How many Earth-like planets have been discovered?

The ESO said the recent discoveries bring to 45 the total number of Earth-like planets with an orbital period shorter than 50 days it has discovered, and suggests one in three stars similar to our sun may harbour such planets.

Last Updated: Monday, June 16, 2008

European astronomers said Monday they have discovered five "super-Earths" — including three orbiting a single star — which they say suggests Earth-like planets could be very common outside our solar system.

The discoveries, revealed at a conference in France, were made using a spectrograph at the La Silla Observatory in Chile, which is run by the European Organization for Astronomical Research in the Southern Hemisphere, or ESO.

Planet hunter Michel Mayor, best known for discovering the first confirmed planet outside our solar system, said in a statement the finds suggest the search for Earth-like planets has just begun.

"Does every single star harbour planets and, if yes, how many? We may not yet know the answer but we are making huge progress towards it," said Mayor, the principal investigator at the ESO's Geneva Observatory.

The term Earth-like planet is a rather broad definition and includes planets with a mass below 30 Earth masses; in other words, planets smaller than gas giants like Jupiter or Saturn. But because they aren't as easy to spot as gas giants, even the largest of these planets evaded detection until recent years.

These planets are often discovered, but not seen directly, by monitoring the slight changes their gravitational pull have on the velocity of their stars. Occasionally a planet will actually pass in front of the Earth's view of the star and provide an even clearer signal of its presence. But in either case, the more frequently the planet orbits the star, the easier it is to spot.

The star with the three Earth-like planets, called HD 40307, is slightly less massive than our sun and is located 42 light-years, or about 400 trillion kilometres, away from Earth.

The planets have masses 4.2, 6.7 and 9.4 times that of Earth, and orbit the star with periods of 4.3, 9.6, and 20.4 days, respectively.

The astronomers discovered the other two Earth-like planets in two separate star systems. One planet with 7.5 times the mass of the Earth, was discovered orbiting the star HD 181433, a star that is already known to host a Jupiter-like planet. The second system contains a planet with 22 times the mass of Earth.

More than 270 planets have been discovered outside our solar system, but as instruments become more precise, research is increasingly focusing on planets similar to Earth, particularly those at a distance similar to our own planet's distance from the sun.

Astronomers see this as the "sweet spot," since the temperature and conditions might allow for the presence of liquid water, a key ingredient for supporting life.

Posted: March 12, 2008

Alpha Centauri, the closest star system to our Solar System, should harbour detectable Earth-like planets, according to a new study by astronomers at the University of Santa Cruz.

Computer simulations of planet formation were performed to show that terrestrial planets are likely to have formed around one of the three stars in the system, Alpha Centauri B. Moreover, the planets would have formed in the 'habitable zone", where liquid water can exist on the planet's surface. Although many different simulations were performed, starting with a variety of different initial conditions, in every case a system of multiple planets evolved with at least one planet about the size of Earth.

Gliese 581 c generated a lot of interest as it was initially reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface and hence, potentially capable of supporting life as we know it. However, subsequent study that took into account the insolation that gives the planet five times the heat the Earth receives and the greenhouse gas effect of the planet’s atmosphere, with at least five times the greenhouse gases, now appears to make this very unlikely.

Gliese 581 d, the third planet in the Gliese system, is now regarded as more likely to harbor life. The planet is astronomically close, at 20.4 light years (193 trillion km or 119 trillion miles) from Earth in the direction of the constellation of Libra

In terms of mass, no Earth-equivalent planet has been discovered yet. Only two extrasolar objects less massive than the Earth have been discovered, both of them relatively small (one has twice the mass of the Moon, the other has less than half the mass of the Moon) and both of them orbiting a pulsar which wouldn't provide a very habitable environment for life. The smallest planet larger than Earth is still fully 3.9 times the mass of the Earth, and the others range from there all the way up to hundreds of times the mass of the Earth (many of them are even larger than Jupiter).

Similarly, I don't think any potential rocky planet has been discovered in an Earth-like orbit around a Sun-like star. The Gliese 581 system is centered around a red dwarf, which creates a different dynamic for the possibility of life than the Sun does. Gliese 581 c stands a good chance of being tidally locked with the Gliese 581 red dwarf itself, which means one side always faces the star while the other side always faces away. While life could exist on such a planet, it is not as hospitable as Earth and most life would probably be either around the twilight belt (where the Sun is perpetually low in the sky, but never sets) or deep under the surface. Scientists also suspect that the planet may host a runaway greenhouse effect like Venus, in which case the chances of life existing there are pretty small. Gliese 581 d (the next planet out in the same system) might be a better environment, but it may be too cold, we really don't know yet. I don't believe any planet has been discovered in the Alpha Centauri system, although computer models suggest that a planet could exist there; if it does, then it would of course be a prime candidate for humanity's first interstellar space mission, when (or if) that happens, due to it being relatively close to the Earth (4.5 light years to 20 or so for Gliese 581).

There are at least two so far, although the only thing Earth-like about them is that they're located in the habitable zone of their parent stars. One is of coure Gliese 581c, the other is a planet still being formed within the habitable zone of a star known as HD113766, about 42 light years distant.

As for Alpha Centauri, no Earth like planets have been found there, but computer models show that there is a likelihood that such could be the case.

i hate like hangnails to align myself with the religious rare-Earthers, but they ARE right.

so far, no Earth-like planets have been discovered.

Perhaps, if your definition is rather loose, you can include Gliese 581C, but I don't.

Sometime in the near future we may get good enough to resolve the terribly small planets, but not yet.

(with our current lack of understanding about planetary formation, a subject which until we actually saw planets extra-Solar, we THOUGHT we knew a lot about, finding real Earth-like planets will be a learning experience, and I look forward to it)

None, but there is a "super Earth" 14 x the mass of the Earth that is supposed to be rocky and possibly in a "life zone". It orbits its star closer than the Earth around our Sun, and takes only 10 days to make a complete orbit (its year is 10 of our days long). The surface temperature is thought to be around 1100°F.

There is another more recent discovery of a 5.5x mass of Earth object (the one you mentioned) that is within a habitable zone that might be friendly towards life as we know it.

Most recently there was one found with a mass of 3.3 Earths (MOA-192 b), but it isn't thought to be rocky, maybe similar to Neptune - gassy and ice. It orbits a brown dwarf which is too small to sustain fusion.

That's about as close as they have come so far.

Well think about this: Does this planet have oxygen, hydrogen, any other types of gases.

if it did have the two types of gases(at least) we would have water and air.

Does it have soil so we can grow plants and the animals can feed of the plants and we feed of the animals?

Is there an O-zone type of thing to protect us from harmful rays that could kill us?

Is there even rain storms so we can have water or an ocean?

Is there a moon that can regulate tides from the ocean, or a moon that can help us decipher from this month or this year.

AND MOST IMPORTANTLY... is there a sun around this plant.. is it even close enothe so we dont freeze...o r is it to close to where we burn to death....

so my point is... God made earth for us..its like a specaially made retainer for someone..im mean we all have a different tooth mark right??

I was under the impression that they have not found Earth's sister-planet yet. Nothing that could sustain human life, anyway. I know they recently found a new galaxy with a solar system that looks promising...though....

they have seen over 200 planets that could "possibly" sustain life

the only one is earth

None I think!