We now know the scale of the universe to within 1% accuracy.  The results have been presented in an arxiv preprint.  It isn’t yet peer reviewed, but it has been submitted to the Monthly Notice of Royal Astronomical Society, and I suspect it will be accepted soon. 

The data comes from the Baryon Oscillation Spectroscopic Survey (BOSS), which has measured the direction and redshift of about 1.2 million galaxies very precisely.  The reason this is important is that it gives us a really good idea of the distribution of galaxies in our universe.  The galaxies measured are not close ones.  Their redshifts (known as z) are from 0.2 – 0.7, which means the light we observe has travelled for about 2.5 to 6.5 billion years.  The galaxies are so distant that the answer to “how far away?” depends on what definition of distance you use.

Regardless of which definition of distance you mean, the greater the z redshift, the greater the distance.  So by measuring this redshift very precisely for more than a million galaxies we know how galaxies are distributed across billions of light years.  This allows us to look at the way galaxies clump together.  The distribution of galaxy clusters lets us determine what is known as Baryon Acoustic Oscillation (BAO).

The basic idea of BAO is that small fluctuations in the distribution of matter and energy in the very early universe cause galaxies to cluster in a particular way.  These are known as baryon acoustic oscillations, because they are variations (oscillations in space) of galaxies (baryonic matter).  The “acoustic” part of the term means they have a sound-like quality to them, not that they have anything to do with sound as we usually mean.

You can get an idea of this if you think of how sound waves affect air molecules.  The sound waves cause the air molecules to be bunched together a bit in some regions (higher pressure), and spread apart more in others (lower pressure).  As a result, sound waves create a variation of clustered air molecules. The clumping of air molecules depends on the speed of the sound waves in air.  In the same way, the distribution of galaxy clusters depends depends on the scale of the cosmic ripples, somewhat like the artistic image below.

These fluctuations depend on the rate at which the universe expands, which depends on the amount of dark energy in the universe.  So making such precise measurements of galaxies allows us to determine the amount of dark energy in the universe.  From this survey, the present scale for clumping (known as the standard ruler) is about 490 million light years.  This puts dark energy at about 69.2% of the universe.

We don’t yet know what dark energy is, but we now have a better measure of its effects.

Paper:  Anderson, et. al. The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Baryon Acoustic Oscillations in the Data Release 10 and 11 Galaxy Samples. arXiv:1312.4877. (2014)

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In 2003 and 2004, the Hubble space telescope looked at a dark patch of sky in the constellation Fornax.  After gathering light for about 275 hours, what it found was an image of more than 10,000 distant galaxies in a patch of sky about the size of a grain of sand held at arm’s length. 

Assuming this patch of sky is typical, we can calculate that there more than 100 billion galaxies in the visible universe.  A typical galaxy has about 100 billion stars, and the vast majority of those stars have planets.  All of that from a single patch of sky known as the Hubble Ultra Deep Field (HUDF).

Of course that assumes the deep field captured by Hubble is typical.  There have been a few other deep field observations made, but not a wider survey of different portions of the sky.  Now there’s a new project known as Frontier Fields that will answer whether the HUDF is typical or not.  The project will make long observations of six dark patches of sky.  These images will be similar to the HUDF, and will give us an idea of whether the distribution of galaxies we’ve seen is typical.

But there’s another aspect to this project that is equally interesting.  The Hubble telescope has different detectors, so it can make multiple observations at the same time.  In this case it is the Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3).  These two detectors look at slightly different patches of sky.  So while one is looking at a dark patch of sky, the other is focused on a cluster of galaxies.  The Hubble is then rotated so that each camera looks at the other patch.  Since the ACS is viewing at visible wavelengths, and WFC3 at infrared wavelengths, we’ll have a wide rage of data for both patches of sky.

The reason the project is also looking at galaxy clusters is because galactic clusters are also clusters of dark matter, and together they can gravitationally lens even more distant galaxies.  So while one camera is gathering data for a deep field observation, the other is gathering data on even more distant lensed galaxies.  Do this for six different locations in the sky and you have a good idea of the distribution of the most distant galaxies in the visible universe.

All that from a space telescope that is nearly 24 years old.

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Every now and then rumors of some companion star or planet to the Sun hits the web.  The most infamous of these is the Nibiru hypothesis, which claims that a large planet will sweep into the inner solar system, sending Earth to its doom.  While claims of Nibiru have always been unfounded, there has been legitimate speculation about a possible companion to the Sun, either a small star or large planet with an orbit that takes it to the edge of our solar system. 

One of these ideas was a companion star known as Nemesis.  The idea for Nemesis came from a periodicity seen in the rates of extinctions in the fossil record.  It was first noticed in 1984 that the rate of extinctions seemed to spike every 27 million years, going back about 500 million years.  This led several astronomers to propose a companion star, which came to be known as Nemesis, as the cause of these extinctions.  The idea was that Nemesis to the Sun could be in an elliptical orbit that takes it into region of the Oort cloud.  Its gravitational interaction would then disrupt bodies in the Oort cloud, leading to a dramatic increase in comets sent toward the inner solar system.  The higher impact rate would then lead to periodic extinctions.

This idea is no longer considered valid for two reasons.  For one, there isn’t any evidence that the periodic extinctions is correlated with higher impact rates in the inner solar system.  For another, a companion star with a period of 27 million years would need to have an orbital radius of about 1.5 light years.  This would put it so far away from the Sun at times that the gravitational interactions with other stars would cause its orbit to fluctuate.  Orbital simulations show that the orbit of such a star could not be stable over 500 million years.  So even if the periodic extinctions were due to higher impact rates, it can’t be due to a Nemesis star.

Another companion idea comes from an analysis of long period comets.  Long period comets are ones that come from the far regions of the solar system, and swing into the inner solar system before heading out again.  Comet ISON was a long period comet, for example.  These comets generally come from all directions, which is why they are thought to originate in the Oort cloud.

But in 1999, John Matese, Patrick Whitman and Daniel Whitmire argued that there was a slight bias in the origins of these comets.  Rather than coming from random directions, they were slightly more likely to originate from directions along the plane of the solar system.  They proposed that this could be explained by the presence of a large gas planet with an orbital radius of about 15,000 AU, or 500 times farther from the Sun than Neptune, which they named Tyche.

Tyche (if it existed) would be detectable by the Wide-field Infrared Survey Explorer (WISE), an infrared space telescope.  Now a new paper in the Astrophysical Journal has presented new results that show Tyche doesn’t exist.  In the paper, the author analyzed WISE data over an extended period of observations looking for the motion of dim objects.

The more massive a planet, the warmer it would be from its own internal heat, and therefore the brighter it would be at infrared wavelengths.  The survey found no companion objects larger than Jupiter closer than 82,000 AU, or Saturn at 28,000 AU.  This is much farther than the proposed distance of Tyche.  It is, in fact in the region of the proposed distance of Nemesis.

So it looks like the Sun doesn’t have any companions larger than Jupiter, at least within 1.3 light years.

Image: Don Dixon. HT to +Hans Havermann for tracking down the source.

Paper:  Adrian L. Melott and Richard K. Bambach. Nemesis Reconsidered. Monthly Notices of the Royal Astronomical Society Letters 407, L99-L102 (2010)

Paper:  Matese J. J., Whitman P. G., Whitmire D. P. Cometary evidence of a massive body in the outer Oort cloud. Icarus 141, 354 (1999)

Paper:  K. L. Luhman. A Search for a Distant Companion to the Sun with the Wide-field Infrared Survey Explorer. Astrophysical Journal 781, 4. (2014)

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I’ve talked quite a bit about planets around other stars, known as exoplanets.  Most of the exoplanets we’ve discovered are Neptune-sized worlds, but we’ve found exoplanets smaller than Mercury.  But in terms of size, that is about our limit given current technology.

Given what we understand about our own solar system, we would expect that these exoplanetary systems also have smaller objects, including asteroids and comets.  We haven’t observed any exo-asteroids, but we have detected exocomets.  This is pretty remarkable, since most of the exoplanets we’ve discovered are through things like transit data and the like.  We have only imaged a few of the larger exoplanets, and then only as a small blur. 

So how can we detect comets around other stars?  Even though comets are much smaller than planets, they vent dust and gas when they are active, which produces the coma and tail of a comet.  It is the vented gas that we can detect.

For example, an upcoming article in Astronomy and Astrophysics looked at data from the High Accuracy Radial Velocity Planet Searcher (HARPS).  The main goal of HARPS is to measure the Doppler motion of stars, and to do that it needs to make good observations of the line spectra from stars.  These line spectra can be used to “fingerprint” the various elements and molecules that exist in the star.

In this case the data was from a star known as HD 172555, which is a young star where a planetary system is still forming.  This means it still has a disk of gas and dust around it.  The HARPS telescope looked at line spectra from this circumstellar disk, and the team found that a few of the spectral lines were transient.  Sometimes being visible in the spectrum of the star, but not seen at other times.  When the team measured the Doppler shift of these transient lines, they didn’t match the overall Doppler shift of the star.  This means the lines were not due to some change in the star itself.

The most likely explanation is that these transient lines occur when gaseous material passes in front of the star, thus absorbing some of the starlight at particular wavelengths.  This is exactly the type of thing you would expect if a comet passes in front of its star.  Thus we have observational evidence of comets around other stars.

One of the interesting things about the exocomets detected so far is that they are all part of young solar systems.  We typically think of comets as icy remnants from the formation of our solar system, the leftover bits that never became part of planets.  That’s true for the comets of our system, because ours is an older solar system.    But as a planetary system forms, clumps of dust and ice form into comets and protoplanetary asteroids, many of which collide to form planets over time.

These exocomets are not remnants of an old solar system, but rather the seeds of new ones.

Paper:  Flavien Kiefer, Alain Lecavelier des Etangs, et al.  Exocomets in the circumstellar gas disk of HD 172555.  arXiv:1401.1365 [astro-ph.EP] (http://goo.gl/8BHKBt)

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Last year I wrote about how Neptune was discovered by analyzing the motion of Uranus.  After Neptune’s discovery, analysis of its orbit showed possible irregularities that some astronomers argued could be evidence of an even more distant planet, which came to be known as Planet X. 

One of the strongest proponents of Planet X was Percival Lowell.  He was famous (or perhaps infamous) for claiming that canals on Mars were evidence of some kind of Martian intelligence.  This led to him being largely ignored by the astronomical community, but he was also wealthy, and funded an observatory in Arizona now known as the Lowell Observatory.  In 1906, Lowell began a search for this distant planet.  He was never successful, but his observatory did take photographs of an object past Uranus.  This new object was discovered by Clyde Tombaugh in 1930 when he analyzed the images below.  This new object became the 9th planet, Pluto.

By 1978, Pluto’s mass was determined, and it was found to be remarkably small.  It was not nearly massive enough to affect the orbit of Neptune in any measurable way.  So further searches for a “Planet X” were undertaken.  But over time analysis of Neptune’s orbit found that there wasn’t any evidence of gravitational influence by a more distant planet.  So, there is no Planet X.  (And no, Nibiru doesn’t exist).

But there are objects beyond Neptune.  Lots of them.  We now know of at least 1,200 such objects, including four dwarf planets (Pluto, Eris, Haumea and Makemake).  None of these is a “planet x” in the way Lowell postulated.  There are still searches for a much larger planet out on the edge of our solar system, but so far there is no evidence for such a planet.

So Percival Lowell’s quest for Planet X was a failure.  But his search for a planet beyond Neptune marks the beginning of our exploration of the outer regions of our solar system.  In that respect, Lowell’s efforts were a great success.

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A new paper in Nature (unfortunately behind a paywall) has announced the discovery of a triple star system consisting of two white dwarf stars and a neutron star.  This has the potential to be a very big deal, because it may allow us to further test general relativity. 

General relativity has passed every experimental test so far, but these tests are of “weak field” effects.  This means we have confirmed the first-order effects of general relativity, but not the higher order ones.  You can get an idea of what this means by imagining the number π.  It is as if Newton predicted π was 3.14 exactly, but Einstein predicted it was 3.14159265, and we have experimentally confirmed that π is 3.14159 give or take a bit.

To measure gravity more precisely, we need to look at situations where strong gravitational interactions occur.  And we need to be able to measure these interactions very precisely.  This new system has two important features that should allow us to do that.

Firstly, the neutron star in the system is a pulsar.  Its strong magnetic field produces a beam of radio waves, and as the neutron star rotates the beam sweeps in our direction.  As a result, we see a burst of radio energy with each rotation of the neutron star.  The neutron star makes about 366 rotations per second, so this allows us to make very precise measurements of its position and motion.

Secondly, the stars of this system are particularly close.  The neutron star and one of the white dwarfs orbit each other at a distance of about 15 times the distance from the Earth to the Moon.  Keep in mind, these are two stars that are orbiting at this distance, so they are remarkably close.  They orbit each other about once every two days.  The other white dwarf orbits this pair at roughly the same distance as the Earth from the Sun.  So this is a system that has three stars within the space of Earth’s orbit.

So this system will allow us to make precision observations of a high gravity situation.  This is close enough that it might also allow us to test the fundamental concept of general relativity, known as the equivalence principle.

The simple version of the equivalence principle is that (neglecting air resistance) any two objects will fall at the same rate, regardless of their mass.  This is sometimes known as the weak equivalence principle, and has been confirmed experimentally.  But general relativity presumes a stronger version of the principle, which states that the inertial and gravitational mass of an object are the same, regardless of the type of material that makes up that object.  If general relativity is correct, then the outer white dwarf will interact gravitationally with the neutron star and the other white dwarf in the same way.

So through close observation of this system, we might be able verify a fundamental aspect of Einstein’s theory of gravity.  Or we might discover that Einstein didn’t quite get it right, and that would be cool too.

Paper:  Ransom SM, Stairs IH, Archibald AM, et al. A millisecond pulsar in a stellar triple system. Nature. (doi:10.1038/nature12917)

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In 1915, Albert Einstein proposed a radical new theory for gravity.  He proposed that gravity could be described by a curvature of space and time, rather than Newton’s theory of forces between masses.  Einstein was already recognized as a prominent scientist for his 1905 papers on the photoelectric effect, brownian motion and special relativity, but no matter how established you are, if you put forward a theory claiming to overturn 250 years of Newtonian physics you’re making an an extraordinary claim.  And as they say, extraordinary claims require extraordinary evidence. 

One of the predictions that Einstein’s model made was that the orbit of Mercury would shift very slightly over time, and effect known as perihelion advance.  While Einstein’s theory gave the correct value for Mercury’s perihelion advance.    This gradual shift had been first observed by Urbain Le Verrier in 1859, so it wasn’t a new result.  Simply demonstrating your model can match observation isn’t enough to supplant a long-established theory.  Besides, there was an alternative model already in place.  If there was another planet closer to the Sun than Mercury, its gravitational pull would produce the same effect.  There were actually ongoing searches to find such a planet.

So that prediction bought him nothing.

To topple Newton, Einstein would need a prediction that hadn’t been observed. One that clearly showed his theory was correct, and Newton’s was wrong. It was Arthur Eddington who devised just such an experiment. Where Einstein’s theory differed significantly was in the way light behaved. If space is truly curved, that curvature would effect light as well as planets. If a beam of light passed near the Sun, the curvature of space would bend the beam, an effect now known as gravitational lensing. If Newton was right, and space wasn’t curved, there wouldn’t be such an effect. Eddington realized that one could observe this effect during a total solar eclipse.

So in 1919, Eddington traveled to the island of Principe off the coast of West Africa to photograph a total eclipse. He had taken photos of the same region of the sky sometime earlier. By comparing the eclipse photos and the earlier photos of the same sky, Eddington was able to show the apparent position of stars shifted when the Sun was near, just as Einstein had predicted.

Eddington’s result made Einstein famous all over the world, but it would take more than one result to establish Einstein’s model as a scientific theory.  But over time more experimental evidence verified Einstein’s model.  In 1959 the Pound-Rebka experiment demonstrated that light can be gravitationally redshifted.  In 1964 Irwin Shapiro demonstrated the relativistic time delay effect.  In 1982, analysis of the Hulse-Taylor binary pulsar demonstrated that they were losing energy due to gravitational waves.  In 2011, the gravity probe B experiment directly measured the effects of frame dragging.  Each time Einstein’s model proved to be correct.

General relativity worked again and again.  Newton’s model, while still very useful for the motions of planets and satellites, simply could not address these new experimental observations.

And that is why Einstein’s general relativity is “just a theory”.

Paper:  Einstein, Albert (1915), “Die Feldgleichungen der Gravitation”, Sitzungsberichte der Preussischen Akademie der Wissenschaften zu Berlin: 844–847 (1915)

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When Pluto was demoted to a dwarf planet in 2006, there was quite a bit of outrage from the public.  But a similar thing occurred with Ceres in the 1800s.

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There is one side of the moon that we always see, and one side that most people have only seen in photographs.  This is because the moon is tidally locked, meaning that one side of the Moon always faces the Earth. 

The term “tidally locked” is a bit of a misnomer.  There is nothing locking one side of the Moon toward the Earth.  It just happens that the time it takes the Moon to orbit the Earth, and the time it takes the Moon to rotate once on its axis are the same.  So as the Moon orbits the Earth it rotates on its axis at the same rate, and the same side of the Moon always faces us.

Part of what makes this possible is the near circular nature of the Moon’s orbit.  If the Moon’s orbit were more elliptical, the faster and slower motion of the Moon in its orbit wouldn’t allow its rotation to sync with its orbit.  This is the case for Mercury, which orbits the Sun twice for every “day” on Mercury.  What also makes it possible is the way gravity acts upon non-spherical bodies.

Gravity acts more strongly upon closer objects than more distant objects (following the inverse-square law).  This means that the side of the Moon facing the Earth is gravitationally pulled a bit more strongly than the side away from the Earth.  If the Moon were perfectly spherical that wouldn’t matter, but the Moon isn’t perfectly spherical.  Because of the difference in gravitational attractions, the Moon bulges slightly, and that means it has a tendency to align one side toward the Earth.  This is known as tidal effect.  Over time, the preferential alignment of the Moon has caused the Moon to become tidally locked with the Earth.

You can see a similar effect with a rolling wheel.  If you have a bicycle tire rolling along the ground, it will roll on a flat surface at a uniform rate.  But if you add a small weight to one of the spokes, making the tire lopsided, the wheel won’t roll at a uniform rate.  Instead it will lurch forward until the weight is toward the bottom, then slow down as the weight is rotated upward.  This lurch-slow-lurch-slow motion is due to the fact that the wheel would like to be aligned with the weight toward the bottom.

Before the Moon’s rotation was in sync with its orbit, the tidal forces would cause the Moon’s rotation to lurch-slow-lurch-slow slightly, until over time its rotational speed matched its orbital speed.  The same forces act on the Earth, but since the Earth is larger than the Moon the effect is much smaller, so there hasn’t been time for the Earth to become locked to the Moon.  A similar effect occurred with Pluto and its largest moon Charon.  In this case, since Charon and Pluto are more similarly sized, the two have become tidally locked to each other.

So the next time you look at the familiar face of the Moon, remember the gravitational dance that got the Moon to put its best face forward.

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One of the assumptions of cosmology is that at sufficiently large scales the universe is essentially the same everywhere (in formal terms, homogeneous and isotropic).  The basic argument in favor of this cosmological principle is that if the laws of physics are the same everywhere (and we have good evidence that they are) then the way the universe appears is basically the same everywhere.  Assuming it’s true, it means that the universe as seen from our vantage point on Earth is not biased in some way.  This is important if we are to have confidence that our understanding of the universe is an accurate representation of how the universe really is. 

Although the idea was considered at least as far back as the ancient Greeks, it was formally stated by Isaac Newton in his Principia, where he proposed that gravity was universal, meaning that it applied everywhere in the universe.  This was a distinct break from the medieval worldview, which presumed the Earth was at the center of the universe.  Of course when Newton proposed the idea, there wasn’t a lot of evidence to support him.  Newton demonstrated that universal gravity worked for the motions of the planets in the solar system, but that’s not the same as assuming our view of the universe is pretty typical.

In the 1900s we began to get observational evidence that the cosmological principle is valid.  We found that our Sun was part of a galaxy, but our galaxy was just one of many.  The distribution of these galaxies looked somewhat uniform.  In 1964 Penzias and Wilson measured the cosmic microwave background.  This thermal remnant of the big bang has a very uniform temperature, with only small fluctuations, just as we would expect from the cosmological principle.

In recent decades we’ve been able to map the distribution of galaxies in the universe.  For example, the APM galaxy survey done in the 1990s, which led to the image below of more than 2 million galaxies in a patch of sky.  The small dark circles are regions that were excluded because a local bright star was in the way, but the rest of the region shows a uniform distribution of galaxies.  At smaller scales there is a degree of clumping, but on large scales the distribution averages out.

Since then there have been even larger galaxy surveys, such as the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey, and these show a similar result.  From the laws of physics, to the cosmic background, to the distribution of galaxies, it really does appear that the universe is homogenous and isotropic.

As far as we can tell, the universe is even steven.

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Sometimes in astronomy a simple question can have a complex answer.  For example, why is the night sky dark?  You might think that has an easy answer:  the sky is dark because the stars are very far away and appear dim.  It’s obvious why the sky is dark, right?

Except it isn’t. 

In actuality, stars are quite bright.  Many of them are as bright or brighter than the Sun.  They only appear dim because they are far away.  But there is an interesting catch.  The apparent brightness of a star decreases with distance according to the inverse square law.  In other words, it two stars have the same absolute brightness, but star A is twice as far away as star B, then star A will appear one fourth as bright.  The catch is that the apparent size of a star also decreases according to the inverse square law.  This means that while a distant star appears dimmer, it also appears equally smaller.  Thus you could argue that a star appears as bright as the Sun, but just over a tiny area.

So if the universe is an endless sea of stars, then if we look in any direction in the night sky we will see a star.  That star may be very, very far away, but in that one precise direction the night sky is as bright as the Sun.  Of course for an endless sea of stars, that should be true for any direction we look.  If the universe goes on forever, then the night sky should be a blaze of light, as seen in the animation below.

This is often known as Olbers’ Paradox, after Heinrich Olbers, who popularized the question in the early 1800s.  For about a hundred years it was a real mystery.

Now you might think the answer to this paradox lies in the fact that the universe is not an endless sea of stars.  Stars are clumped into galaxies, after all.  But this doesn’t solve the paradox.  Instead it simply pushes the paradox from stars to galaxies.  The apparent brightness and size of galaxies follows the same inverse square law, so an endless sea of galaxies would also produce a blazing bright sky.

What about the fact that there is lots of dust in the universe.  The dust doesn’t give off light like stars, so that should solve the paradox.  It doesn’t because the dust would absorb light and heat, so if the dustless regions of sky were as bright as the Sun, the dust would be heated to the same temperature and would therefore give off just as much light as the stars.

Olbers’ paradox makes two assumptions about the universe.  The first is that it goes on forever, the second is that it has always existed.  While the first assumption might be true, the second assumption is not true.  The universe is expanding, and it began about 13.77 billion years ago with the big bang.  Since the universe has a finite age, and light travels at a finite speed, there is a limit to how far away visible stars and galaxies can be.  Light from more distant galaxies simply hasn’t had time to reach us.  Also, since the universe is expanding, the light from distant stars is redshifted, making them appear cooler than they actually are.  So the night sky is dark precisely because the universe is expanding and has a finite age.

When you look at a dark night sky, you are seeing evidence of the big bang.

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There are now more than 1,000 confirmed exoplanets.  The majority of these planets have been found by the Kepler telescope using what is known as the transit method.  The transit method looks at stars over long periods of time, measuring any changes in the brightness of the star.  If a planet passes in front of the star (transits), then the star will appear slightly dimmer.  From this we know a planet orbits the star.

The main downside of this method is that it requires the planet’s orbit to be along our line of site so we can observe it transit the star.  But if that happens to be the case, then we can gather a wealth of information about the planet.  By measuring the time between transits we can determine the orbital period of the planet, from which we can determine its orbit.  By measuring how much the star dims, we can determine the size of the planet relative to its mass.  We can even learn things about the atmosphere of a planet by observing how it filters the star’s light (http://goo.gl/WAcfnW).

One thing we haven’t been able to determine from a planetary transit is the planet’s mass.  During a transit, the star is dimmed in proportion to the planet’s size relative to the size of the star.  Knowing the temperature and brightness of a star we can get a really good measure of the star’s size, and from that we can determine the size of the planet.  But size is not the same as mass.

We can make a rough estimate of a planet’s mass from its size.  If a planet is about the size of Neptune, for example, then it probably has about the same mass.  But the mass of a planet depends upon its size and its density.  A high-density planet will have more mass than a low density planet of the same size.  And density matters when it comes to planetary makeup, since a rocky planet will have a higher density than an icy or watery one.

For some transit planets we have other information, such as the Doppler motion of the star, and from this we can determine the mass of a planet.  But from transit observations alone we haven’t been able to determine mass… until now.

A new paper in Science (unfortunately behind a paywall) has outlined how transit data can be used to determine the mass of a planet directly.  The method relies on the fact that the a planet’s atmosphere (actually what’s known at it’s pressure profile) depends upon its surface gravity (which depends on size and mass).  The effect of this pressure profile can be observed in the transit data, as seen in the image below.

Of course the pressure profile also depends upon the planet’s temperature and the density of the atmosphere, so to take these into account the team drew upon the work of 18th century mathematics.  It turns out all these parameters are connected by a constant known as Euler–Mascheroni constant.  This constant plays a central role in the way an atmosphere’s temperature, density and pressure profile affect light passing through the planet’s atmosphere.

Of course this idea is fine in theory, but to prove it works you need to compare the mass of an exoplanet determined by this new method with the mass determined by traditional means.  The authors did just that with an exoplanet known as HD 189733b.  From Doppler methods, the mass of this exoplanet is known to be 1.14 Jupiter masses.  Using this new method, the authors got a mass of 1.15 Jupiter masses.  We’ll want to do similar comparisons with other exoplanets, but it seems clear this method works really well.

This method has a huge potential, because future exoplanet telescopes, such as the James Webb telescope scheduled to be launched in 2018, have all the necessary equipment to make these kinds of measurements.  This method doesn’t require any fancy new equipment, simply an analysis of data we already planned to gather.

Image:  De wit J, Seager S., from their paper.

Paper:  De wit J, Seager S. Constraining exoplanet mass from transmission spectroscopy. Science. 2013;342(6165):1473-7.

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