A Brief History Of Gliese 581d and 581g, The Planets That May Not Be

Goldilocks Zone
Artists impression of Gliese 581g. Credit: Lynette Cook/NSF

Two potentially habitable planets in the Gliese 581 system are just false signals arising out of starstuff, a new study said. Gliese 581d and 581g are (study authors said) instead indications of the star’s activity and rotation. It’s the latest twist in a long tale about the system as astronomers struggle to understand how many planets could be orbiting the star.

“Our improved detection of the real planets in this system gives us confidence that we are now beginning to sufficiently eliminate Doppler signals from stellar activity to discover new, habitable exoplanets, even when they are hidden beneath stellar noise,” stated Paul Robertson, a postdoctoral fellow at Penn State University, in a press release.

“While it is unfortunate to find that two such promising planets do not exist, we feel that the results of this study will ultimately lead to more Earth-like planets.”

Planets were first announced around the system in 2007 (by a research team led by Geneva’s Stephane Udry) including Gliese 581d. The system has been under heavy scrutiny since a team led by Steven Vogt of the University of Santa Cruz announced Gliese 581g in September 2010. Both 581d and 581g were considered to be in the “habitable” region around the dwarf star they orbited, meaning the spot that’s not too far or close to the star for liquid water to exist.

Potentially habitable exoplanets and exoplanet candidates as of July 3, 2014. Gliese 581d and 581g are crossed off in the catalog. Click for larger version. Credit: PHL @ UPR Arecibo
Potentially habitable exoplanets and exoplanet candidates as of July 3, 2014. Gliese 581d and 581g are crossed off in the catalog. Click for larger version. Credit: PHL @ UPR Arecibo

About two weeks after the discovery, another team led by Geneva University’s Francesco Pepe said it could not find indications of Gliese 581g in data from HARPS (High Accuracy Radial Velocity Planet Searcher), a telescope instrument frequently used at the European Southern Observatory to confirm exoplanets. It also cast doubt on the existence of Gliese 581f, announced by a team led by Geneva’s Michel Mayor in 2009. Other researchers examined the system, too, with mixed results.

Two years later, Vogt led another research team saying that analysis of an “extended dataset” from HARPS did show Gliese 581g. But in a press release at the time from the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo, its director (Abel Mendez) said the discovery would continue to be controversial. At the time he added the planet to the list of potentially habitable exoplanets the laboratory maintains. As of yesterday, both 581d and 581g are crossed off.

The uncertainty arises from the delicacy of looking for signals of small planets around much larger stars. Astronomers typically find planets through watching them pass across the face of a star, or measuring the tug that they exert on their parent star during their orbit. It is the nature of the tug on Gliese 581 that is so interesting astronomers.

Orbital Period
The orbits of planets in the Gliese 581 system are compared to those of our own solar system. The Gliese 581 star has about 30 percent the mass of our Sun, and the outermost planet is closer to its star than the Earth is to the Sun. The 4th planet, G, is a planet that could sustain life. Credit: Zina Deretsky, National Science Foundation

“These ‘Doppler shifts’ can result from subtle changes in the star’s velocity caused by the gravitational tugs of orbiting planets,” wrote Penn State in the press release yesterday.  “But Doppler shifts of a star’s ‘absorption lines’ also can result from magnetic events like sunspots originating within the star itself — giving false clues of a planet that does not actually exist.”

The researchers now say that only three planets exist around this star. It’s impossible to fully represent the debate in a single short news article, so we encourage you to look at some of the original literature. Here is a list of papers related to Gliese 581g and another for Gliese 581d. The new paper is available online in Science.

Also, here are some past Universe Today stories about the system:

Exoplanet Gliese 581g Makes the Top 5

Exoplanet Gliese 581g is back, and “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.

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)

“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

Buzz About Gliese 581g: Doubts of Its Existence; Aliens Signals Detected

Goldilocks Zone
Artists impression of Gliese 581g. Credit: Lynette Cook/NSF


Ever since the announcement of the discovery of exoplanet Gliese 581g, there has been a buzz in the news, on websites, Twitter – pretty much everywhere, about the first potentially habitable extrasolar planet. But the past couple of days there has been a different sort of buzz about this distant world. Two stories have surfaced and they both can’t be true. The first one is fairly off the deep end: an astrophysicist from Australia claims that while doing a SETI search two years ago, he picked up a “suspicious signal” from the vicinity of the Gliese 581 system, and a couple of websites have connected some dots between that signal and a potentially habitable Gliese 581g.

The second one is more sobering. At an International Astronomical Union meeting this week, other astronomers have raised doubts whether Gliese 581g actually exists.

Unless you’ve been under a rock the past two weeks, you likely know that this newest and most promising of potential habitable extra solar planets was described by the scientists who discovered it as a rocky world about 3 times the mass of Earth, and it orbits within the red dwarf star’s habitable zone, the place that is just right for water to remain as a liquid on a planetary surface. And it is fairly close to us, too, at about 20 light years away, located in the constellation Libra.

Also announced was the discovery of planet ‘f’, a 7-Earth mass planet with a 433-day orbit around Gliese 581.

Astronomer Steven Vogt announced the discoveries by his team, which used the HIRES instrument on the Keck I telescope in Hawaii. They also used 119 measurements from the HARPS instrument on the La Silla telescope at the European Southern Observatory in Chile.

On Monday, Steinn Siggurdson broke the news on his Dynamics of Cats blog that an astronomer who works on HARPS data at the Geneva Observatory, said at the IAU meeting this week that his team could not confirm the existence of Gliese 581 g.

In an article on the Astrobiology Magazine website today (Tuesday) the astronomer, Francesco Pepe, said that not only can they not confirm the existence of planet ‘g’, but also the ‘f’ planet.

In 2009, the Geneva team announced the discovery of planet ‘e’ in the Gliese 581 solar system. At approximately 1.9 Earth masses, this ‘e’ planet is the lowest mass extrasolar planet found at that time, and has a 3.15-day orbital period around the star.

Pepe said they have studied this planet-rich system frequently, gathering a total of 180 data points in 6.5 years (with about 60 of those data points since 2009) and they can only see evidence of the 4 previously announced planets b, c, d, and e.

There is a signal which could possibly be f, but the signal amplitude of this potential fifth planet is very low and basically at the level of the measurement noise, said Pepe.

The planets in the Gliese 581 system were discovered using spectroscopic radial velocity measurements. Planets ‘tug’ on the star they orbit, causing it to shift in position (stars and planets actually orbit a common center of mass). By measuring the star’s movement in the sky, astronomers can figure out what sort of planets are orbiting it. Multi-planet systems create a complicated signal, and astronomers must tease out the spectral lines to figure out what represents a planet, and what is just “noise” – shifts in the star light not caused by an orbiting planet. Astronomers have developed various ways to reduce such noise in their telescopic observations, but it still creates a level of uncertainty in detecting extrasolar planets.

The Geneva team plugged the HARPS data on Gliese 581 into computer models, and the models show “the probability that such a signal is just produced ‘by chance’ out of the noise is not negligible, of the order of several percents,” Pepe said. “Under these conditions we cannot confirm the presence of the announced planet Gliese 581 g.”

While this doesn’t definitively mean Gliese 581g doesn’t exist, it certainly casts doubt on it. More teams will be looking at the Gliese 581 star to try and determine what is really out there. This story is not over yet.

As for the alien signal, this news has met some pretty harsh criticism — even from Dr. Frank Drake, a leader in SETI community. Astronomer Ragbir Bhathal, a scientist at the University of Western Sydney, said he detected an unusual pulse of light nearly two years ago from the same region at Gliese 581, and with the news of the potential habitable world there, his claims came up again. In an article in Space.com Drake said is suspicious because Bhathal would not share his data with anyone.

You can read an article published in 2009 in the Australian about Bhathal’s claimed discovery.

Could Chance for Life on Gliese 581g Actually Be “100%”?

Orbital Period
The orbits of planets in the Gliese 581 system are compared to those of our own solar system. The Gliese 581 star has about 30 percent the mass of our Sun, and the outermost planet is closer to its star than the Earth is to the Sun. The 4th planet, G, is a planet that could sustain life. Credit: Zina Deretsky, National Science Foundation


The announcement yesterday of the discovery of the closest Earth-sized planet found so far that also exists in the habitable zone around its star is certainly exciting (read our previous article for all the details). 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.”

Yes, that is an exact quote. He really used those words. He also said that it would be pretty hard to imagine that water wouldn’t exist on the planet, given the ubiquity of water in our solar system and beyond, and the habitable region in which this planet orbits.

Also participating in the briefing was Paul Butler of the Carnegie Institution of Washington, which provided funds for the observations at the Keck I telescope, and his comments were more tempered.

“Any discussion of life on at this point is purely speculative,” Butler said. “What we know is that this planet exists at the right distance for liquid water it has the right amount of mass to hold on to its atmosphere and any liquid water on the surface. So any subsequent discussion of life there is purely speculative. That being said, on the Earth anywhere you find liquid water you find life in overwhelming abundance. The question should be, if this planet has liquid water, how can you rule out life doesn’t exist? It is pretty probable that anywhere you find liquid water pooling, that you would find life existing.”

Are Vogt’s claims too extreme, or were they made in exhilaration during an exciting announcement? This has been a topic of debate on Twitter this morning. Some wondered if Vogt had been misquoted, and many expressed that Vogt’s words may fuel off-the-deep-end speculation about the certainty of life on another world.

“Until we know more about this planet and the origin of life itself, any claim of certain habitation is idiotic and does not serve science,” said Dr. Stuart Clark (@DrStuClark), author and astronomy journalist. To clarify, he wanted others to know that he thinks just the claim is idiotic, not the discovery or the people involved.

“As cool as it is, please realize that right now all we really know about it is its orbit and estimated mass. That’s it.” said Lee Billings (@leebillings), editor at Seed Magazine. “In other words, barring observational evidence that may still be a generation away, Gliese 581g is ‘Earth-like’ only in terms of mass/orbit.”

From our pal Phil Plait, the Bad Astronomer (@badastronomer): “I understand what he meant – he thinks it could have life – but it was phrased unfortunately, and the media have jumped on it, of course.”

From David Masten (@dmasten), CEO of the commercial space company Masten Space Systems: “I have an opinion or 3 about life on anything in Gliese 581! And I’d dare say much closer to zero chance. But I’m not an astrobiologist.”

“Claiming a 100% chance of life on Gliese 581g is definitely an overreach,” said astrophysicist Juan Cabanela (@Juan_Kinda_Guy) at Minnesota State University Moorhead, “given we currently have a sample of 1 planet with life.”

“Vogt’s extrapolation was certainly quite a leap. On the other hand, the media might finally get it that some scientists really do think life everywhere is possible – but not bug-eyed aliens” said Robert Cumming, (@maltesk), journalist at the Swedish magazine “Populär Astronomi.“. “Then we can also discuss why there might not be life everywhere after all.”

Mark Thompson (@PeoplesAstro), Astronomy presenter on BBC’s the One Show said the Vogt’s quote was “absolutely and totally inappropriate. We can’t even be 100% sure it’s made of rock!!!”

From astronomer, educator and journalist Nicole Gugliucci (@noisyastronomer): “The public seems to have enough trouble trusting science these days without scientists making bold statements like that.”

“100% is ridiculous,” Tweeted frequent image contributor to Universe Today, Stu Atkinson (@mars_stu). “No possible way anyone could know that, surely?”

Many expressed excitement over the discovery, and Stu articulated perhaps the most colorful, which was re-tweeted several times yesterday: “Ah, a PROPER planet!” Not a great fat bloated sweaty “Who ate all the pies” ‘hot Jupiter’ tearing insanely around its star.”

What are your views?

*all Tweets used by permission.

Here’s an article about abiogenesis, theories about how life got started here on Earth.