Starship Asterisk*

Orbital diagrams comparing the Gliese 581 system to our own. [i](Credit: NSF/Zina Deretsky)[/i]

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A team of astronomers that includes the University of Hawaii’s Nader Haghighipour has announced the discovery of a planet that could have liquid water on its surface.

The planet, which is probably 30 percent larger than Earth, was discovered using one of the telescopes of the W. M. Keck Observatory on Mauna Kea. It orbits a relatively small star, Gliese 581, that is 20 light-years from Earth in the constellation Libra.

“By determining the orbit of this planet, we can deduce that its surface temperature is similar to that of Earth,” said Haghighipour. This means that at least some of any water on the surface of the planet and in its atmosphere will be in liquid form rather than ice or vapor. The discovery of liquid water in space is an important step in the search for extraterrestrial life.

The team estimates that the new planet, called Gliese 581g, has a mass three to four times that of Earth, and orbits its star in just under 37 Earth days. Its mass indicates that it is probably a rocky planet with enough gravity to hold on to its atmosphere. It is one of six known planets orbiting the star.

To discover the planet, the team looked for the tiny changes in the star’s velocity that arise from the gravitational tugs of its planets. They used 238 separate observations of Gliese 581 taken over a period of 11 years.

The star Gliese 581 hosts an Earth-sized planet (foreground) that orbits in the star's habitable zone. [i](Artwork: Lynette Cook)[/i]

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A team of planet hunters led by astronomers at the University of California, Santa Cruz, and the Carnegie Institution of Washington has announced the discovery of an Earth-sized planet (three times the mass of Earth) orbiting a nearby star at a distance that places it squarely in the middle of the star's "habitable zone," where liquid water could exist on the planet's surface. If confirmed, this would be the most Earth-like exoplanet yet discovered and the first strong case for a potentially habitable one.

To astronomers, a "potentially habitable" planet is one that could sustain life, not necessarily one that humans would consider a nice place to live. Habitability depends on many factors, but liquid water and an atmosphere are among the most important.

"Our findings offer a very compelling case for a potentially habitable planet," said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. "The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common."

The findings are based on 11 years of observations at the W. M. Keck Observatory in Hawaii. "Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exoplanet revolution," said Paul Butler of the Carnegie Institution. "Our ability to find potentially habitable worlds is now limited only by our telescope time."

Astronomers have found a new, potentially habitable Earth-sized planet. It is one of two new planets discovered around the star Gliese 581, some 20 light years away. The planet, Gliese 581g, is located in a “habitable zone”—a distance from the star where the planet receives just the right amount of stellar energy to maintain liquid water at or near the planet’s surface. The 11- year study, published in the Astrophysical Journal and posted online at arXiv.org, suggests that the fraction of stars in the Milky Way harboring potentially habitable planets could be greater than previously thought—as much as a few tens of percent.

The new study brings the total number of planets around Gliese 581 to six and, like our own solar system, they orbit their star in nearly circular orbits. The scientists, members of the Lick-Carnegie Exoplanet Survey, collected 11 years of radial velocity data on the star. The radial velocity method looks at a star’s tiny movements in response to the gravitational tug from orbiting bodies. The team tracked the motion of the planets to a precision of about 1.6 meters per second.

The amplitude and phasing of the star’s subtle gravitational reactions allow researchers to determine a planet’s mass and orbital period. The planet’s radius is estimated by making assumptions about its composition, and its surface gravity is calculated from its mass and radius. Astronomers can also determine the planet’s equilibrium and surface temperatures, which help to determine the potential for habitability. Equilibrium temperature reflects the balance between the energy emitted from the planet and the thermal energy received from the star. The surface temperature is estimated by the planet’s distance from the star and a range of guesses about the composition of its atmosphere. To be habitable, the temperatures must not be too hot, which would vaporize water, nor too cold.

A team of planet hunters including scientists from the NASA Astrobiology Institute has announced the discovery of a planet with three times the mass of Earth orbiting a nearby star at a distance that places it squarely in the middle of the star’s “habitable zone,” an area where liquid water could exist on the planet’s surface. If confirmed, this would be the most Earth-like exoplanet yet discovered, and the first strong case for a potentially habitable one.

Source: [NASA Press Release]

Discovery suggests our galaxy may be teeming with potentially habitable planets

A team of planet hunters led by astronomers at the University of California, Santa Cruz (UC Santa Cruz), and the Carnegie Institution of Washington, and supported by the National Science Foundation (NSF) and NASA, has announced the discovery of an Earth-sized planet (three times the mass of Earth) orbiting a nearby star at a distance that places it squarely in the middle of the star's "habitable zone," where liquid water could exist on the planet's surface. If confirmed, this would be the most Earth-like exoplanet yet discovered and the first strong case for a potentially habitable one.

View a webcast with Steven Vogt and Paul Butler, leaders of a team that discovered the first potentially habitable exoplanet.

• arXiv.org > astro-ph > arXiv:1009.5733 > 29 Sep 2010

Astronomers have announced the discovery of a planet with about three times the Earth’s mass orbiting the nearby red dwarf star Gliese 581. That in itself is cool news; a planet like that is very hard to detect.

But the amazing thing is that the planet’s distance from the star puts it in the Goldilocks Zone: the region where liquid water could exist on its surface!

First, a few things:

1. Gliese 581 is a dinky, cool red dwarf about 20 light years away. That’s pretty close as stars go; only a handful are closer. Bear in mind it’s still 200 trillion kilometers (120 trillion miles) away, and that’s still a bit of a drive.
2. The planet is one of six now known to orbit the star [that link goes to a PDF of the journal paper]. Apparently, all the planets have neat, circular orbits, so the system seems to be stable. This new planet takes 37 days to orbit the star once, and orbits at a distance about 1/6 the distance of the Earth from the Sun. As far as we know, it’s the fourth planet from its star.
3. The planets have all been found by the Doppler method: as they orbit the star, they tug on it. This causes a shift in the wavelength of emitted light from the star. The mass of the planet, its distance from the star, and the shape of the orbit all determine how the light shifts, which is how astronomers found those properties of the new planet.

OK, so that’s what we know. Now let me be clear here about stuff we can be fairly sure about.

If you’re too close to a star, it’s too hot to support liquid water. If you’re too far, it freezes. This defines a rough region from the star — the Goldilocks Zone, for obvious reasons — where liquid water can exist on the surface of a planet. This depends on the star, of course, but also on other factors like the planet’s atmosphere; Venus could have liquid water, but its super-thick atmosphere produces a runaway greenhouse effect which has heated it to 460° C (900° F). If Mars had a thick atmosphere, it might support liquid water! So the planet itself matters here too.

Gliese 581g, as the new planet is called, is in the zone where the temperature is just right. And with a mass of just three times that of the Earth, it’s unlikely to be a gas giant.

However, this does not mean the planet is habitable, or even very Earthlike. It may not even have any water on it at all. For now, we can’t know these things, so beware of any media breathlessly talking about life on this planet, or how we could live there.

There are some things we can speculate on with some solid footing. The orbital period of 37 days puts it pretty close to the star – since the star is a red dwarf, it’s cooler than the Sun, so being closer doesn’t necessarily mean you overheat. But it does mean the star exerts strong tides on the planet, which have the effect of slowing the planet’s rotation until it equals the orbital period. This has almost certainly happened to this planet, so in other words, one day on this planet = one year, and the planet always shows the same face to its star like the Moon does to the Earth.

That makes things a bit dicier for habitability. The side facing the star may get very hot, while the dark side gets very cold. If the planet has an atmosphere that gets mitigated somewhat (the hot air on the day side will flow over to the night side and vice versa, smoothing out the highs and lows in temperature), and may make the planet more clement. However, we have no clue if this planet has an atmosphere at all.

I also want to note that the mass found (3x Earth) is the minimum mass of the planet! It may be more massive, though it’s unlikely to be much more. The Doppler method doesn’t give an exact mass, only a lower limit. That’s frustrating, but that’s the way the math works out.

I’ll note that one of the other planets, Gliese 581d, orbits farther out but if it has a thick atmosphere like Venus it may also be able to support liquid water on its surface. All in all, this could be a very interesting system to study!

But perhaps the most interesting and exciting aspect of all this is what it implies. The Milky Way galaxy is composed of about 200 billion stars, and is 100,000 light years across. The fact that we found a planet that is even anything like the Earth at all orbiting another star only 20 light years away makes me extremely optimistic that earthlike planets are everywhere in our galaxy. 20 light years is practically in our lap compared to the vast size of our galaxy, so statistically speaking, it seems very likely it’s not unique. I don’t want to extrapolate from a data set of two (us and them), but if this is typical, there could be millions of such planets in the galaxy. Millions.

So we don’t know if this planet is all that much like Earth — the surface gravity may be quite high if it’s dense and small, for example, or it may not have any air, or it may have a thick atmosphere like Venus — but what it’s telling us is that smaller, lower mass planets at the right distance from their star for liquid water are almost certainly common in the galaxy.

And that’s big enough news for me.

makc wrote:that's a cold rock, and the habitable zone is shaped like a ring, so I guess they would be like us in a way that they fight for territory. if they have a technology, there are wars all over the planet

Two weeks ago, U.S.-based astronomers announced the discovery of the first Goldilocks planet circling another star: just the right size and just the right temperature to harbor alien life. But yesterday at an exoplanet meeting in Turin, Italy, Switzerland-based astronomers announced that they could find no trace of the prized planet in their observations of the same planetary system.
...
But at this week's Astrophysics of Planetary Systems meeting, astronomer Francesco Pepe of the Geneva Observatory and the Swiss group reported that he and his colleagues could find no reliable sign of a fifth planet in Gliese 581's habitable zone. They used only their own observations, but they expanded their published data set from what the U.S group included in its analysis to a length of 6.5 years and 180 measurements. "We do not see any evidence for a fifth planet ... as announced by Vogt et al.," Pepe wrote Science in an e-mail from the meeting. On the other hand, "we can't prove there is no fifth planet." No one yet has the required precision in their observations to prove the absence of such a small exoplanet, he notes.

Things don’t look good for Gliese 581g, the first planet found orbiting in the habitable zone of another star. The first official challenge to the small, hospitable world looks in the exact same data — and finds no significant sign of the planet.

“For the time being, the world does not have data that’s good enough to claim the planet,” said astro-statistics expert Philip Gregory of the University of British Columbia, author of the new study.

The “first habitable exoplanet” already has a checkered history. When it was announced last September, Gliese 581g was heralded as the first known planet that could harbor alien life. The planet orbits its dim parent star once every 36.6 days, placing it smack in the middle of the star’s habitable zone, the not-too-hot, not-too-cold region where liquid water could be stable.

Planet G was the sixth planet found circling Gliese 581, a red dwarf star 20 light-years from Earth. A team of astronomers from the Geneva Observatory in Switzerland found the first four planets using the HARPS spectrograph on a telescope in Chile. The team carefully measured the star’s subtle wobbles as the planets tugged it back and forth.

Two more planets, including the supposedly habitable 581g, appeared when astronomers Steve Vogt of the University of California, Santa Cruz and Paul Butler of the Carnegie Institution of Washington added data from the HIRES spectrograph on the Keck Telescope in Hawaii. They announced their discovery Sept. 29.

Just two weeks later, the HARPS team announced they found no trace of the planet in their data, even when they added two more years’ worth of observations. But it was still possible that the planet was only visible using both sets of data.

Now, the first re-analysis of the combined data from both telescopes is out, and the planet is still missing.

“I don’t find anything,” Gregory said. “My analysis does not want to lock on to anything around 36 days. I find there’s just no feature there.”

Unlike earlier studies, Gregory used a branch of statistics called Bayesian analysis. Whereas classical methods are narrow, testing only a single hypothesis, Bayesian methods can evaluate a whole set of scenarios and figure out which is the most likely.

Gregory wrote a program that analyzed the likelihood that a given planetary configuration would produce the observed astronomical data, then ran it for various possible configurations.

For the HARPS data set, he found that the best solution was a star with five planets, which orbit the star once every 3, 5, 13, 67 and 400 days. The 36-day habitable world wasn’t there.

When he looked at the HIRES and the combined data sets, the best solution was a star with two planets. Only when he included an extra term in the HIRES data did Gregory find more, which he suspects means the HIRES instrument isn’t as accurate as thought.

“There may be something in the telescope…that’s contributing to the error,” he said.

Gregory’s model finds the probability that the six-planet model is a false alarm is 99.9978 percent. None of the planets Gregory’s analysis turned up lay in the habitable zone. The results are in a paper submitted to the Monthly Notices of the Royal Astronomical Society and published on the physics preprint website arXiv.org¹.

Other astronomers seem impressed with Gregory’s analysis.

“That’s the right way to do it,” said exoplanet expert Daniel Frabrycky of the University of California, Santa Cruz. “I think everyone would agree that that is the most sophisticated analysis that you can do, and as much as you could hope to do.”

“The Gregory paper is by far the most complete statistical analysis to date that has been made public,” said exoplanet and astro-statistics expert Eric Ford of the University of Florida. “It’s by far the most rigorous analysis.”

But most astronomers are not yet ready to close the book on Gliese 581g.

“I’m not going to admit that it’s a dead planet yet,” said exoplanet expert Sara Seager of MIT. “No one will be able to sort this out today … it will take some time.”

Vogt, for his part, still firmly believes the planet is there. “I’m standing by our data,” he told Wired.com.

He said there are two ways to interpret the signals from Gliese 581. Sometimes a single planet with an elongated, or elliptical orbit can look the same as two planets that trace perfect circles around their stars. One of Gliese 581’s planets, planet D, could be one of these “eccentric impostors,” hiding an extra planet within its signal.

Part of the reason it’s so difficult to tell these two scenarios apart is that spotty observations make fake signals in the data. These signals, which show up because the telescope can’t watch the star continuously, look like they could actually be planets, but would disappear if we could observe round the clock.

In a paper that’s still in preparation², astronomer Guillem Anglada-Escudé and Harvard graduate student Rebekah Dawson tackle these issues, and conclude that the habitable planet still has a chance. “With the data we have, the most likely explanation is that this planet is still there,” Anglada-Escudé said.

Everyone agrees that the problem can only be resolved with more data. In particular, astronomers are anxious to see the extra data that the HARPS group used to conclude Gliese 581g is a mirage.

“I don’t think anything will change significantly until the Swiss publish their data,” Anglada-Escudé said. “Nobody else has seen their data. We’re waiting to see that, just to settle down the problem.”

1. Bayesian Re-analysis of the Gliese 581 Exoplanet System - Philip C. Gregory
   • arXiv.org > astro-ph > arXiv:1101.0800 > 03 Jan 2011
2. Aliases of the first eccentric harmonic : Is GJ 581g a genuine planet candidate? - Guillem Anglada-Escudé
   • arXiv.org > astro-ph > arXiv:1011.0186 > 31 Oct 2010

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[i]Diagram of the Gliese system. The green area is the habitable zone, where liquid water can exist on a planet’s surface. (PHL @ UPR Arecibo)[/i]

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Exoplanet Gliese 581g is back, “officially” ranking #1 on a list of potentially habitable worlds outside of our solar system thanks to new research from the team that originally announced its discovery in 2010. Orbiting a star 20 light-years away, the super-Earth is now listed alongside other exoplanets Gliese 667Cc, Kepler-22b, HD85512 and Gliese 581d in the University of Puerto Rico at Arecibo’s Habitable Exoplanets Catalog as good places to look for Earthlike environments… and thus the possibility of life.

First announced in September 2010 by a team led by Steven S. Vogt of UC Santa Cruz, the presence of Gliese 581g was immediately challenged by other astronomers whose data didn’t support its existence. Vogt’s team conducted further analysis of the Gliese system in which it appeared that the orbits of the planets were circular, rather than elliptical, and it was in this type of scenario that a strong signal for Gliese 581g once again appeared.

Read: Could Chance For Life on Gliese 581g Actually Be “100%”?

“This signal has a False Alarm Probability of < 4% and is consistent with a planet of minimum mass 2.2M [Earth masses], orbiting squarely in the star’s Habitable Zone at 0.13 AU, where liquid water on planetary surfaces is a distinct possibility” said Vogt.

And, located near the center of its star’s habitable “Goldilocks” zone and receiving about the same relative amount of light as Earth does, Gliese 581 g isn’t just on the list… it’s now considered the best candidate for being an Earthlike world — knocking previous favorite Gliese 667Cc into second place.

Read: Billions of Habitable Worlds Likely in the Milky Way

The announcement was made on the PHL’s press site earlier today by Professor Abel Méndez, Director of the PHL at UPR Arecibo.

• “The controversy around Gliese 581g will continue and we decided to include it to our main catalog based on the new significant evidence presented, and until more is known about the architecture of this interesting stellar system”
  
  – Prof. Abel Méndez, UPR Arecibo

• arXiv.org > astro-ph > arXiv:1207.4515 > 18 Jul 2012

Nancy Atkinson wrote, on September 30, 2010 (http://www.universetoday.com/74679/coul ... ly-be-100/):
Gliese 581g is surely a potential habitable planet where liquid water could exist on the planet‘s surface, and many are touting the old adage of where there’s water, there’s life. However, some quotes from one of the scientists involved in the discovery might be feeding some wild speculation about the potential for life on this extrasolar planet and elsewhere. “Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent,” said discoverer and astronomer Steven Vogt during a press briefing yesterday. “I have almost no doubt about it.”

Ann wrote:
What would Steven Vogt have said if he had been sitting a few hundred light-years away
and had discovered an interesting inner planet in our Solar system?

• HEELLLPPPPP