The Planet That is No More

A new analysis confirms that an exoplanet thought to orbit in the habitable zone of the star Gliese 581 actually doesn’t exist.

A long-debated exoplanet signal has finally gone poof. A new analysis of stellar activity for the M dwarf star Gliese 581 shows that its planet d is probably just a ghost in the data.

GJ 581 has been a poster child for multiplanet systems for several years. In 2005 a Geneva-led team using the ESO’s HARPS spectrograph discovered the first planet, b; in 2007 they reported c and d, and in 2009 e. All these planets have orbits smaller than Mercury’s, but around a star 30% the size of the Sun and 60% as hot.

habitable zone of GJ 581

Conservative (bright green) and optimistic (dark green) estimates for the size of the habitable zone around the star Gliese 581. This diagram from the Habitable Zone Gallery has the planets e, b, and c (from inmost out), plus the now-debunked planet d, which astronomers thought crossed into the star's habitable zone.Credit: Habitable Zone Gallery

A second team used a combo of observations from HARPS and Keck’s HIRES spectrograph to suggest the presence of two more planets, f and g, but these were quickly questioned and have basically fallen away. Only b, c, d, and e remain.

Some astronomers have also questioned whether d actually exists, notably Roman Baluev (Pulkovo Observatory, Russia), who in 2012 did a careful analysis of noise in the observations and raised a red flag. But analyses of stellar activity turned up no big starspots that could masquerade as a planet, and d stayed in the catalogs — and on the list of potentially habitable planets: much of its predicted orbit fell in GJ 581’s habitable zone.

The new work by Paul Robertson (Penn State) and colleagues makes clear that d needs the boot, too. The team took another look at the HARPS spectra, focusing on hints of stellar activity from hydrogen-alpha emission. This emission comes from hydrogen atoms being hit by particles accelerated along magnetic field lines in the star’s atmosphere; stronger magnetic fields mean more acceleration, which means more emission. But stronger magnetic fields also mean more stellar activity, which can create shifts in starlight that look much like the wobbles in a star’s position created by a planet.

Robertson’s team found that, during times of higher magnetic activity, the strength of d’s signal went up. When the star was less active, the signal weakened.

They also revised the star’s rotational period from previous calculations, revealing that it was twice as long as planet d’s orbital period of 66 Earth days. That suggests d’s signal is from the star itself.

Taken together, the observations support only one likely conclusion. “I think the evidence shown by Robertson et al. is strong enough to remove GJ 581d from the list of exoplanets,” says Xavier Bonfils (Grenoble Observatory, CNRS/UJF, France), who coauthored the original discovery paper.

Although Bonfils and his colleagues searched for starspots, the activity that’s to blame is more subtle than what the team was looking for. Active regions on a star can look like a planet because of their effect on the Ferris-wheel-like motion of plasma (called convection) in the star’s photosphere. As a star rotates, its magnetic field lines are twisted and rearranged. When these magnetic fields are concentrated in a small region, the boost in field strength diverts the surrounding ionized gas from its normal convective motion, Robertson explains. So instead of the star’s light uniformly seeming to move toward us (as that part of the star rotates into view) and away from us (as it rotates out of view), an extra signal will appear that seems to move differently. This signal looks much like a planet.

But GJ 581 appeared to be starspot-free during the observations used to discover planet d. Robertson’s team found that, although no big, dark blemishes showed up on the star’s surface, the H-alpha emission clearly indicates that the star had magnetically active regions. It’s these regions that mucked up the spectra.

Stellar activity is a known problem with exoplanet searches, particularly with M dwarfs, which are notoriously noisy. Bonfils says that HARPS has actually found more false positives due to active regions than true planets. But given the signal strengths involved, he thinks that only a few reported exoplanets at most might prove to be from active regions like those Robertson’s team found. Accounting for these shifts in starlight will prove especially important for low-mass planets in moderate to long orbital periods, he adds.

P. Robertson et al. "Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581." Science. Published online July 3, 2014.

Read our March 2014 issue to find out how you can help find an exoplanet — even without a telescope.


11 thoughts on “The Planet That is No More

  1. Tom Hoffelderrocksnstars

    How many more are data ghosts? All science is so out of control. First there is this planet, then there isn’t. First there is evidence from BICEP2 that the CMB shows inflation, then maybe it’s really dust. Can there be any question why more and more people seem to be slipping back to the pseudosciences? I’ll never do that, but I might quit reading science news stories, which means I wouldn’t be reading any type of news stories, and wait for books to be published on any science subject of interest. Usually that doesn’t happen until all the questions are resolved.

    But thank you Camille for telling us the real story. Someone has to!

    1. Martin B. Vestergaard

      “All science is so out of control.” exactly and luckily.
      Societies all through history have imposed various degrees of control over science, some have dictated what was to be researched, some what should be proved, some what was suitable to be known to the masses, etc. etc. and all have had disastrous implications for science. Most notably it has hampered scientific development, it also lead to fancy theories to support the “scientific truth” of the ruling classes, that in many cases had no base in reality.

      Whenever a society has let the scientific community discuss their ideas freely and openly (meaning including the non-scientific population) science have flourished, and new discoveries have been made. Naturally this open discussion will include data and scientific theories that does not hold up, but that is part of the process, an important part. Sometimes a team or individual scientist can see no flaw in their theory, only to have it taken apart by another team that thought of a different explanation, and a third group will take the new theory apart, by yet another explanation. Sometimes a theory does stand up to centuries of scrutiny, and becomes the accepted truth for generations. Even these truths are not seen as absolute truths by most scientists, and occasionally someone will make observations that does not fit, and a new theory has to be formulated, or the data will have to be disproved.

      A recent example of this was when a group of scientists at CERN measured neutrinos that travelled faster than light. They didn’t believe it themselves, but could not find any error either in their data or the set-up, so they published their findings, with the stated intent of promoting further inquiry and debate. This news hit the media, in most cases not including the reservations of the original group. What was not so widely published in the media was that several other groups tried to repeat the experiment, but non of them got the same result, while some scrutinised the data and the original set-up, and eventually someone was able to find two errors in the original set-up that had caused the results.

  2. Drew L.

    rocksnstars, your characterization of this situation couldn’t be more off the mark!!! This is NOT an example of “science out of control” but rather a perfect example of how the scientific process works. Science is seeks to provide natural explanations for observed phenomena. Part of this process is the formulation of a scientific hypothesis which not only seeks to provide an explanation for current observations but makes testable predictions of what future observations will find in order to prove or disprove that hypothesis. In this case, the new observations failed to support the hypothesis for the existence of GJ 581d and g. Instead the signals appear to have a different cause – in this case magnetic activity modulated by the rotation of GJ 581. Speaking as a professional scientist with decades of experience, this sort of thing happens every day and it is this constant reassessing and self-correction that makes science the powerful tool that it is. Before you “quit reading science news stories”, I would instead suggest that you read up on how the scientific process actually works before passing judgement.

  3. Peter WilsonPeter Wilson

    First there is a planet, then there is no planet, then there is
    First there is a planet, then there is no planet, then there is

    Astronomer sheds his dust to find a g-wave sig within
    Astronomer sheds galactic dust to find a g-wave sig within

    First there is a signal, then there is no signal, then there is
    First there is a signal, then there is no signal, then there is

    Oh connector, oh connector, oh connector, I curse your name

    The knock upon my L H C’s a Higgs boson, that’s what it is
    The knock upon my L H C’s a Higgs boson, that’s what it is

    First there is a planet, then there is no planet, then there is
    [repeat & fade…apologies to Donovan]

  4. Tom Hoffelderrocksnstars

    Gentlemen, I totally agree that science works! However, I would submit the difference between 50 years ago, when I started reading science articles of all types, even 25 years ago, and now was that the scientists got together first and verified results before anything was sent to the news media. Now as soon as someone comes up with some results that sound exciting, it’s sent out to the world. Maybe some people like these roller coaster rides; I do not, which is why I mentioned waiting for the book because that usually doesn’t get published until the science is actually done. Maybe some of the general public that has always doubted science and only needs a little “help” to slip back to the dark side won’t be influenced negatively, but I doubt it. And how many people only see the first story which turns out to be wrong? There’s not much sensationalism in saying sorry, that interesting planet doesn’t exist after all. And much of the news media is interested mostly in sensationalism as opposed to the whole truth, which does obviously appear on websites like this. But what percent of the people look here for their news?

    1. Drew L.

      rocksnstars, speaking as a professional scientist (with 30+ years of experience), a science news reader (with 45+ years of experience) and a science writer (with 25 years of experience), I’m not sure where you got your ideas about how science works or how scientific results are shared with the public. Good science has been and continues to be about scientists getting together first and verifying results before anything is sent to the news media. It’s called the peer review process and ALL of the results discussed in Camille’s story about GJ 581 were subject to this process before press releases were issued. The story about the planets of GJ 581 and how the interpretation of the data has changed over the last decade is a perfect example of how science is really done. Observations are made, hypotheses are proposed to explain those observations, new observations are made to test the hypotheses and the process starts over again. Nothing about science is ever “settled before a book is published”. In fact nothing about science is ever really settled. Unlike religion or other static knowledge-based systems, science by its very nature is constantly changing as new information becomes available. This has been reflected in science reporting for decades as new discoveries after another are made resulting in changes in the scientific view of the universe. And while it can be argued that there is far too much sensationalism in media reporting of science along with other bad habits, this is hardly a new phenomenon and has been around as long as there has been a free press. If the public does not know how science really works, it is a failure of our educational system and the lack of properly honed critical think skills not a failure of science or the scientific process.

  5. jaromir

    I see a few interesting sides to this:

    – I suppose by “out of control” you meant something more like “unreliable” and the radial velocity method IS more unreliable than we thought and I am also wondering how many MORE “pseudoplanets” are going to be retracted soon, especially those which have already been done to death in the news but also in habitable planet lists and diagrams, I see more “diappearing planets” coming
    – I totally agree with you that journalism tends to be less interested in truth nowadays which counters its very DEFINITION
    – I totally agree with you that the WAY we do science has become a bit inadequate by going away from trying to UNDERSTAND towards just trying to MEASURE things and this might be one of the reasons why people are turning towards pseudoscience which seems much more attractive to them

  6. KevinKevin

    There’s one fact that everybody here seems to be missing–science is self-correcting! No matter what the field of study, the method is always the same: One person offers a theory, others rigorously test it and if it fails to pass the “reality check”, it is dumped and the search starts over to find the true answer. If science was never mistaken, then we would never learn from our errors and would end up proceeding in our studies using incorrect information which would taint everything that came after it. For example, if you get the speed of light wrong, or misinterpret the Doppler Shift, then you can forget accurate measurements in any astronomical studies.

    Exoplanet detection is still in its infancy and as with any new pursuit, theories and findings will change more often and more rapidly. If you prefer a “field” that never questions itself then you should stick with religion. It will never change because it has no true facts to question in the first place. That’s the joy of fiction after all! You can put forth any idea, no matter how implausible or fantastical and nobody can argue with it.

  7. RodRoderick

    Exoplanets come and go if folks track reports over the years and watch the list rise and fall as members are removed. Gliese 581 d in May 2011, computer models were developed that indicated the exoplanet may support oceans – “First Habitable Exoplanet? Climate Simulation Reveals New Candidate That Could Support Earth-Like Life”, Now this 2014 report shows the exoplanet likely is not real. Always a good idea not to jump to hasty conclusions in science.

  8. Peter WilsonPeter Wilson

    Astronomers have answered the basic questions that everybody looking up at the sky asks: What are those points of light? Why do some move? Why are there seasons? What powers the sun? But the man-on-the-street does not look up and wonder: What’s holding galaxy-clusters together? What’s pushing them apart? Can gravitational waves can be detected? Etc. You have to be a student of science to even understand the questions being asked today. It’s an uphill battle maintaining the public’s interest in topics becoming increasingly obtuse. Maybe there’s more temptation today to hype one’s results?

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