Kepler-78b: An Earth-sized Inferno

Even though NASA's Kepler spacecraft is likely no longer able to continue its search for exoplanets due to the failure of two of its reaction wheels (which help keep its optics pointing steadfastly at the same region of sky), it has accumulated a truly vast mountain of data containing 3277 potential-planet signals over its 3.5 year mission. That's enough to keep mission scientists busy confirming these signals for years to come, despite the probable end of Kepler's main science mission. Recently, a team of researchers at MIT working with Kepler data discovered a small planet with a radius of just 1.16 Earth-radii orbiting a sun-like star. However, the planet has one of the shortest orbital period of any planet ever discovered. This world, called Kepler-78b, whips around its parent star in just 8.5 hours. 

A single year on this planet takes about as long as a good night's sleep.

Of course, orbiting that close to its sun has some pretty extreme consequences. The surface temperature of Kepler-78b is estimated to be 2300-3000 K, which is far in excess of the melting points of most metals and silicate minerals, meaning that the entire surface of the planet is a roiling ocean of lava. If the planet has any kind of atmosphere, it would be made of rock or metal vapor. The size of its orbit (just 3 stellar radii across) also means that Kepler-78b is tidally locked to its star, always showing the same side to the star like our Moon does to the Earth.

The light-curve data for Kepler-78, from Sanchis-Ojeda et.al.

What's more interesting is what the light-curve reveals. A light-curve is a plot of a star's brightness over time. The Kepler spacecraft uses them to search for exoplanets by looking for tiny periodic changes in the star's brightness as a planet transits across the face of the star. The minimum point on the curve (the dips) represent when the planet is directly between us and the star. But what's interesting here is the smaller dip between the two transit events, starting at around the 3.5-4 hour mark. What's going on here is that as the planet is going around the star, the angle at which we view it is changing. We're actually seeing the change in illumination on the surface of the planet as it goes around the star. We can actually see the phases of this planet! This is a direct measurement of starlight reflected off the planet's surface. That can tell us a lot about the kind of surface we're looking at, especially things like composition.

But you also must remember that this is still a very small change in brightness; around 10 parts per million. That small amount of reflected light can still be detected through the glare of the host star! That's pretty amazing!

For all that, we don't really know that much about this strange new world. The transit method can only tell us so much. We have its radius and orbital period; 1.16 Earth-radii and 8.5 days, respectively. From the orbital period, we can calculate the average distance from the star with Kepler's Third Law; P^2 = A^3, where P is the period of the planet in years and A is the distance in astronomical units, or AU. This gives an average distance of just 0.010 AU, or just 1.5 million kilometers. We can also estimate the surface temperature based on the brightness of the star and the distance of the planet, which yields the 2300-3000 K estimate. And that's really about it. The MIT paper posits an upper mass bound of 8 Earth-masses, but the actual value is most likely far lower.

While this planet is far from being habitable, the fact that it and several other exoplanets have been detected with diameters about the same size as Earth means we now have the ability to detect planets in the same size range as our own. It is only a matter of time before we find one at the right distance from its star for liquid water to be stable. And there very well might be one sitting in that vast mountain of planet candidates, just waiting to be uncovered.

Nova Delphini 2013

Ah, the inaugural posting. It's always so difficult to come up with a way to start off a blog. In many ways the first post can set the tone of blog for many posts to come. So what do I have planned for this cozy little corner of my virtual world? The goals for this blog are outlined in the About page, but for the sake of redundancy, I'll review them here.

I'm a junior at Penn State University. I'm a fresh transfer student coming in from a local community college, so it's somewhat of a big transition for me. My major right now is Geoscience, but that's simply a placeholder until I earn enough credits to switch to my desired major which is Astronomy and Astrophysics. This blog is, in part, a project to document my journey as a student through the world of academia as I start my scientific career and begin the transition from undergrad to grad student.

The other mission of this blog is to post about topics in astronomy, space science, and space exploration which I find interesting or relevant. This can be anything from a recently discovered exoplanet to discussions musing about why Titan's methane seas are mysteriously flat and waveless. So with that in mind, let's jump right into it, shall we?

A nova has recently gone off near the constellation of Delphinus. The nova was discovered by Japanese amateur astronomer Koichi Itagaka on the night of August 14^th, using a 7-inch reflector telescope. A few hours after its discovery, the nova was confirmed at magnitude 6.8, which is just visible to the naked eye under a dark sky. You'd have better luck seeing it clearly in binoculars or a small telescope. Unlike a supernova, where a massive star destroys itself in a massive explosion, novae occur in binary systems where one component is a small but massive white dwarf star siphoning material away from it's partner - usually a giant star, but it doesn't have to be - forming a disc of matter around itself. As this stolen gas falls onto the surface of the white dwarf, it gets compressed due to the dead star's high gravity. Once all that hydrogen reaches 20 million K, it undergoes fusion and creates a violent nuclear explosion on the white dwarf's surface, which can be seen from a great distance away. The white dwarf survives this explosion, and can often experience several novae as it collects more matter from its partner star.

Animation of Nova Delphini 2013, from the Remanzacco Observatory Blog.

The event is now being referred to as Nova Delphini 2013. On August 16^th, Nova Delphini 2013 reached a peak brightness of 4.3 and has been rapidly declining since, at a rate of almost 1 magnitude per day. Novae are classified by the amount of time it takes for them to dim. There are four subtypes; Fast (NA), Slow (NB), Very Slow (NC), and Recurrent (NR). Fast novae, such as this one, typically fall rapidly in brightness over a period of days, falling to 1/16^th maximum brightness in 100 days or less. The slowest novae can last over a decade, which makes them quite unusual. Recurrent novae experience periodic outbursts of brightness, typically separated by many decades. It is not yet known if Nova Delphini 2013 will erupt again, but we will no doubt be keeping an eye on this region of the sky for the next few decades.