Gliese 581d is an Ex-Planet

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.

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?

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.

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

To do this, the team took all 239 spectra of Gl 581 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.

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

Gaia: Planets and Parallax

In six hours’ time, A Soyuz rocket will blast of from Guyana with the hope of delivering a €1billion Christmas present to astronomers across the world. That present will be Gaia, ESA’s flagship science mission, which hopes to revolutionise how we look at the galaxy around us by providing a 3D map of a billion stars and finding hundreds of new exoplanets.

So what is Gaia? It is essentially the most sensitive camera ever to be pointed at the heavens. That may sound the same as most space telescopes, but its specifications mean it will be able to pinpoint the location of stars with accuracy previously only dreamed of. Using a 1.5m mirror and a Gigapixel CCD camera, it will image more than a billion stars at least 70 times over a 5 year mission to provide the most accurate catalogue of stars in the Milky Way ever seen.

It is not the sensitivity of the telescope that is extraordinary, however, but rather its angular resolution. Consider the previous such mission, Hipparcos. It was capable of resolving objects tens of thousands of times closer together than the human eye, for example even from 200km away, it’s camera was capable of spotting two lights placed only a millimetre apart. This corresponds to the order of milliarcseconds, or 1/3600000th of a degree. Gaia, on the other hand, will be able to resolve stars mere microarcseconds apart. That is equivalent to being able to read 20pt text from 30,000km above Earth, or resolving two bright lights only 170m apart at the distance of Pluto.

Astronomical Parallax

What this amazing technological shift means is that Gaia will not only be able to compile the most accurate catalogue of star positions in history, it will also be able to map them in 3D. It may seem strange, but measuring the distance to a far-away point source like a star is nearly impossible. For nearby stars the shift of Earth’s position during the course of the year can act as a sort of cosmic depth perception, with the location of nearby stars wobbling subtlety between July and January, depending on how far away they are. It is this Parallax effect that, thanks to the incredible resolution of Gaia, will enable the distance to 1% of the stars in our galaxy to be precisely measured.

But when this effect due to the motion of Earth is corrected for, what motion is left? It’s likely the star will be moving in some direction through the galaxy relative to our solar system. This straight-line speed is the star’s ‘proper motion’ and can be as high as 10.3 arcsecs per year. But that’s not the only thing Gaia might spot. Stars are also tugged at by the gravitational pull of all nearby objects. This is most prominently done by planets in the stars vicinity. For example, an observer 30 lightyears away would see the sun shift by nearly 500µas due to the orbit of Jupiter. That means Gaia would see the Sun perform a slow ellipse across the sky every 5 years each time Jupiter orbits.

Planets Gaia can detect bounded in Blux lines. Upper line: Sun-like star. Lower line: M-dwarf

The biggest signals come from Gas Giant planets circling far from their stars, and Gaia will be able to search the nearest 400,000 stars for such worlds. Due to its 5-year mission, it will find these Jupiter analogues between 1 and 4AU. With any luck, more than 1000 candidates will be found; potentially doubling the current crop of exoplanets. And with Kepler dead and TESS still on the drawing board, Gaia may well become our best tool to mine the skies for new planets.

Even more interesting for exoplanet astronomers is that Gaia will find planets missed by other detection techniques. Both the transit and radial velocity methods are more sensitive to close-in planets, and have such discovered hundreds of bloated Hot Jupiters circling close to their star. Gaia, on the other hand, will be able to scan regions much further from the star. This will potentially answer the question of whether these Hot Jupiter systems are common or if other solar systems are more like our own stellar back

Another remarkable feat that Gaia will be able to achieve is pinning down the exact mass of some exoplanets. Worlds discovered by radial velocity give us an estimate of their size based on the to-and-fro motion of the star due to planets. Astrometry by Gaia will be able to give the side-to-side motion and determine in what precise inclination the planets are in. By tying down the planets orbit like this, their mass can be precisely determined.

Gaia, if successfully launched in the next few hours, will be capable of incredible feats. First and foremost, it’s incredible parallax measurements will turn astronomy from a two-dimensional star map into a complex three dimensional system where the distances to almost every object is known precisely. And tagged on for free are another thousand potential exoplanets to add to the exponentially growing list of alien worlds! If all goes well in Guyana at 9am, a collective sigh of relief will emanate from astronomers worldwide, and it might just signal the start of a new era of astronomy.

Infographic on Gaia:

Related articles

• Countdown to GAIA (telescoper.wordpress.com)
• ‘Billion star mapper’ set for launch (bbc.co.uk)
• Galaxy-Mapping Gaia Spacecraft Set for Launch Thursday: How to Watch Live (space.com)
• How the Gaia Telescope Will Scan the Entire Sky (gizmodo.co.uk)
• Space probe Gaia searches for galaxy’s dark energy (theguardian.com)
• A Planet for Every Star? (lostintransits.wordpress.com)

A Planet for Every Star?

Astronomers have now found an astonishing 1000 exoplanets. But that pales in comparison to the 100 billion stars in our galaxy. So how can we say whether planets are the norm? And is it possible to find a star that is definitively a planet-free zone?

The current crop of alien worlds comes from a limited selection of well-studied stars. Rather than try to directly spot what is the equivalent of a fleck of dust in a spotlight, astronomers use changes in the light of the star itself to tease out the signal of a planetary companion.

This can be done in a variety of ways, each of them with their own shortcomings. Often the method of discovery itself means that only a tiny selection of flukily-aligned planets will have the potential of being discovered.

For example, the Kepler spacecraft was staring at over 100,000 stars to try to detect the drop in light as exoplanets crossed their star. However, the probability of the average planet making this crossing is extraordinarily low. A planet orbiting at 1AU (the same distance from its star as Earth) will be found in only 1 in 200 such systems! To put that in perspective; for each Earth-like planet found by Kepler, 199 more stars with planets exactly like our own will have been be tossed aside.

The other common detection technique, known as radial velocity, is marginally less wasteful. This uses the to-and-fro of the star imprinted in the colour variations (or spectra) to find the delicate gravitational tug of a planet. While this works for planets in most orbits, if they happen to circle their star in a face-on orientation, no signal will be received at all. For both cases, this means that even if no planetary signal is detected at all, we can’t definitively say there isn’t one there.

These techniques are also only sensitive to planets larger than a certain size. While the Kepler mission was able to find Earth-sized worlds, similar transit surveys from the ground will only ever be able to find large Gas Giants. Any Mars or Mercury-sized planets will be missed entirely. Radial Velocity is also limited by size, with Neptune or Super-Earth-sized worlds the current limit. These searches are both also bias towards planets close to their stars. To detect worlds at Earth distances is a much trickier prospect than those scraping the surface of their stars.

So many planets will be missed entirely. How can we talk with any certainty about the number of planets in the whole Galaxy?

Well, because the exact problems with these techniques are known, astronomers can estimate how many planets we expect to find. If we know the number of stars studied and the probability of an orbit being perfectly aligned, we can use the number of planets found to estimate the number of planets around all stars.

For example, a study of gravitational lensing by planets showed that on average every star has a planet larger than 5 Earth Masses from 0.5 to 10AU. Similar studies have also been done with Kepler, finding basically the same number: More than one planet bigger than Earth from 0 to 2AU around every star. It should also be noted that these results also only cover a tiny portion of potential planets. Distant Jupiters or low-mass rocky planets were missed completely. So, as our searches become more and more sensitive to small and distant worlds, those numbers can only go up. It’s likely that on average every star in the Milky Way has its own Solar System with multiple planets.

But what about lonely, planet-less worlds? There are certain to be stars without any planetary material wandering the cosmos. For example, those dislodged from triple-star systems, as can happen due to gravitational resonance and scattering, might not hold onto any planetary material. But until we’re able to study a star in perfect detail and definitively say no planets exist, we are forced to stick with what has become the default setting: all stars have planets, and it’s just a matter of time until we find them.

Related articles

• Kepler: Not All That Glitters Is Gold (lshugh.wordpress.com)
• Alien Super-Earth Planets Plentiful in Exoplanet Search (space.com)