The closest rocky planet ever has been found…. so what?

A new planet has been discovered that is —to slightly mock the NASA press release— a “mere 21 light years away”. I gently mock because such a distance would take around 380 thousand years to reach, assuming we travelled at the speed of the Voyager space craft when it left the solar system at 37,000 miles per hour.

The planet has a mass of 4.5 x Earth and orbits so close to its star, that a year takes a (genuinely) mere 3 days. This makes the world a likely molten cookie and life —most definitely— is out of the question. The planet’s size is 1.6 x Earth, giving a density that strongly suggests a rocky (rather than gaseous) composition. It's data that pushes the discovery into first place for the nearest rocky planet to Earth. Yet, given the conditions on such a scorching world, is this find even interesting?

The PC term in this case is that it's a world with "great potential". 

Last week, NASA announced the discovery of exoplanet Kepler-452b: a planet that swiftly gained the label “Earth 2.0.” This was as daft as a brush, and I went on at some lengths about it here:

The main problem with describing exoplanets as “Earth-like” is we know nothing about their surface conditions. The renowned planet finder, the Kepler Space Telescope, is great for statistics: it has taught us there are a myriad of worlds in all different positions from their star, and in all different sizes. Yet because we know nothing of the planet’s atmosphere, we cannot tell what it would be like to stand on the planet’s surface. Indeed, in many cases (including Kepler-452b) we can’t even get a bulk density measurement, meaning that the planet may not even have a surface. If Kepler-452b was actually a Neptune-like gas giant or a water-less planet with a graphite ground, would we still say it was “Earth-like”? 

…. I’m going to hope you all said “no” at this juncture.

Our current major missions allow us to find these worlds, but they don’t let us find out what these worlds are like. This is where this new planet comes into play. 

The planet’s name is HD 219134 b and while 21 light years might not be an afternoon trip, it’s a hell of a lot closer than Kepler-452b’s 1,400 light year distance. This means its star is bright. Bright stars mean easier measurements and easy measurements mean accuracy. 

The planet was initially found by a wobble in the star’s position. This is known as the ‘radial velocity’ technique and gives a handle on the planet’s mass. The exact mass cannot be pinned down by this measurement alone, because the angle of the planet’s orbit needs to be known. If the Earth, the star and the planet are all beautifully aligned, then the planet drags the star directly towards us. On the other hand, if the planet’s path is tilted, then it will pull the star at a slight angle, causing us to underestimate the strength of its tug and so its mass. The radial velocity method therefore provides a minimum planetary mass. 

The star’s wobble actually indicated a system of four planets. Closest to the star was HD 219134 b, with an orbit taking 3.09 days. Slightly further out but still decidedly steamy, were planets with orbits taking 6.76 days and 46.78 days and finally —in an effort to over compensate for the squiffy periods of its hot siblings— a Saturn-sized world taking 1190 days. 

The proximity of HD 219134 b to its star suggested an exciting option. There was a high chance that the planet would not only wobble its star but also transit its surface; that is, pass directly between the star and Earth to produce a dip in the starlight. For planets orbiting further out, the chance of finding a transit decreases. Any slight tilt in the orbit would lift the planet out of the Earth’s view. But so close to the star, HD 219134 b’s orbit would need to be seriously tilted to not block out the star’s light.  

OK, so maybe “serious” is taking it a bit far. There was a 9% chance that the planet would transit and that was good enough to check out. 

The instrument that did the checking was the Spitzer Space Telescope, which did indeed confirm the transit. The transit detection nailed down the angle of the orbit to provide a confirmed mass and radius. Due to the proximity of the system, these could be measured to a great 6% and 9% accuracy, and produce a tight constraint on the planet’s bulk density. It rolled in at 5.89 g/cm3 which (allowing for its larger size with some interior modelling) pointed to an Earth-like composition with an iron core and magnesium silicate rock. 

Thus far, the planet’s proximity has nailed us a good value for the bulk density. This is great since larger errors lead to pretty massive variations in how solid the planet might be. But this still doesn’t tell us a whole lot. The composition is suggestive of Earth, but we’re basing that on a giant average. Given that formation models suggest the planet did not form in its current position, a more detailed look at the planet composition could tell us a whole load about its history. 

(This is personally pretty exciting to me, since the most common planet found by Kepler are giant Earths snuggled up close to the star and —really— what the hell are they doing there? Our solar system didn’t do anything like that.)

This is where HD 219134 b’s potential comes into play, along with a hefty dose of patience. The fact it the planet is relatively close and transits means that it makes a fantastic candidate for the next generation of missions that are already being prepared. These will look particularly at planet atmospheres. By examining the starlight as it passes through the gas above the planet’s surface, the different molecules in its atmosphere can be picked out.  Admittedly, as a rocky world so close to its star, HD 219134 b isn’t likely to have a whole lot of air, but what is in the gas can tell us what elements might be in the rock below or when indeed, it acquired that atmosphere. 

This is pretty neat since the actual record holders for the closest confirmed planet are not candidates for such missions. GJ 674b and GJ 15 Ab sit closer at 14.8 and 11.7 light years, but they do not transit their star. This leaves the worlds without a radius measurement, meaning nothing is known about their composition.

So HD 219134 b may not have a Starbucks but I agree with this NASA press release that such a world is a “potential gold mine of science data”. 

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From the NASA press release below, showing the location of  HD 219134 b.

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NASA press release:

Journal paper:

#exoplanets   #superearths #NASA
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