What do you do when you see the flash? In the case of a dying star, you know a supernova is coming. 

A core-collapse supernova occurs when a large star runs out of hydrogen and other elements to fuse. The fusion of elements in a star’s core creates the heat and pressure necessary to prevent a star from collapsing under its own weight. When a star stops fusing elements, its core cools rapidly and collapses. The heavy elements in the core slam into each other and recoil, creating a shockwave that rips the star apart. This causes a dramatic brightening of the star over a period of days, which is what we observe as a supernova. But before the star explodes the shockwave ripples through the layers of the star. It first causes instabilities in the star’s surface, such as plasma jets. When the full shockwave reaches the surface, it liberates a tremendous amount of photons from the star’s interior, causing an initial flash of light before the star begins to brighten.

At least that was the theory. The problem is the initial flash only lasts a few minutes, and it occurs before the star swells into a supernova. Usually we don’t notice a supernova until the star has already brightened a bit. To see the initial shockwave flash, you need to be watching a star before it goes supernova. Since we have no way of predicting when a particular star will begin to explode, we haven’t been able to catch the initial flash. That is, until now, when the Kepler space observatory happen to observe a supernova in its earliest moments.

The Kepler space observatory was designed to find exoplanets. It does this by observing stars for long periods of time, measuring their brightnesses to look for small dips in brightness. Such dips can indicate that a planet is passing in front of the star. It just so happened that a star in its field of view began to go supernova, and so Kepler caught the initial flash of the shockwave. It was really just blind luck, but it confirms the shockwave of a core-collapse supernova.

As we continue to make large scale sky surveys, the chances of observing the early stages of a supernova such as this become more likely. That’s important because it’s only by studying the early stages of a supernova that we will gain a better understanding of their triggering mechanisms.

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There’s news this week of an “impossible” triple star system recently discovered by astronomers. One that “defies known physics.” Needless to say, there’s no need to abandon physics quite yet.

It all comes from a new paper being published in MNRAS titled “KIC 2856960: the impossible triple star.” Despite the overly-hyped title, it is interesting work. It’s based upon data gathered from the Kepler satellite, which looked at the brightness of stars over time looking for exoplanets. Kepler finds exoplanets via the transit method, where the brightness of a star can be seen to dip when a planet passes in front of it. But the method can also be used to study multiple star systems if they happen to have the right alignment.  Just as a planet can cause a star to dip in brightness when it passes in front, one star passing in front of another can have a similar effect.

The team looked at the data from KIC 2856960, for which Kepler gathered data over 4 years. In the data we see a small dip in brightness about 4 times a day, and a larger dip every 204 days. From this, it looks like a close binary of smaller stars (with orbital periods of 0.26 days) orbiting a third star with a period of 204 days. So it is a fairly common triple star system. Not a big deal, move on to other data.

But this team wanted to determine some of the characteristics of this system, such as their exact orbits and masses, so they looked at the data in more detail. Determining the details of a system can be tricky. There are all sorts of things that can add to noise in your data, such as starspots and other stellar activity. This is why exoplanets are divided into confirmed planets and candidate planets. Once you’ve eliminated the noise you can, you try to match the observed fluctuations to particular orbits, and then see if those orbits are stable. Sometimes the results can be deceiving.

What the team found was that the more they looked at the data for KIC 2856960, the more confusing things got. At first glance it looks like a triple star system, but when they tested candidate orbits, none of them seemed to fit. Several of them kind of fit, but there was always some unexplained fluctuation in the data. So the team tried other models, and found a 4-star system that basically worked, but it required the orbits one binary system to be in exact resonance with the other, which seems highly unlikely.

In other words, the Kepler data is inconclusive. It could be a strange 4-star system, or it could be a triple-star system with something else buried in the data. We can’t be certain at this point. This does not make KIC 2856960 an “impossible” system. There’s no evidence that it is defying known physics, just that the data is odd and we don’t understand it.

And that in itself makes it interesting. It is clear that this system is not a simple, boring triple system. It’s a mystery at the moment, but it’s a mystery that could be solved with more work and more data. And that makes it a mission possible.

Paper: T. R. Marsh, et al. KIC 2856960: the impossible triple star. MNRAS (2014)

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Kepler-421b is a cold world orbiting a star about 1,000 light years away. At least according to a new paper announcing its discovery. This work hasn’t yet been peer reviewed, but it looks quite promising.  What makes the discovery a big deal is that it is the longest period planet to be discovered by the transit method.

The transit method of exoplanet discovery (as many of you might recall) is where the brightness of a star is measured over time, looking for small dips at regular intervals.  When a planet passes in front of its star, it blocks some of the starlight, making the star appear slightly dimmer.  The advantage of the transit method is that it is relatively straight-forward, and can be used to discover lots of planets.  The method has two big disadvantages, however. The first is that it requires the orbit of the planet to be along our line of sight.  If it is tilted slightly relative to us, then we won’t observe a transit. The second is that it favors close orbiting planets over more distant ones.  The more distant a planet from its star, the less likely its orbit is to be along our line of site, and the longer its orbit, the less transits we can observe in a given period of time.

Credit: Kipping et al.

For Kepler-421b, only two transits have been observed.  It’s orbital period is 704 days, so that means it is nearly two Earth-years between transits.  If the observation period for the Kepler telescope had been earlier or later by a few months, we’d only have gathered one transit, which wouldn’t be enough to verify the planet.  Normally even two transits isn’t enough to be sure, but these particular transits are a particularly good fit.  You can see this in the image, where the blue dots are from the first transit and the red dots from the second.

Given the distance from its parent star, Kepler-421b is also past what is known as the “snow line” of the system. This line marks the distance where volatiles such as water, methane and other molecules are cold enough to condense or freeze.  It is thought that gas planets form beyond the snow line, which is how they can gain such mass and volatiles.  Often, gas planets will form beyond the snow line, but then migrate inward toward the star, which is why we find planets such as “hot jupiters” close to a star.  Kepler-421b is a Neptune-sized world, so it likely formed near its current location.

Paper: Kipping et al. (2014), “Discovery of a Transiting Planet Near the Snow-Line”, arXiv:1407.4807 [astro-ph.EP]

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Just how difficult is it to discover a planet moving around another star? The Kepler space telescope finds planets by observing the brightness of stars over long periods. If a planet passes in front of its star, the light will dim slightly. But it doesn’t dim very much, so it takes some serious data analysis to discover.

You can get an idea of this challenge in the image below. It is an image of discovered planets plotted as shadows on the star they orbit. We don’t observe the stars as disks like this, only as points of light, but you can see that larger planets are easier to see than smaller ones. How many planets can you see in the image?

Oh, and that one star all by itself? That is the Sun in comparison to the other stars. You can see the shadow of Jupiter pretty clearly, but can you see Earth as well?

Credit: Jason Rowe, Kepler Mission

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Launched in 2009, Kepler was intended as a “planet hunter” telescope. It finds planets by observing stars for long periods of time. To make long observations, Kepler needs to be able to point in the same direction very precisely, and it must be able to adjust its direction if it starts to drift. So how do you keep a telescope oriented?

On Earth, a telescope can be mounted to the ground, and any change in direction can be made by orienting the telescope relative to its mount on the ground. But for a space telescope there is nothing to mount it to. This means there are only two ways to adjust the orientation of a space telescope: thrusters and gyroscopes.

Thrusters are basically small rockets. They release a small amount of propellent, and the telescope moves a bit in the opposite direction. It’s Newton’s third law of motion in action. With multiple thrusters you can adjust the orientation of the telescope. But there are two disadvantages to this method. The first is that every time you make a thrust you lose a bit of fuel. The second is that it is difficult to make thrusts with the precision necessary for Kepler.

Gyroscopes use a different approach. A gyroscope is basically a spinning wheel. When the wheel is spinning, it resists changing direction (a property known as conservation of angular momentum). You can see this effect in the video. With three gyroscopes you can orient a telescope in any direction, and you can keep it in a specific direction very precisely.

The Kepler spacecraft has 4 gyroscopes (called reaction wheels), but about two years ago one of them started acting wonky and was shut down. This wasn’t a huge deal, since the telescope can get along just fine with 3 gyroscopes. But last year a second reaction wheel malfunctioned, and that put the mission at risk.

So what could be done? They basically tried two options. The first was to start up the first reaction wheel in the hope that it would function well enough to be used. The second was to use a combination of thrusters and the two remaining gyroscopes. But neither of these solutions were successful.

So it was feared that Kepler’s planet hunting days were over. There are other projects the telescope could be used on, but its orientation simply wouldn’t be precise enough to detect planets. This would have been really disappointing, but it wouldn’t mean a failure of the mission. Kepler completed its mission in 2012, and then entered an extended mission phase because it was still functioning well. It has gathered tons of data that has yet to be fully analyzed, so there is plenty to keep astronomers busy.

But it turned out the failure of the reaction wheels wasn’t the death of Kepler. Using some clever tricks, a new project known as K2 was devised.  But that’s a story for another time.

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NASA and JPL have announced the discovery of an Earth-sized planet orbiting in the habitable zone of its star.  This has led to some popular press announcements that Earth’s twin has been discovered, but these planets are twins more in line with Danny DeVito and Arnold Schwarzenegger than identical twins. It should also be emphasized that being in the “Goldilocks Zone” of a star does not mean the planet harbors life, or even liquid water.  So what do we know about this planet so far?

A comparison of the Kepler-186 system and the inner solar system. Credit: NASA/JPL

The planet is named Kepler-186f, and is the 5th planet from its star, Kepler 186. The star itself is a red dwarf about half the mass of the Sun. The Kepler study shows now evidence of large solar (stellar) flares over the 4-year observation period, but we do know the star is active, and even that it has starspots. This is an important factor when considering habitability, because red dwarf stars tend to have strong flares and stellar wind, which would act to strip closer planets of their atmosphere.  Red dwarf stars also tend to be much hotter in their youth, only later cooling down to their reddish orange hue.  So planets of such a star may be baked dry as well.

But it’s not all bad news. Since the star is smaller and thus cooler than the Sun, it’s habitable zone is closer to the star.  In this case, Kepler-186f has an orbit roughly the size of Mercury’s orbit in our own solar system. At that distance the planet might not be tidally locked, so it could have a daily cycle similar to Earth’s. It is only about 10% larger than Earth, and while we can’t determine its mass directly, if it is about the same composition as Earth it would have a mass about 1.4 times that of Earth.  That would give it a surface gravity only 15% stronger than Earth’s, so it isn’t likely to have a thick hydrogen-helium atmosphere.

If Kepler-186f has a strong magnetic field, then it is possible that it has a more Earth-like atmosphere, and would be capable of having liquid water on its surface.  Given its size, it could also be geologically active.  It is possible that the planet is the most Earth-like exoplanet discovered so far.  However the most likely scenario is that it is dry and cold.  More Mars-like than Earth-like.

Of course none of this should minimize the importance of this discovery.  It shows that Earth-sized planets do exist within the habitable zones of their stars.  We figured they must exist, but now we know. It is a first step toward discovering a truly Earth-like planet around another star.

Paper:  Elisa V. Quintana et al. An Earth-Sized Planet in the Habitable Zone of a Cool Star. Science, Vol. 344 no. 6181 pp. 277-280 (2014)  DOI: 10.1126/science.1249403 

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The Kepler mission has announced the addition of 715 new exoplanets to the official list of known worlds.  This almost doubles the number of confirmed planets to the list.  

The number of planets discovered each year. Credit: NASA

It’s hard to understate just how big of a leap this is.  If you look at the figure you can see how this latest announcement dwarfs past annual tallies.  What’s more, most of these new planets are not large, Jupiter-type worlds.  The number of Earth-sized worlds has increased by a factor of 4, and the number of “super-Earths” (up to twice Earth size) has increased by a factor of 6.  So most of these new planets are rocky and Earth-like.

So how were we able to nearly double the number of planets in a single go?  Usually data for an exoplanet has to be examined by hand.  This means going through the candidate planet data and determining if there is strong enough evidence to confirm a planet.

But this time a team looked at candidate planets that are part of a possible system of multiple planets.  They then applied statistical analysis on the data to pull out the best possible solutions that would be part of a stable system.  This poses its own challenges, as I’ve talked about before.  But the advantage is that when planetary systems are confirmed they yield multiple planets at once.  Looking at planetary systems also helps distinguish signal from noise.

Kepler has finished its primary data gathering period, but there are still more than 2,000 candidate exoplanets that could still be confirmed.  Then there are other spacecraft gathering data that will lead to still more exoplanet discoveries.  We’re just getting started.

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The Kepler telescope has led to the discovery of more than a thousand exoplanets.  But there are still several thousand “candidate” planets.  An exoplanet can be in candidate status for various reasons.  The data may not be solid enough to confirm the planet, or what looks like a planet doesn’t seem to have a stable orbit, or half a dozen other reasons.  Sometimes the data might look good, but it’s just…strange.  As an example, consider the data of an exoplanet known as KIC 12557548 b.

Kepler is not capable of observing planets directly.  Instead Kepler gathered long term data on the brightness of about 150,000 stars.  If a planet orbiting a particular star passes in front of the star (transits the star), then the brightness will dip slightly.  So by looking at the brightness variations of these stars, we can look for transiting planets.  So far we’ve found about 3,500 candidate planets in the data.

The data on KIC 12557584 b was reported in the Astrophysical Journal.  Like other candidate planets, the data shows a periodic variation in brightness.  In this case the dips in brightness occur every 15.6 hours.  The parent star of the planet is a K-class star about 70% the mass of the Sun.  With an orbit of 15.6 hours, that would put the planet extraordinarily close to the star, with an orbital distance of about two stellar diameters.  By comparison, the orbit of Mercury is about 40 solar diameters.

What makes the data unusual is that the periodic dip in brightness is not consistent.  The amount the brightness dips can vary from about 0.2% to 1.3%.  This variation is far too much to be caused by a single transiting planet.  So what could cause such a wide variation?

The authors of the paper looked at several possible solutions.  One idea was that the dip is caused not by a planet, but by a companion star with an accretion disk.  As the accretion disk varies in orientation the amount of starlight blocked would vary, thus accounting for the fluctuation in brightness.  But the observational data limits the mass of the planet to no more than three Jupiter masses, which is far too small to be a companion star.  Such a mass is also too small to have an accretion disk.  Even a ringed planet wouldn’t be sufficient to account for the variation.

Another possibility is that we aren’t observing one planet, but two.  A double planet of two Jupiter-sized worlds could account for the variation in brightness.  When lined up they would block less light than when side-by-side.  But two mutually-orbiting Jupiter-sized planets so close to the star would be unstable, so this doesn’t look like a good solution.

Artist rendering of the dying world model. Credit: NASA/Kepler

A third possibility seems more reasonable.  A planet so close to the star would be quite hot, about 1,800 degrees Celsius.  If it is a small planet, then material from its surface could be boiled off, which would give the planet a trail of dust.  The planet and dust would then transit the star, and variations in the dust would explain the variations in stellar brightness.  For this to be the case, the planet would need to have a mass about 10% that of Earth (or two times that of Mercury).  Much larger and the evaporating surface material wouldn’t be able to escape the planet’s gravity.  If this is the case, then we may have found a dying planet.  With its estimated mass and the amount of material loss, it would only be around for another 200 million years or so.

As the authors point out, the dying planet model is both reasonable and agrees with current data, but proving the model will take more observational data.  For example, infrared observation of the star to look for absorption features could prove there is indeed a cloud of silicates trailing the planet.

For now it looks like Kepler has found a dying world.  Further observations will either bring this planet into the confirmed column of exoplanets, or open the door to something much more strange.

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The planets in our solar system all orbit in the same general plane.  They aren’t exactly aligned, but their orbital planes are all within a few percent of each other.  Some of the minor planets, such as Pluto, have orbits that are tilted differently than the main planets, and objects such as comets can have orbital orientations in all directions, but the main part of our solar system is essentially flat.

This makes a bit of sense.  As a star forms, a protoplanetary disk also forms, out of which the planets coalesce.  Since the protoplanetary disk is generally aligned with the equator of the star, the planets form with that same basic orientation.  This is what we see in our solar system and in many exoplanetary systems.

Of course planetary interactions over the history of a solar system can lead to changes in the orientation of certain planets.  In our solar system, for example, Neptune’s moon Triton was once a minor planet like Pluto.  But early in the solar system’s history Triton was captured by Neptune in a retrograde orbit.

Planets that aren’t captured by a larger planet can be thrown off into tilted orbits.  Before exoplanetary systems were discovered, it was generally thought that such interactions would just send minor planets into tilted orbits.  But we soon found exoplanetary systems where some larger planets are in tilted orbits.

These systems happen to have what is known as a hot Jupiter.  That is, a Jupiter-type planet that is particularly close to the star.  Computer simulations of forming planetary systems show that large planets tend to form early and tend to migrate inward over time.  This process results in the hot Jupiters we see in many exoplanetary systems.  Since planetary systems with tilted orbits have hot Jupiters, it’s generally been thought that the gravitational interactions between the large forming planet and smaller forming planets leads to the smaller ones being thrown into tilted orbits.

But now a new discovery casts a bit of doubt on that idea.  A recent article in Science finds an exoplanetary system known as Kepler 56, with two smaller planets in a common plane and a third planet in a tilted orbit.  The interesting thing about this system is that it doesn’t seem to have a hot Jupiter, and the outer planet in the tilted orbit is actually the most massive of the three.  However this system got this way, it wasn’t by the hot Jupiter mechanism.

It gets more interesting because having your largest planet tilted relative to the others should make the system somewhat unstable.  Gravitational interactions should cause the inner planetary orbits to start tilting, such as through the Kozai mechanism.  But the authors ran computer simulations of the system and found that the two inner planets reinforce each other’s orbits.  So the system remains stable, despite a large tilted planet.

As we find more exoplanetary systems, we’re discovering that they have a much wider diversity than we expected.  And we’re learning that gravitational dynamics is even more complex than we had once thought.

Visualization of the Kepler-56 system. Credit: Daniel Huber/NASA’s Ames Research Center

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Previously, I’ve talked about binary stars, and how you can observe the orbits of some stars directly to determine their mass.  This time we will up our game and look for planets orbiting other stars.

As you might suspect, detecting planets is much more difficult than detecting binary stars.  For one, planets are smaller than stars, so their gravitational effects are smaller.  For another, they don’t shine like stars, so they tend to be hidden in the glare of their sun.

There are several ways to detect planets, but one of the more interesting methods is known as the transit method.  You might remember the transit of Venus, when Venus passed in front of the Sun as seen from Earth.  Imagine the same thing happening around a star, where a planet transits that star from our point of view.

When a star’s planet passes between us and the star, some of the starlight is blocked by the planet, making it slightly dimmer.  The change isn’t much, only a small fraction of a percent, but it is enough to measure.  The figure below (taken from http://kepler.nasa.gov/Mission/discoveries/) has a plot for a planet known as Kepler 20b, and you can see the effect of the transit is pretty clear.

What is particularly impressive is just how much data we can gather from watching transits such as this one.  By measuring multiple transits, we can determine the shape and size of its orbit, which tells us the planet’s mass (as well as the mass of the star).  By observing how much the star dims during transit, we have a measure of the planet’s size.  With size and mass, we can determine its density, which tells us something about the type of planet it is (gas giant, rocky, etc.).  Knowing its distance from the star gives us a handle on its surface temperature.

Most of the planets we’ve discovered so far by the transit method have been large planets close to their star.  As we gather more data, it will be easier to detect smaller planets with more Earth-like sizes and orbits.  Already we’ve detected planets that lie within their star’s habitable zone.  Soon we’ll likely discover an Earth size planet in an orbit similar to ours.  All from light curves like the figure below.

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