Tag Archives: TNG

The Solar System’s has Four New Neighbors

The number of worlds discovered around other stars is now counted in the thousands. But, if you were to go out on a dark night and try to spot those planet-hosting stars with your own eyes, you would struggle – only 6% of planets orbit stars bright enough for our eyes to pick out. This is especially true of transiting planets; those that pass in front of their star relative to our line of sight. Of more than 1000 such planets known, only one (55 Cancri) is bright enough to see in the night sky. That is, until today…

Position of HD219134 in Stellarium

HD 219134, nestled between Cassiopeia and Cephus, is remarkable in so many ways. It was first studied with HARPS-N, during it’s Rocky Planet Search. This instrument, a spectrograph on the TNG telescope in the Canary Islands, is able to measure the motion of stars so precisely that it can spot the to-and-fro wobble caused by planets.

Amazingly, this instrument found not just one but four planets around this star; a mini solar system just like our own. The outermost is a gas giant on a 3-year orbit, while the inner three are between the size of Earth and Neptune orbiting once every 3, 7 and 47 days.

And the prize for funkiest Colour scheme goes to...
And the prize for funkiest Colour scheme goes to…

ssc2015-02b_Inline[1]At this point, astronomers had no idea if these new worlds transited. But a planet on a 3-day orbit has pretty good odds to pass in front of its star so, taking control of the Spitzer space telescope, they pointed it and hoped. And sure enough, exactly when predicted, the innermost planet blocked out 0.036% of starlight. This fraction is just the surface area of the star covered up, giving a precise measure of the radius of the planet.

Now, with the mass of the planet measured by HARPS and the radius of the planet measured by Spitzer, it’s density can be found. While many similar sized worlds have turned out to be fluffy gas-balls rather than true super-Earths, a density of 5.89gcm-3 puts HD 219134b bang on Earth-like composition. If there was a surface, it’s gravity would be just under twice what we experience on Earth (18.8ms-3). With an orbit of only three days, though, the planet’s star-facing surface is likely to be hot enough to melt!


At only 20 light years away, the newly-discovered solar system around HD219134 is also the closest transiting exoplanet ever found, and one of the 20 closest bright star systems to our Sun. With transiting planets extremely rare, there’s even a chance that this could actually be the closest transiting planet around a bright star (K & G-type).

HD219134’s brightness is also important for astronomers. The brighter & closer a planet, the more interesting ways we can study it. For example, this new world has jumped to the top of the list for those trying to study exoplanet atmospheres. We can also measure the path it takes as it crosses it’s star to determine just how the planet orbits. The outer 3 planets might peturb the orbits of the inner one, causing detectable variations in transit timing (TTVs).

It has truly been a remarkable week for exoplanet astronomy, beginning with the discovery of habitable-zone super-Earth Kepler-452b, and now the detection of the brightest, closest, awesomest transiting planet ever found. And, thanks to a huge array of exciting follow-up options, this will not be the last you’ve heard of HD219134b,



Here’s how you can find the star in the sky (and a very neat animation of the transit):


The paper by Montelabi can be found on arXiv here

Other coverage includes:

Elizabeth Tasker’s piece on “The closest rocky planet ever has been found… so what?

Sci-News: HD 219134: Three Super-Earths Found Orbiting Star 21 Light-Years Away

Daily Mail: Star discovered with THREE Super-Earths, and one is the closest rocky planet ever found outside our solar system Read more: 

Kepler’s Last Stand: Verification by Multiplicity

TNG_LaPalmaFor 3 months a year, the TNG telescope on the island of La Palma turns its high-precision spectrometer (HARPS-N) towards the constellations of Cygnus and Lyra. This is the field of view that NASA’s Kepler space telescope stared at for more than 3 years, detecting thousands of potential new exoplanets using the transit method. There the TNG scans hundreds of Kepler’s potentially planet-holding stars looking for tiny changes in their radial velocity. If detected, this signal will indicate the presence of a real planet, confirming once and for all what Kepler first hinted at many months before. This is the process that, up until now, has been used to definitively find the majority of Kepler’s 211 planets.

New ‘discoveries’ in context

That appeared to change in the blink of an eye this week with the confirmation of 715 new planets using a new catch-all statistical technique. But how did the Kepler team confirm all these new worlds, and can they really be considered real planets?

Without further observations with instruments such as HARPS, Kepler’s 3000 planetary candidates cannot usually be called definite planets. This is because a number of other signals could mimic the transit signal of a star, including tightly bound double-stars that graze one other as they orbit, or unseen dim stars that have binary companions. Alternatively the cameras themselves could be acting up, producing periodic, transit-like signals in the data. Last year a team used simulations of the Kepler data to estimate that around 10% of the candidates were likely to be such false positives.

Kepler Candidates by size

So how can more than 700 worlds be confirmed at once, without any manual work from telescopes on the ground? The answer is through performing statistics on Kepler’s planets. Of a zoo of 190,000 stars observed, Kepler discovered 3000 potential planets, of which 10% are likely to be spurious signals. As a rough estimate then (and the Kepler team go into much more effort than this), the random probability of finding a false positive is 300/190,000, or a rate of only 0.16%.

That number on its own cannot help confirm planets. The trick comes when thinking about Kepler’s hundreds of multiple planet systems. The likelihood of a single-planet system randomly having another false positive also in the data is extremely low. In fact, applying that rough number to the 1000 best single-planet candidates tells us only around 2 of those multiplanet systems should have a spurious planet. Similar calculations can be done for even rarer systems with two false positives, two planets and a false positive, etc.

Possible False Positve Signals

This rate can also be significantly improved by excluding any targets more likely to give these spurious signals. For example, the authors removed more than 350 potential planets from the initial sample for many reasons. Some had instrumental artefacts seen in other stars or had transits close to the limit of detection. Others with V-shaped transits were eliminated as these are more likely to be grazing binary stars. The team also studied the images Kepler took to check for possible transits on a secondary star, eliminating anything where the transit did not in the star’s central position.

Using these cuts, the study narrowed down the search to 851 planets around 340 stars. Applying statistics and using the estimate that 10% of currently detected planets might be false positives, the team found that 849 of the 851 planets were likely to be planets. This corresponds to a certainty of 99.8%,  just greater than 3σ, which in astronomy is usually enough to constitute a detection. This is how “verification by multiplicity” works.

Confirmed Kepler Planets by Size

Of these, 715 are previously un-confirmed worlds. Nearly all are relatively small planets, with radii going from the same as Earth up to that of Neptune. Four of these new planets may also reside in their star’s habitable zone, the region where liquid water could exist on the surface.

As amazing as it would be to nearly double the number of exoplanets overnight, some doubts remain about this method. By eliminating astronomical follow-ups, no extra information can be gleaned. For example, without performing radial velocity measurements, the mass of these planets will never be known. And without other accurate astronomical studies, we cannot accurately determine the nature of the star, and therefore the radius of the planet.

The main difference, though, comes from the impersonal nature of verification by multiplicity. Previous confirmation methods assessed the probability of each candidate being a planet individually. By performing the confirmation in bulk we will know, thanks to the statistics, that at least 2 planets are imposters*. But if exoplanet astronomers can learn to live with that doubt, such planets may well be accepted as confirmed worlds and this simple idea will see the single biggest influx of validated exoplanets in history.

* Here’s another way to compare those statements. Imagine you have two pills. One produces a 0.2% chance of death. The other causes the loss of two fingers (0.2% body mass). By adding these planets to the list of exoplanets, we may well gain a whole new body of worlds, but there will be painful amputations to come in the future.

The two papers, which will be released on March 10th in ApJ, can be found here (Lissauer, 2014) and here (Rowe, 2014).

UPDATE: The new planets are proving reasonably contentious. The exoplanet counter on NASA’s planetquest sits at 1690 , wheras the Paris-based exoplanet.eu remains on 1078. Time will tell whether astronomers accept these as true planets or simply string candidates.