National Geographic News
Image of the habitable-zone planets discovered to date alongside Earth.

Planets one to four times the size of Earth (at right) could be amenable to life.

Photograph by NASA Ames/JPL-Caltech

Dan Vergano

National Geographic

Published January 6, 2014

Earth-size planets circling nearby stars come in two flavors, either rocky or gassy, astronomers reported on Monday. And more than three-quarters of stars likely host at least one of these alien Earths. (See also: "Earth-Like Worlds 'Very Common.'")

"We are talking about worlds barely larger than our own," says astronomer Geoff Marcy of the University of California, Berkeley, speaking at the American Astronomical Society meeting in Washington, D.C. "That's how far we have come." (See "MacArthur Genius Searching for Signs of Life on Exoplanets.")

When astronomers began reporting the discovery of planets orbiting nearby stars in 1995, the few worlds they detected were as large or larger than Jupiter. Now measurements from NASA's Kepler space telescope—which has discovered 237 of the more than 1,000 planets detected, according to Michele Johnson of NASA's Ames Research Center in Mountain View, California—are helping us learn what worlds in Earth's weight class are made of.

The findings narrow the range of planets on which we might expect to see alien life, Marcy says, to the smaller ones nearest to Earth in size. However, none of the planets reported in the new Kepler data orbited well enough inside the "habitable zone" of their stars to be amenable to oceans and life, he noted. (Related: "Newfound Earth-Size Exoplanet Doomed.")

On the plus side, that still leaves a lot of planets for future alien hunters to investigate, as roughly three-quarters of the 3,538 still-unconfirmed candidate planets detected by Kepler appear to be Earth-size, Marcy reported at the meeting. And roughly one in five stars are orbited in their habitable zone by a planet one to two times as wide as Earth.

Planetary Dividing Line

"We are finding a dividing line between two classes of Earth-size planets," says astronomer Yoram Lithwick of Northwestern University in Chicago, who presented a study of 60 "Super-Earth" worlds, ones roughly one to four times as wide as Earth. "Many are even fluffier than Neptune and Uranus."

In the study presented by Lithwick and a separate study of 42 planets presented by Marcy, the cutoff is between planets more or less than two times as wide as Earth.

Those less than two times as wide as Earth are either rocky or are draped with an outermost layer of cloudy hydrogen and helium gas haze, while those more than two times as wide as our planet all have densities that suggest they are gassy worlds. These "mini-Neptunes" likely look more like Uranus and Neptune in our solar system.

Planetary Chemistry

What causes the dividing line between rocky Super-Earths and mini-Neptunes? Basically, rocky planets can't get much wider than twice the size of Earth, says Marcy. Once they reach that size, added rock is just compressed further, making the planet more dense but leaving it at the same width.

Gassy worlds, in contrast, grow wider as more gas is added, because the thin gas allows them to balloon out.

"It's a reasonable conclusion, supported by theory, and the observations seem solid," says astronomer Stephen Maran, author of Astronomy for Dummies. "The thing that really stands out is that we live in an oddball solar system that doesn't have one of these mini-Neptunes."

18 comments
Arup Ghosh
Arup Ghosh

very much interesting about planet and wish to know that  have any prospect  water and life any planet or about mars. 


Jacqueline Muñoz
Jacqueline Muñoz

Cuándo encontrarán indicios de vida en otro(s) planeta(s)???

Gul Ahmed
Gul Ahmed

this is cool!

we are finally making progress in our quest for other habitable planets :)

keep up the good work guys (and girls)

Andrew Roberts
Andrew Roberts

It amazes me how arrogant these scientists are- "on which we might see alien life"  Who are we to say what can and cannot support life.  We fall under a very narrow definition of life.  Who is to say that life elsewhere in the universe is not totally different- even to be including the air.

Chase Mistrot
Chase Mistrot

this is cool!

we are finally making progress in our quest for other habitable planets :)

keep up the good work guys (and girls)!

Michael Brito
Michael Brito

It's interesting that as we discover more and more worlds the criteria for worlds which may be habitaal to the known type of "life as we know it", has narrowed in some respects and widnened in others. The possibility that moons around gas giants may be able to support life as we know it due to tidal effects has given science new places to look. At the same time rocky worlds like ours are contrained by the type of star that they orbit, in conjunction to their distance, and now plus a limited size. What makes this last point interesting is that it means that the varieties of life on those world like ours although different in size shape color would for the most part be easily recognizable to us.

alisa coplin
alisa coplin

for real did not know there was grass on other plans I thought we were still looking for water on other plants

Dusty Rockets
Dusty Rockets

Rocky or Gassy huh? Aren't those the ONLY options for planets?

Marcos Toledo
Marcos Toledo

Could Mercury be the remnant of a hot Jupiter or Neptune because is so dense a planet.

Sue Jobs
Sue Jobs

I guess there must be aliens somewhere!

Mireia Estrany
Mireia Estrany

Then, perhaps, is there fossils or living beings elsewhere? Nonetheless, isn’t the emergence and maintenance of life a process of radical contingency? That is, is a unique and unrepeatable past totally necessary? Or does life emerge through space like mushrooms when some conditions are present? So, how many conditions are necessary: three, four, trillions, infinite? Only one, water-rocky-planet or any sort of God? Is God the word that means infinite conditions, absolute necessity? Anyway, how did the life that emerge in a given conditions resist when switching to a different  moment? How does life resist time itself, entropy? But, is it possible for human beings to recognise a simpler life than their own brain only? On the other hand, beyond likeness, is it possible to recognise a complex life than their brain, is this the alien life that some people are searching unsuccessfully? However, is there an origin of life or would it be as finding a cut in the material history of the universe, an infinite void that human language patches now? Is it the same cut between life and death?  Along these lines, there is a  book, a short preview in http://goo.gl/rfVqw6 Just another suggestion, far away from dogmas or axioms.

Will Bodnaryk
Will Bodnaryk

@Andrew Roberts I agree with your comment completely, with the exception of the first sentence. At this time, in an already vast and challenging search through what we can see of the universe, it is completely logical - not arrogant - to begin by searching for something we would know how to recognize.

Jason DeLand
Jason DeLand

@Andrew Roberts I hope for your sake that you are joking.  That was one of the stupidest comments I've ever seen.  They are scientists and ou are arguing who they are to make theories about something they have probably been studying their entire lives.  Shut up.

Luka Znid
Luka Znid

@Marcos Toledo Not likely, if it would have been a gas giant once then wouldn't have smaller planets around it (like Earth, Venus and Mars) be sucked in it? Or at least knocked off course and on a dive into the Sun?

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