and storms.

The second effect of these tides would be to heat up the exoplanet, similar to the tidal heating of Io, a moon of Jupiter that shows global vulcanism.

Finally, tides can cause the rotational period of the planet (the planet’s “day”) to synchronize with the orbital period (the planet’s “year”). This situation is identical to the Earth-moon setup: the moon only shows Earth one face, the other side being known as “the dark side of the moon.” As a result one half of the exoplanet receives extreme radiation from the star while the other half freezes in eternal darkness.

The habitable zone around low-mass stars is therefore not very comfortable — it may even be uninhabitable.

Here is my previous post on the factors needed for a habitable planet. Now we just have one more. I actually find this article sort of odd, because my understanding of stars was that only high-mass stars could support life at all. This is because if the mass of the planet was too low, the habitable zone wouldbe very close to the star. Being too close to the star causes tidal locking, which means that the planet doesn’t spin on its axis at all, and the same side faces the star. This is a life killer.

This astrophysicist who teaches at the University of Wisconsin explains it better than me.


Higher-mass stars tend to be larger and luminous than their lower-mass counterparts. Therefore, their habitable zones are situated further out. In addition, however, their HZs are much broader. As an illustration,

  • a 0.2 solar-mass star’s HZ extends from 0.1 to 0.2 AU
  • a 1.0 solar-mass star’s HZ extends from 1 to 2 AU
  • a 40 solar-mass star’s HZ extends from 350 to 600 AU

On these grounds, it would seem that high-mass starts are the best candidates for finding planets within a habitable zone. However, these stars emit most of their radiation in the far ultraviolet (FUV), which can be highly damaging to life, and also contributes to photodissociation and the loss of water. Furthermore, the lifetimes of these stars is so short (around 10 million years) that there is not enough time for life to begin.

Very low mass stars have the longest lifetimes of all, but their HZs are very close in and very narrow. Therefore, the chances of a planet being formed within the HZ are small. Additionally, even if a planet did form within the HZ, it would become tidally locked, so that the same hemisphere always faced the star. Even though liquid water might exist on such a planet, the climactic conditions would probably be too severe to permit life.

In between the high- and low-mass stars lie those like our own Sun, which make up about 15% percent of the stars in the galaxy. These have reasonably-broad HZs, do not suffer from FUV irradiation, and have lifetimes of the order of 10 billion years. Therefore, they are the best candidates for harbouring planets where life might be able to begin.

This guy is just someone I found through a web search. He has a support-the-unions-sticker on his web page, so he’s a liberal crackpot. But he makes my point, anyway, so that’s good enough for me.

Maybe the new discovery is talking about this now, but I already knew about the tides and habitability, because I watched The Privileged Planet DVD. Actually that whole video is online, and the clip that talks about the habitable zone and water is linked in this blog post I wrote before.