As Mission Dies, Gaia Revises Kepler Earth-sized Exoplanet Count

As NASA’s Kepler Space Telescope ends its mission once and for all, ESA’s GAIA Space Telescope data shows that some Kepler planets need revising.

2962 Views | Published on 1st Nov, 2018

Hello Space Fans and welcome to another edition of Space Fan News. In this episode, as NASA’s Kepler Space Telescope ends its mission once and for all, ESA’s GAIA Space Telescope offers new data prompting a reassessment of the estimate of rocky habitable Earth-sized worlds found by Kepler. This new information reduces the number of potentially habitable rocky worlds considerably.

On the same week that the Kepler Space Telescope finally ran out of fuel, ending its nine year mission, new light (so to speak) has been shed on some of Kepler’s finding requiring a reassessment of the number of potentially habitable rocky worlds that Kepler found while it was in operation.

GAIA is an astrometric space telescope that measures the brightness and positions of billions of stars. Launched in 2013, Gaia is creating an ultraprecise 3D map of stars in our Milky Way. This map includes position information for 1.7 billion stars and distance data for 1.3 billion stars. Gaia is providing the most accurate data of its kind every produced and it is adding valuable insight into our knowledge of the Milky Way Galaxy.

OK, so what has Gaia done to Kepler? Remember that Kepler finds exoplanets using the transit method and from the tiny dips in brightness astronomers can learn several things. First, the amount of the dip is directly related to the brightness of the star. The size of a exoplanet is derived from the percentage of the stellar disk that is blocked. Big planets block a larger amount of light than smaller ones. Makes sense, right?

Secondly, how many times you see this dip over time tells you something about its orbit. If Kepler was far away and looked at the Earth passing in front of the sun this way, it would see a dip that occurred every 365 days. That tells astronomers something about its distance from the star. And this is usually the number people use to see if it’s in the habitable zone or not.

So couple these light curves from Kepler with what we already know about the star: it’s type, size, temperature and all that good stuff, astronomers piece together a picture of what the planet may be like: things like how big the planet is as well as how far away it is from the star. That’s not a lot of information to go on to give us a complete picture, like whether there is an atmosphere or not, but it’s a good start.

What Gaia did was give better information on the brightnesses of the stars in Kepler’s original field of view. Remember Kepler stared for five years at the constellation Cygnus to specifically look for earth-sized transits. And over those five years, Kepler found 30 roughly Earth-sized worlds in orbit around stars located in that stars’ habitable zone.

When Gaia looked at those stars, it returned observations that some of those 30 candidate worlds orbited stars much brighter that previously thought. And since the light curves measured by Kepler are proportional to both the stars brightness and the planets size, if the stars are brighter, that means the planet must be bigger.
Why? The brighter a star is, the hotter it is and it pumps out more heat. If the star is brighter and hotter, then the light curves seen by Kepler must have come from bigger planets, so they are larger than Earth-sized. Hotter stars also mean the habitable zone has moved further out and the planets Kepler THOUGHT were in the habitable zone, actually aren’t any more.

So what’s the damage? Of the 30 planets that Kepler thought were earth-sized and in the habitable zone, now, with these new Gaia observations, only between 2 and 12 are thought to be.

Yeah, I know, that’s disappointing, but let’s remember that it was Kepler who showed us that there are more planets in the galaxy than there are stars over 50 percent more! So there is still plenty of opportunity here, this doesn’t really affect some of the other candidates we’re all juiced about, like TRAPPIST-1, Ross 128b and all those that weren’t found by Kepler. I’m interested to see how, if at all, the Gaia data affects those estimates. I would imagine not much since they are pretty close by and we know a lot about those stars. Still, you never know.

I’m really impressed by what Gaia is doing, on the surface its mission isn’t sexy, measuring brightness and position is pretty boring stuff, but that information is vital in getting to the more interesting stuff, like whether a planet is in a habitable zone and if it’s Earth-sized.

Another interesting thing is that astronomers aren’t sure how big a planet can get and still be rocky. I guess past a certain point, rocky planets cannot be sustained and they are still trying to get a handle on those limits.

Well, I just want to say so long Kepler and thanks for all the exoplanets. I have a shirt made to commemorate the Kepler mission on Teespring, the shirt is down below the video if you want to take a look. I had one shipped to me but I haven’t gotten it yet. I’ll probably have it for the next video though.

That’s it for this episode Space Fans, I want to thank all of Deep Astronomy’s amazing and supporting Patreon Patrons who, along with OPT Telescopes - a leader in telescopes worldwide - make sure these episodes are produced and sent to you with no advertising and free of charge. Thank you all so much.

Thanks to all of you for watching and as always, Keep Looking Up!

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