How common are terrestrial, habitable planets around sun-like stars?

Sep 28, 2011 By Ray Sanders, Universe Today
Artist concept of the Kepler telescope in orbit. Credit: NASA

Once again news from the Kepler mission is making the rounds, this time with a research paper outlining a theory that Earth-like planets may be more common around class F, G and K stars than originally expected.

In the standard stellar classification scheme, these type of are similar or somewhat similar to our own (which is a Class G star); Class F stars are hotter and brighter and Class K stars are cooler and dimmer. Given this range of stars, the habitable zones vary with different stars. Some could orbit their at twice the distance Earth orbits our Sun or in the case of a dim star, less than ’s orbit.

How does this recent research show that small, rocky, worlds may be more common that originally thought?

Dr. Wesley Traub, Chief Scientist with NASA’s Exoplanet Exploration Program outlines his theory in a recent paper submitted to the Astrophysical Journal.

Based on Traub’s calculations in his paper, he formulates that roughly one-third of class F, G, and K stars should have at least one terrestrial, planet. Traub bases his assertions on data from the first 136 days of Kepler’s mission.

Initially starting with 1,235 exoplanet candidates, Traub narrowed the list down to 159 exoplanets orbiting F class stars, 475 orbiting G class stars, and 325 orbiting K class stars – giving a total of 959 exoplanets in his model. For the purposes of Traub’s model, he defines terrestrial planets as those with a radius of between half and twice that of Earth. The mass ranges specified in the model work out to between one-tenth Earth’s mass and ten times Earth’s mass – basically objects ranging from Mars-sized to the theoretical super-Earth class.

The paper specifies three different ranges for the habitable zone: A “wide” habitable zone (HZ) from 0.72 to 2.00 AU, a more restrictive HZ from 0.80 to 1.80 AU, and a narrow/conservative HZ of 0.95 to 1.67 AU.

After working through the necessary math of his model, and coming up with a “power law” that gives a habitable zone to a star depending on its class and then working out how many planets ought to be at those distances, Traub estimated the frequency of terrestrial, habitable-zone planets around Sun-like (Classes F, G and K) stars at (34 ± 14)%.

He added that mid-size terrestrial planets are just as likely to be found around faint stars and bright ones, even though fewer small planets show up around faint stars. But that is likely because of the limits of our currently technology, where small planets are more difficult for Kepler to see, and it’s easier for Kepler to see planets that orbit closer to their stars.

Traub discussed how the quoted uncertainty is the formal error in projecting the numbers of short-period , and that the true uncertainty will remain unknown until Kepler observations of orbital periods in the 1,000-day range become available.

Explore further: An unprecedented view of two hundred galaxies of the local universe

More information: Check out our previous coverage of exoplanet detections using the Kepler data at:… -extrasolar-planets/

arXiv:1109.4682v1 [astro-ph.EP]

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User comments : 8

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1.6 / 5 (14) Sep 28, 2011
Thanks for the story.

Our studies over the past fifty years (1961-2011) indicate that very natural events produced our habitable planet Earth orbiting the Sun - the remains of a supernova that gave birth to the solar system five billion years (5 Gyr) ago:

1. "Elemental and isotopic inhomogeneities in noble gases: The case for local synthesis of the chemical elements", Trans MO Acad Sci 9, 104 (1975)

2. "Strange xenon, extinct super-heavy elements, and the solar neutrino puzzle", Science 195, 208 (1977)

3. "Neutron Repulsion", The APEIRON Journal, in press (2011)

4. "Origin and Evolution of Life Constraints on the Solar Model", JMP 2, 587-594 (2011)


The observations are not in dispute, although widely ignored.

With kind regards,
Oliver K. Manuel
Video summary of career (1961-2011)

1 / 5 (5) Sep 28, 2011
"Habitable", but to what?

A ten Earth mass planet with 2 Earth radius is habitable?

I don't know, maybe for very, very fit humans.

Now that I think about it, there's some obese people weighing 400 or more pounds who somehow manage to function in life. So apparently a very unhealthy human can survive at 400 pounds on earth. Maybe a fit human could survive at 400 pounds on a super-earth...if their internal organs aren't crushed under their own weight....

I guess divide 400 by 2.5g, and you'd have 160 pounds, so maybe very fit, light weight males and petite women could actually survive, if the atmospheric composition and pressure was suitable?

The habitable planet topic continues to evade a concrete definition.

Habitable to what?

What are the limits of "healthy" ranges for mammalian life in terms of gravity, air pressure, and atmospheric composition?

Can't we do some lab experiments with rats and monkeys at varying atmosphere pressures and simulated gravity?
1 / 5 (4) Sep 28, 2011
Why not put some monkeys in an environment in an air-tight box with some food, pressurized (or depressurized) and spun on a gravitron and see how many days the monkeys live under various conditions?

They do worse things with lab animals...

I guess NASA, U.S.S.R./Russia, and ESA have probably done this already, but where would you get access to the actual data from such experiments?

Somebody define the limits of "habitable", at least in terms of known reptilian and mammalian multicellular life.

I have seen on some scientific programs the theory that if a planet had a low enough surface gravity, yet a dense enough atmosphere, there could theoretically be flying creatures up to around the size of Earth's Blue Whales.
5 / 5 (9) Sep 28, 2011
Habitable does not mean suitable for the organisms that have evolved for billions of years to thrive on Earth.

Habitable means habitable to carbon-based life such as we understand it. There may be countless other types of life forms that exist and thus countless types of habitable conditions, but in the absence of any knowledge of these potential types of life, there's no point in spending time on it.

You can look at any place in the universe and dream of fantastical forms of existence that could call it home, but where does that get you? On the other hand, we know that carbon/water based life exists. So, we're looking for places that could harbour that life.

If we find a planet on which the conditions are suitable for life as we know it. The next step is to try to detect that life. If we do that, it will be an epochal moment in Human history.

More on this and the other wonders of Kepler here:

A Plethora of Planets
5 / 5 (1) Sep 29, 2011

Some race of beings, on some 10 earth mass planet in a faw away galaxy. They are sitting there at their computers right now, discussing the possibility of life forms evolving on planets with X-10 times the mass of their own planet.

Some of them may be wondering how anything could live in a 1/10th standard mass planet that is a mere 93 million miles from a scorching G class star.
not rated yet Sep 29, 2011
Habitable??? The size of the planet is only a fraction of the problem. How many planets are in the right zone but have the wrong atmosphere?

If Earth and Venus swapped places, what would each be like?

What about earth and Mars and Venus and Mars? have them all swap places and see if they would be habitable.

That is just a sample of three we know. What about other planets the right size in these general positions. Look at some of the larger moons in the solar system and swap places with them and Earth or Mars or Venus. Would any of them produce habitable domains?
1 / 5 (2) Sep 29, 2011
Given a genome of a finite length, and therefore Chromosomes and genes of finite length, then there is a finite limit to the number of combinations of bases of RNA or DNA.

There is also finite limit to the number of protiens and amino acids that can be made, and thus a finite limit to the types of lipids, fats, protiens, minerals and vitamins that can be made. Thus a finite limit to the size and strength of membranes, organelles, cell walls, matrixes, fluids, bones, skins, scales, teeth, and other structures which could be made.

So it's not like you'd reasonably expect any DNA or RNA based life form to be substantially different than anything on earth in terms of the limits of physical structures or chemistry.

You're not going to find a walking rock, or a liquid metal "T-1000" organism on another planet, nor any other such absurdity.

On earth, we have viruses, fish, bactieria, prions, blue whale, condor, zebra, insect, amoeba, etc...
5 / 5 (2) Sep 29, 2011

The sheer variety of life on earth would suggest that any mundane differences like heavier gravity would be trivial to overcome. Look at complex higher order mammals that can survive the scorching heat of deserts, the frozen wastelands of the arctic, and the crushing depths of the deep ocean, (not to mention the more exotic animals that can live in the incredibly high temperatures of deep ocean vents).

Adaptability to differing gravity can be illustrated on Earth. While we all feel the same gravity, creatures of differing sizes feel much different forces. As the size of a creature goes up, it's mass (and thus the force of gravity it feels) goes up by a cubic relationship, while the strength of its load-bearing parts increases only by a square relationship.

You won't find human size and shaped creatures on a 2.5G world, but you may find human size, elephant shaped ones. We're engineered for the environment we inhabit. There's a reason you don't see a 6' spider: Physics.