Tag Archives: gj 581

Gliese 581d is an ex-planet

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Exoplanet poster child

If, in 2009, you asked 18-year-old me to name an exoplanet, then Gliese 581d would have been it. Discovered by an American team of astronomers in 2007, it was, for a long time, the poster child for exoplanetary science. Not only was the first rocky world ever found in the habitable zone of its star where life-friendly temperatures are found, it was also relatively nearby (for astronomy standards) at only 20 light years.

Astronomers used the radial velocity technique to find the first planet around Gliese 581 as far back as 2005. This method relies on the gravitational pull that a planet has on a star as it orbits. This wobble is detectable in the spectra of the starlight, which gets doppler shifted as the star moves back-and-forth, allowing the period and mass of an orbiting planet to be determined. While the first planet, ‘b’, orbited close to the star with a period of only 5.4 days, it was joined by two cooler (and more habitable) planets, ‘c’ and ‘d’ in 2007. This was soon followed in 2009 by Gliese 581e, the smallest planet in the system on an even shorter (3.1d) orbit.

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Movie credit: ESO

Things started to get even more confusing in 2010 when observers at the Keck observatory announced two more planets (‘f’ and ‘g’) orbiting at 433 and 37 days respectively. This would put ‘g’ between ‘c’ and ‘d’ and right in the middle of the star’s habitable zone. However, new observations of the star with a Swiss telescope showed no such signal. Was there a problem with the data, or could something else be mimicking these planets?

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Other stars, just like our sun, have extremely active surfaces
One problem comes when we consider the star itself. Just like our own sun, most stars are active, with starspots skimming across the surface and convection currents in the photosphere causing noise in our measurements. These active regions can often mimic a planet, suppressing the light from one side of the rotating star and shifting the spectra as if the star itself were moving back-and-forth. Add to that the fact that, like planets, activity comes and goes on regular timescales and that cool stars such as Gliese 581 are even more dynamic than our pot-marked sun, and the problem becomes apparent.

The first planet to bite the interstellar dust was ‘f’. At 433 days, its orbit closely matches an alias of the star’s 4.5-year activity cycle, and it was quickly retracted in 2010. Similar analyses with more data also suggested Gliese 581g was also likely to be an imposter, but the original team stuck by this discovery. For the last 3 years, this controversy has simmered, until last month all the data available for Glises-581 was re-analysed by Paul Robertson at Penn State. This showed that not only is Gliese 581g not a planet, but that the poster child itself, Gliese 581d, was also an imposter.

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The signal strength of any potential planets with (red) and without (blue) activity correction.

To do this, the team took all 239 spectra of GJ581 and analysed not just the apparent shift in velocity, but the atomic absorption lines themselves. Using the strength of the Hα absorption line as an indicator for the star’s activity, they compared this to the residual radial velocity (after removing the signal from planet b). This showed that there was a relatively strong correlation between activity and RV, especially over three observing seasons when the star was in a more active phase. They also found that this activity indicator varied on a 130 day timescale.

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The new system with only 3 planets
When the team removed the signal from stellar activity, they found that planets ‘c’ and ‘e’ were even more obvious than in previous searches. However the signal for planet ‘d’ dropped by more than 60%, way below the threshold needed to confirm a planet. Even more remarkably, ‘g’ does not appear at all. So what exactly caused this ghostly signal. The planet’s orbital period of 66 days gives us a clue -it is almost exactly half that of the star’s 130 day rotation cycle, so with a few fleeting starspots and the right orientation, a strong planet-like signal at 66 days results.

This case of mistaken identity is a sad one, but thanks to the incredible progress of our field in the last 5 years, their loss barely makes a dent in the number of potentially habitable exoplanets known. Instead, it acts as a warning for planet-hunters: sometimes not all that glitters is gold.

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The results are also explained in exquisite detail at Penn State University’s own blog, including an excellent timelapse showing how our understanding of the Gl 581 system has changed over time

What’s In A Name?

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Hundreds of astronomers across the globe are currently searching nearby stars for a fleeting glimpse of astronomical gold dust: exoplanets. I am also part of the search, scanning through terabytes of data taken by the WASP and NGTS telescopes looking for the distinctive signal of a distant world crossing its star. Thanks to the mountains of data from NASA’s Kepler probe, it is now even possible for amateurs to go online and help out. And thousands of people have taken part, spurred on by the chance to become the first person in history to lay eyes on a new part of the universe.

It is a thrilling quest, but the question on everyone’s lips is this: do you get to name it? Surprisingly enough, the answer is a ‘No’. Or maybe a ‘Not yet’…

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Here on Earth it has long been custom that, for whatever it may be, the discoverer becomes the namer. Columbus, Cook and Magellan all took pleasure in naming new lands, doctors such as Alzheimer or Asperger gave their names to their respective disorders, even some recently named animal species include Attenborosaurus conybeari and Heteropoda davidbowie in honour of the researcher’s heros. Chemists discovering new elements are given a relative freedom over naming their discoveries. Even in astronomy, comets are named after their discoverer with names such as Lovejoy or McNaught often gracing comet codes. Exoplanets, on the other hand, are a very different kettle of fish.

The problem with naming planets comes from the stars they circle. As nice as it would be to name every object something eye-catching like ‘Permadeath’ or ‘Baallderaan’, to avoid confusion the name of the star must be listed first. This is much like the way biological names come with both genus (Homo) and species (sapiens). So how do we end up with names like HD80606b whereas biologists get Bushiella beatlesi? The first part comes down to how we name stars.

Too Many stars to Count

Unlike islands or animals, there exist a near infinite plethora of stars. Our galaxy alone has more than 100 billion. Attempt to name each in the Linnaean style and you would quickly run out of words (and sanity). Early sky-watchers soon realised this and, after giving a few hundred stars colloquial names such as Vega or Pollux, settled for simply numbering the stars by brightness in a certain area. This ‘Bayer’ designation, cooked up in 1603, ranked the stars from alpha down to omega and beyond. For example the brightest in the Centaurus constellation is Alpha Centauri, our Sun’s nearest neighbour. With limited telescopic power and Greek and Latin characters, Bayer gave up after about 1500 stars.

More recent surveys have used telescopes to attempt to sweep the rest of the sky into some sort of order. This has resolutely failed, with the majority of stars having numerous names under many different catalogues (HD, HR, Gliese, or HIP to name but a few). Each of these official catalogues simply orders the stars by number, giving rise to the cumbersome alphanumeric system we see today. {NB: Despite what some might insist, naming a star has never been done via gift subscription companies}. So, thanks to the sheer number of star systems, the sky is a mess and there would seem little hope of sorting it out. 

GJ581 Planets

But forgetting the star for a second, once a planet is found we do get to add a ‘species name’ to the stars, right? Dont get your hopes up: this is normally the lower-case letter b. The lower case shows it to be a planet (as opposed to ‘B’ which would designate another star) and the ‘b’ designates it as the second object in the system after the star itself. In multi-planet systems things get even more confusing, with the order of names increasing not outwards from the star but simply in order of which was discovered first. For example GJ581e circles within the orbit of ‘b’ and GJ 581g is sandwiched between ‘c’ and ‘d’. However, this fundamentally makes sense: planets in the same solar system are given names reflecting their sibling nature.

It may be a dysfunctional system that results in far-from eye-catching names, but it is one at least partly grounded in reason. The alternative, of letting discoverers name the planet whatever they want (my personal choice would be Hughtopia), would ultimately end in confusion and a lot more angry shouting matches at conferences.

Even worse, a whole host of recent crowd-sourced websites have sprung up attempting to get the general public to name the 100-strong list of current exoplanets (for money, of course).  The International Astronomical Union (IAU), who ultimately decide on the names of everything in space, have even given support to public-generated naming systems. The feeling among astronomers, though, is that such a move might not be such a good idea.

But is there a middle way? Could the ordered nomenclature remain intact while giving at least some naming rights to the discoverers? The Planetary Habitability Laboratory recently proposed a system that would retain the star name but allow free reign over the planetary name, for example allowing Alpha Centauri B b to become Alcen-B Rakhat. It is an intriguing idea, and one that could help improve the public perception of astronomy. I, for one, am still hopeful that ‘Betelgeuse Hughtopia’ can become a reality.

[Relevant XKCD:]

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