Habitable Earth-Like Exoplanets Might Be Closer Than We Think

by Nancy Atkinson on March 13, 2013

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The graphic shows optimistic and conservative habitable zone boundaries around cool, low mass stars. The numbers indicate the names of known Kepler planet candidates. Yellow color represents candidates with less than 1.4 times Earth-radius. Green color represents planet candidates  between 1.4  and 2 Earth radius. Credit: Penn State.

The graphic shows optimistic and conservative habitable zone boundaries around cool, low mass stars. The numbers indicate the names of known Kepler planet candidates. Yellow color represents candidates with less than 1.4 times Earth-radius. Green color represents planet candidates between 1.4 and 2 Earth radius. Credit: Penn State.

Size might matter when it comes to stars having habitable environments for planets, and in this case smaller might be better, as well as closer to Earth. A new study indicates that low mass stars may be the most abundant planet hosts in our galaxy. And since these smaller stars like M-dwarfs are plentiful, the number of potentially habitable planets could be greater than previously thought.

“We now estimate that if we were to look at 10 of the nearest small stars we would find about four potentially habitable planets, give or take,” said Ravi Kopparapu from Penn State University. “That is a conservative estimate,” he added. “There could be more.”

Kopparapu has published a new paper where he recalculated how common Earth-sized planets in the habitable zones of low-mass stars, also known as cool stars or M-dwarfs. Since the orbit of planets around M-dwarfs is very short, this allows scientists to gather data on a greater number of orbits in a shorter period of time than can be gathered on Sun-like stars, which have larger habitable zones.

Additionally, since M-dwarfs are more common than Sun-like stars, it means more of them can be observed.

Moreover, there are M-dwarfs located relatively close to Earth, which makes it easier to study any planet that may be orbiting these stars.

“The average distance to the nearest potentially habitable planet is about seven light-years,” Kopparapu said. “That is about half the distance of previous estimates.”

Kopparapu said there are about eight of these cool stars within 10 light-years of Earth, and the thinks, conservatively, we should expect to find about three Earth-size planets in the habitable zones.

His paper follows up on a recent study by researchers at the Harvard-Smithsonian Center for Astrophysics which analyzed 3,987 M-dwarf stars to calculate the number of Earth-sized planet candidates in cool stars’ habitable zones. That study used habitable zone limits calculated in 1993, but recently, a group of astronomers that included Kopparapu developed a new model for identifying habitable zones around stars based on water and carbon dioxide absorption (see the Habitable Zone Calculator here). Now Kopparapu has applied the new model to the Harvard team’s study, and found that there are additional planets in the newly determined habitable zones.

“I used our new habitable zone calculations and found that there are nearly three times as many Earth-sized planets in the habitable zones around these low mass stars as in previous estimates,” Kopparapu said. “This means Earth-sized planets are more common than we thought, and that is a good sign for detecting extraterrestrial life.”

Read Kopparapu’s paper.

Source: Penn State

About 

Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

Philip Wilson March 13, 2013 at 7:31 PM

Earth like planet is a gross misnomer for the vast majority of these planets which would have to be so close to the numerous late K and M dwarfs that they would be tidally locked with an eternal sun overhead, unmoving forever with the back side forever dark. Imagine the storms and winds. Imagine that every object, mountain, hill, “tree” casts a shadow forever.

Torbjörn Larsson March 13, 2013 at 8:45 PM

It is Earth like in mass, if it’s around a G/F star its an Earth analog.

These planets will be habitable, and to compensate for the uneven heat flow (less wind) you only need the range of denser atmospheres. (Which admittedly will tend your planet towards the large side, but there are still many of those.)

Chris Bowman March 13, 2013 at 7:34 PM

Incredible. With the launch of Hubble’s replacement, does that mean we can start to study a potential habitable planet way more closely?

Torbjörn Larsson March 13, 2013 at 8:40 PM

Yes, while the JWST isn’t optimized for it, the IR capability can look at planetary system formation (from cool gas to hot system), characteristics (dust ring heat signatures) and planetary atmospheres (molecule absorption).

Maybe someone expert in JWST can fill in. One problem is that this generic observatory will be a busy instrument.

krenshala March 14, 2013 at 4:31 PM

All the more reason to have more than one in orbit. Ignoring any VLBA capabilities having more than one would allow.

Olaf2 March 13, 2013 at 8:04 PM

This is interesting. One day we were the only planet in the universe, and then in a few years time the universe is filled with planets. The same happened with black holes.

Torbjörn Larsson March 13, 2013 at 8:37 PM

Same for bacteria, viruses, dark matter, dark energy. When we acquired the requisite resolution in our observations & theories, a lot of stuff suddenly fell into the field of view.

I love when that happens!

Me March 14, 2013 at 5:43 PM

Planets, yes. Many, many of them are out there. But just like Eden Earth? Not even close, …as of YET. This planet Eden Earth is so special, so extremely highly special that I feel deeply it will take them hundreds of years to find another Eden Earth. Then again, maybe not. Until that day they find a duplicate Eden Earth. I am content to say, ..so, so what if they find other rocky & gas planets. Take care Olaf2…

Torbjörn Larsson March 13, 2013 at 8:48 PM

Good catch! It stands to reason the new HZ definition will spawn reviews.

bdlaacmm March 13, 2013 at 9:35 PM

Why does Kopparapu say there are “about eight” red dwarfs within 10 light years of the sun, when there are precisely seven? (Proxima Centauri, Barnard’s Star, Wolf 359, Lalande 21185, BL Ceti, UV Ceti, and Ross 154) Such unnecessary imprecision is rather odd.

But in any case, nothing is going to make an Earth-sized planet around one of these stars “Earthlike”. The skewing of an M-class star’s spectrum toward the red (as compared to the Sun) means anything resembling our planet’s fauna would not survive there. That, plus the fact that most, if not all, of these stars are “flare stars”, which would periodically bathe any planets in a would-be Habitable Zone in lethal radiation, thus repeatedly sterilizing them. Strike three would be the likelihood that an Earth-sized world that close to its sun would almost certainly be tidally locked to its star, meaning one hemisphere would be in perpetual daylight and the other in eternal dark. Any atmosphere would freeze out on the night side, leaving the sunlit side as bare as own own moon.

“Earthlike”? There’s more to that concept than size alone.

lcrowell March 14, 2013 at 2:39 AM

The flaring of M-class stars is a problem. Some flare more than others, but we really do not know if a relatively quiet M-class star is just between episodes of flaring.

Tidal locking is likely a problem for complex life. It is possible that on an annular region where the star would appear on the horizon to an observer there that conditions might permit some form of biology. I do though suspect this is a small percentage of cases, and I also would suspect that complex life and ecosystems as seen on Earth probably occurs on very few or none on these planets.

LC

krenshala March 14, 2013 at 4:29 PM

Don’t forget the nearest variable star to Earth … a certain G2 we are in very close proximity of.

meekGee March 14, 2013 at 5:44 AM

I am sure that life forms living on the “equator” of tidally locked planets, for the life of them, can’t imagine how life is possible on a rotating planet – I mean, how in the world is complex life going to deal with temperature cycles in the tens of degrees C?

Flares? If the planet is locked, than the back portion of the annulus is naturally protected. Brightness variations? They are less than the difference between day and night on Earth. They are longer, for sure, but now we’re just talking about a different type of variability.

Spectral shift towards Red – are you seriously claiming that since our plants are optimized for the Sun’s spectrum, then all plants in the universe are too?

None of these is even remotely a show stopper for life.

bdlaacmm March 15, 2013 at 3:34 PM

“Spectral shift towards Red – are you seriously claiming that since our
plants are optimized for the Sun’s spectrum, then all plants in the
universe are too?”

Not at all. You didn’t read my posting very carefully. I was objecting to the adjective “Earthlike”. If some other, non-earthlike form of vegetation can thrive elsewhere, that does not make that world “Earthlike”.

Me March 14, 2013 at 7:57 AM

Earth is a real Eden. Earth is a real garden. All they have found is make believe earths/Edens. They want to find life so badly they are willing to play make believe. So far it is fiction. So far, our earth-Eden is the rarest of all. The only one so far. Until they point one out, our earthly Eden is singular, …period. So far all we have done is just started to scratch the universe’s surface. Take care.

campbell s w March 14, 2013 at 12:44 PM

“habitable” presumably means habitable for us which is fine if you’re looking for an alternative Earth for us but it means nothing in the search for different life forms whose physical and biological properties we know nothing about.

Dav_Daddy March 16, 2013 at 1:46 PM

Habitable in this sense means liquid water is possible on the surface given the distance from the star.

From the outside looking in at our solar system Earth, Venus, and possibly Mars would all be considered potentially habitable by this definition.

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