Tuesday, May 29, 2012

Like a Diamond in the Sky?

Spitzer measurements suggest the atmosphere of planet WASP 12-b has carbon monoxide, excess methane, and not much water vapor. Image credit: NASA/JPL-Caltech/CFHT/MIT/Princeton/UCF

I have learned that there could be some astronomical truth in the picture conjured when singing the refrain of "Twinkle, Twinkle Little Star". David and Edward love this song and so do many other generations of children.  I love it, too and I am glad to have an excuse to include it in a post in some form.

"Twinkle, twinkle, little star,
     How I wonder what you are.
     Up above the world so high,
     Like a diamond in the sky."
              (Ann and Jane Taylor, 1806)

  Do stars have a diamond-like composition?... well,  not as far as I know, but some planets do. At the end of April I went to a seminar on the chemical characterization of Extraplanetary Atmospheres by Nikku Madhusudhan, a prize postdoctoral fellow at Yale University. I found out that the atmosphere and the core of some planets have more Carbon than Oxygen. In fact, this is not so new anymore, but it is the first time I have heard of it.

It is important to remember that planets do not have any light of their own, but they do masquerade as stars because they reflect the light of the sun. However, in general, they do not appear to twinkle in the sky.  The stars twinkle because we see them through many layers of thick, turbulent air known as the Earth's atmosphere. As the light from the star travels through the atmosphere the light is bent or refracted. This bending of light many times and in random directions causes the twinkling effect. Planets do not appear to twinkle unless the air is extremely turbulent because they are much closer to us. Stars do not twinkle when viewed from the Moon, for example, because the Moon does not have an atmosphere or from the International Space Station.
Hubble Image of a young sun-like star surrounded by a disk
  Let's get back to what Nikku said. He pointed out that models for planet formation and planetary atmospheres assume solar abundance ratio. Our star, the sun, has Carbon to Oxygen ratio of 0.5, i.e., twice as much oxygen as Carbon.  If we assume this ratio, water should be the most abundant element in planetary atmospheres. Nikku and his collaborators found several planets with atmospheres that are Carbon rich vs. Oxygen rich. So, this assumption needs to be relaxed.

How do scientists measure the composition of atmospheres of extrasolar planets? Telescopes use spectroscopic measurements of light reflected by the planetary atmosphere, i.e., they measure the intensity of the light as a function of wavelength, to find the chemical composition. One opportunity to make this type of measurements is when we observe the planet transiting across its parent star. The planet blocks some of the star's light and produces a noticeable dip in the measured light output. The advantage of this method is that scientists can probe the atmospheres of planets discovered in this way, and obtain information on their chemical composition, in addition to that of the star. They can also look at the depth of the transit and estimate the ratio of the radius of the planet with respect to that of the star. The disadvantage of this method is that there are relatively few sources because we can only detect planetary systems that are close to edge-on with respect to the observer.

 Another way to study the atmospheres of planets is to directly image them. However, planets are extremely faint sources relative to their parent stars. So, telescopes need to block the light from the star while leaving the planet detectable in order to detect the planets this way.  So, while the number of planets detected this way is growing, this method remains very challenging. 

What about planets in our solar system?
The Galileo satellite was destroyed during a controlled impact with Jupiter in 2003. Its findings are consistent with a Carbon to Oxygen ratio greater than or equal to 1. So, we could have diamond planets/moons in our own solar system. However, it is believed that the satellite landed in a particular dry spot on Jupiter.  There are several missions that will tell us more about Jupiter and its moons in the near future. The Juno mission, which was launched last year, will test this hypothesis in a few years. It takes 5 years to get to Jupiter in its current configuration. There is another mission that will be launched to study Jupiter's moons by the Europeans by 2022.  It's called Jupiter Icy Moon Explorer (JUICE).  However, it is important to remember that Jupiter is very different from extrasolar planets because it is cold. Jupiter is cooler than the water condensation curve, which means water would be condensed, i.e., buried deep in the atmosphere. Most of the discovered extrasolar planets are Hot Jupiters, Hot Neptunes and SuperEarths.

The first diamond planet and possible consequences
The first giant planet believed to have its atmosphere and core dominated by Carbon, WASP-12b, was discovered by astronomers in 2008. Nikku and his collaborators showed that this planet has low levels of water and high levels of methane. The carbon to oxygen ratio (C/O) was bigger than or equal to one by three standard deviations from the mean. If a planet with this composition is found orbiting a sun-like star with a C/O = 0.5, then this means that there are planetesimals with both this composition and a C/O grater than or equal to 1 composition.  In this case, the protoplanetary can not have the same composition everywhere - it must fluctuate as a function of distance. An alternative model proposes that the water condensed early on in the protoplanetary disk, while the C/O in the gas is higher (Oberg et al. 2011). Giant planets form by accreting this gas and hence would have a composition that is Carbon rich.

Can rocky planets have diamond cores?
Some authors (Rogers and Seager 2010, Fressin et al. 2012) show that rocky planets like our Earth could have Carboids instead of Si at their center. However,  SuperEarths are smaller and their atmospheres are harder to observe. Their chemical composition needs to be studied in order to understand which planets can support life and which can not. So, it's a question that hopefully will be answered sometime in the future.

Overall, the main message of the talk was that it is not OK to assume that all planets and stars have the same composition as our sun. This ratio varies and the variation can have impact on our predictions for habitability and on our understanding of planet formation.

Saturday, May 12, 2012

Afraid of Supermoon?

Birds against Supermoon. Credit: Taken by Flickr photographer Don Kittle
I am sorry to report that we did not see "The Supermoon" in Zurich this year. There were two reasons for this. The first was that David decided he was afraid of the Supermoon and the second was that we could not find it. We tried to go out at night to see it and climbed up the mountain in the back of our (rented) house to find the Moon. On the way there David refused to go any further and started crying. I failed to convince my 5 year old nephew that there was absolutely no reason to be afraid of the Moon - now that it's a little bigger. In the end he became so upset I had to give up.

It later occurred to me that the tides are stronger when Supermoon happens and so his fear might not have been unreasonable if we had lived on a house close to the ocean or been on a boat vs. in Switzerland. Of course, his fear is not unreasonable anyway because he is a five year old child and it's OK to be afraid of a bigger Moon if you are five. It just surprised me that I could not convince him to not be afraid.

When does "Supermoon" happen? and how big is it?
Source: http://oceanservice.noaa.gov; Sun: bright yellow, Moon: purple, Earth: Blue
The Moon is Earth's satellite and has an elliptical orbit around the Earth. The apogee is the point where the moon is the furthest from Earth and the perigee is the point when it's closest to Earth. Supermoon happens when the Earth, the perigee-Moon, and the Sun are all in line. In other words, the Supermoon is a full moon that is also closest to Earth (a full Moon happens when the moon is on the opposite side of Earth from the Sun).

Supermoon happens once a year. In general, the full moon and the perigree moon do not coincide exactly. So, the size of Supermoon does vary. However, Supermoon is not huge. NASA says the 2012 Supermoon is only about 14% larger than a typical full moon at its furthest point and about 30% brighter than the other full Moons of 2012,  which makes it seem bigger.

Does David have a point? Should we be afraid of Supermoon?
 The combined effects of the Sun and the Moon will be the strongest of the year when Supermoon happens. The tides are stronger than at any other time of the year. Supermoon did ground a few ships last year. However, no correlations have been found between Supermoon and Earthquakes or Tsunami or any other natural disaster. So, while we should be cautious if we are on a ship or on the beach and it's Supermoon night, Supermoon does not seem to be something we should fear. David did relax and forgot about his fear when we were back inside. He and Edward spent the rest of the evening jumping around the house and screaming "jumping monkey" until they were tired enough to go to sleep.

Friday, May 11, 2012

Behind The Light

This is not a post about death and life after death. It's about a different light - the bending of light caused gravitational lensing that increases the brightness of the source. This post is inspired by several talks on the subject at the ITP. In particular, Dr Sebastiano Calchi Novati (from Salerno University; a former postdoc) had "behind the light" as part of his title of his talk on microlensing and galactic astrophysics.

 What is Gravitational Lensing?
Gravitational lensing occurs when an object comes in-between the source of light and the observer and bends the light from the source affecting the amount of light that the observer sees. Sometimes multiple images of the source appear. The light rays that we would not have seen otherwise are bent from their path towards (or away from) the observer causing the far away source to change its brightness. There are several types of lensing: weak lensing, strong lensing and microlensing.

Strong Lensing

Strong lensing: five images instead of one
The most extreme bending of light occurs when the lens is massive - galaxy or cluster of galaxies - and relatively close to the source of light.  If the source is a quasar, multiple point images of the quasar will appear due to its small radius.  When the background source is another galaxy, the observer can see giant arcs or rings. Strong lensing produces some of the most beautiful images in astronomy.

Weak Lensing

Light from far-away galaxies can be bent by closer galaxies or galaxy clusters. The sources will be stretched and magnified, but the distortions are only a few percent. Very many sources have to be analyzed by scientists to look for coherent distortions. These distortions give us information about the mass distribution of the lenses. This way clusters nearby have their mass distribution measured, i.e.,  are "weighted" against far way galaxies. One of the most striking of weak lensing examples is the bullet cluster, which shows a subcluster passing through the cluster. The weak lensing contours map where most of the mass is, which is the dark matter. The Bullet cluster observations show that dark matter, stars, and gas behave differently during the collision. The gas interacts electromagnetically and slows down the most. The stars collide and slow down, and the dark matter clumps just go through each other since dark matter is collisionless. This is the first direct evidence for dark matter that it makes it more likely that dark matter is formed from particles vs just a modification of gravity.

 Light from stars in distant galaxies is bent by closer planets or other compact objects causing the distant stars to become brighter for a period of time, i.e., the planet acts as a magnifying glass for the distant star. This time period can be weeks, days or even hours. The duration of the event depends on the mass of the lens, the distance to the lens and its velocity. It is also important to point out that these are "one-time" events that do not repeat. Microlensing first emerged as a technique to find compact objects made from dark matter. However, no evidence for such dark stars exists to date.

Microlensing later morphed into a successful planet-finding method. It can find Earth-mass planets that are relatively far from their star. The distance dependence is stronger than the dependence on the mass of the lens. In fact, gravitational microlensing was the first method to find Earth-mass planets orbiting main sequence stars.  The first extragalactic exoplanet in the Andromeda Galaxy A was also found through microlensing.

Does the title make sense?
We can use the light from distant objects to learn more about closer object though the lensing effect. So, the "behind" in the title here has an allegoric meaning, i.e., we learn about what is behind the change in brightness. However, do not be confused, the lens (closer star/planet/etc) is between Earth and the further away (and older) source, and the duration of the event depends on how long the lens spends across our line of sight.

Wednesday, May 2, 2012

From Quasars to Black Holes

Last week Scott Tremaine, a professor at the Institute for Advanced Study in Princeton, visited the University of Zurich & ETH. He is the only person I know who has an asteroid (3806 Tremaine; no worries it's quite small) named after him. He also wrote a standard textbook called "Galactic Dynamics" that is still used by many universities around the world.  I went to two of the three seminars he presented. This post is inspired by his talk on Black Holes in Nearby Galaxies (the slides are available on his website). 

Quasar core. Credit: Hubble Telescope
 What are quasars?
The word Quasar comes from quasi-stellar radio source. Quasars are among the most luminous and most energetic objects in the universe. They are about 10 000 000 000 000 times more luminous than the sun or 100 to 1000 times more luminous than a galaxy. This huge luminosity is produced by gas that heats up due to friction and glows brightly as it gets closer, and closer to the really dense object at the quasar core.

Why Quasars require black holes?
These are the top three reasons:
A. The radio jets that come from quasars are absolutely straight over timescales of 100 000 years. The fact that they can maintain their orientation over such a long timescale suggests that the source could be a spinning black hole - which acts like a gyroscope.  Needless to say, I was glad to hear that the mechanism that helps my toy helicopter run also governs spinning black holes.

B. The velocity of the radio jets is relativistic, i.e., close to the speed of light. These jets actually appear to be superluminal, but this is just an optical illusion due to the jet making a very narrow angle with the line of sight of the observer. This is consistent with the theory that quasars contain black holes because black holes are tiny and can eject mass at relativistic velocities.

C. This enormous amount of light comes from a tiny area in the sky of less than a few mili-parsecs. A mili-parsec is about 200 times the distance between our Earth and the Sun.
On the right we can see what is call "Einstein's cross". Four images of the same quasar appear around a foreground galaxy due to strong lensing. Gravitational lensing occurs because heavy objects bend the space-time, and as a result light rays coming from a background source are bent. When the lensing mass in complex such a galaxy, the observer can see multiple images of the same source. The fact that we see these four images puts a limit on the size of emitting source, which is the quasar - it has to be less than something called the Einstein Radius (Wambsgans 2006).

Of course, they could also be other kinds of compact objects like boson stars that do not contain a singularity, but, so far, no particles that could condense to make up these stars have been detected.

Where are these Quasars today?
The number of observed quasars per volume (or more precisely their co-moving number density) peaks at redshift of 2. The universe was a little more than 3 billion years old at the time. By now 13 billion years have passed since the Big Bang and we don't observe any quasars, but we observe massive black holes in the center of many galaxies and we believe they are 'dead' quasars.

Why are massive black holes at the center of galaxies? and how about multiple black holes?
 Dynamical Friction causes the orbit of massive bodies to spiral towards the center of the galaxy. In fact, more than one black hole could spiral towards the center of galaxy, which causes scientists to think that binary black holes and hopefully black hole mergers are common at galactic centers. I say hopefully because it can take a very long time (sometimes more than the age of the universe; this is called the final parsec problem) for these black holes to get closer and closer together due to gravitational wave emission and eventually merge.  The rate of black hole merges is still unknown. A gravitational wave observatory in space such as LISA would be the only way to get some statistics on these binaries.

How many black holes at galactic centers have been observed so far?
About 40-50 with masses ranging from millions to billions of solar masses.

Which galaxies have black holes?
Roughly speaking, most large galaxies have black holes at their center. More technically, almost all galaxies that have a "hot" component, which is the bulge of elliptical and some types of spiral galaxies, also harbor black holes at their centers. Their masses are correlated with properties of the host galaxy - in particular with velocity dispersion.

Why are black holes at the centers of galaxies important?
They play an important role in galaxy formation.

What do we know about the black hole at the center of the Milky Way, our own galaxy?
The Milky Way's black Hole: Sagittarius A*; Image credit: Chandra

The mass of 'our' black hole is about 4 million solar masses in an area of 0.5 mili-parsecs, which is about 3 x the distance to Neptune or about 100 times the average distance between the Earth and the Sun. The radius of a black hole of this mass would be more than one thousand times smaller. The presence of the black hole is suggested by the fact that the orbits of the stars around the center of the galaxy are closed ellipses (Ghez et al. 2008, Gillessen et al. 2009).  The smallest orbital period is only 16 years.

Since there is still so much to learn about quasars, I end with the poem "Quasar" by George Gamow. I sometimes recite it to my nephew, David and to my son, Edward instead of the standard "Twinkle, Twinkle Little Star" song and even they get confused:

"Twinkle, twinkle quasi-star
Biggest puzzle from afar
How unlike the other ones
Brighter than a billion suns
Twinkle, twinkle, quasi-star
How I wonder what you are."
George Gamow, "Quasar" 1964.