Kepler-62e/f

[asterroc]

Okay, here's the skinny on Kepler-62e and f, the latest potential "Earth-like" planets. I'm numbering my points in case you want to refer to them elsewhere.

Enough detail for a science fan, more detail than most news sources will have, not enough detail for an astronomer.

1. Every star has a Habitable Zone (HZ). This is the range of distances from the star where you could theoretically have liquid surface water. Any closer to the star and it's so hot that the water evaporates. Any further and it's so cold the water freezes. In our solar system, the HZ includes the orbits of Venus, Earth, and Mars. Venus doesn't actually have surface water because it also has a really thick atmosphere which results in a higher temperature (greenhouse effect) and the water boiled off. Mars doesn't actually have surface water because it has a really thin atmosphere and the water froze out (it has polar ice caps).


2. Kepler is a space-based telescope that watches the brightness of many different stars over a long period of time. If a planet happens to be orbiting one of those stars in such a way that it sometimes passes in front of the star (called a transit), the planet will periodically block out a little bit of the light from the star. If you want to get a feel for what Kepler is looking for, try out PlanetHunters, a citizen science project where YOU (yes, you) can examine Kepler data and try to find planets. Kepler is run by a group of astronomers, some of whom are employed full-time by NASA, some of whom receive partial funding from NASA and work full-time for a university or some other research institute like the Harvard-Smithsonian Center for Astrophysics (the CfA), some of the work is farmed out to peers who receive no funding whatsoever from NASA, and of course there's also PlanetHunters which has made one planet discovery so far (the first planet in a four-star system, in fact).


3. The Kepler team found evidence of five (transiting) planets around the particular star called Kepler-62 (that is, the 62nd star in the Kepler mission's list). The star is always designated "A" (i.e., Kepler-62A is the star), and the planets follow alphabetically after that from biggest/brightest/first discovered to smallest/dimmest/last discovered, written in lower case (i.e., Kepler-62b, c, d, e, f). This means that Kepler-62e and f are the smallest planets in this system. This is good news if you're looking for Earth-like planets - bigger planets tend to be gas giants (like Jupiter, Saturn, Uranus/Neptune).


4. Their small size is also bad news. We want to confirm that these planets even exist in the first place, not to mention getting more details about them. Kepler can only determine the planets' period from how often the transits take place (and thus distance from their star, which is what makes us say they're in their star's HZ), and the planets' radii from how long the planet blocks the star's light (which is what makes us say they're a bit bigger than Earth). Small planets are really difficult to confirm by other methods. Other methods (than the transit method used by Kepler) include
   
   
   • Direct imaging - small planets are faint enough that they're washed out in the glare of their parent star. Think of the famous Pale Blue Dot photo from Voyager, but even worse.
   
   
   • Radial velocity (RV) - when a big planet like Jupiter orbits its star, it actually makes the star wobble a little bit. Because the star wobbles, we can observe a continually changing Doppler shift of the light coming from the star. A bigger wobble will be caused by a planet that is big (massive) and/or close to the star. Unfortunately, these planets (assuming they exist) are not only small (low mass) but are also far from their star. In addition, the star is actually variable, which means that it pulses in size which also results in a continually changing Doppler shift. In this case, the small wobble of the star is not distinguishable from either the pulsations or from instrumental noise.
   
   
   • Other methods exist (such as gravitational lensing of a background star), but none are applicable to this situation.
   
   So in other words, although we have a lot of data from the Kepler mission about this system, we do not have any other confirmations, neither from the same group using different methods or instruments, nor independent confirmation from an entirely different group.


5. Assuming these planets exist, all we know about them is that their radii are somewhat larger than Earth's. What we do not know is a veritable laundry list: mass, chemical composition, state/phase (solid/liquid/gas), atmosphere, ... The two main scenarios for the main body of the object are mostly rock (like our terrestrial planets, Mercury through Mars, plus the Moon and a few other objects), or a rock core covered with water ice (like Jupiter's moons Europa, Ganymede, and Callisto, and Saturn's moon Enceladus).

So that's my summary. If you want to read other articles, here's the Nature layperson's article which does a good job of not overhyping things. And here's the peer-reviewed journal article in Science (really prestigious!, though my dept has a running gag that 50% of Science and Nature articles turn out to be wrong). I believe the Science article is behind a paywall if you're not at a university, so if you want to read it and can't access it, let me know and I can email it to you.

Originally posted on Dreamwidth. comments there. Comment here or there.

Comments

[hitchhiker.livejournal.com]

thanks, that was an interesting read!