Other Cool Science with ATLAS

Habitable Planets

The habitable zone near a white dwarf star is shown shaded in blue as a function of the white dwarf's age and distance from the star.

Two decades ago astronomers believed that most stars have planets but not one extrasolar planet had been discovered. Today, a variety of techniques have uncovered over a thousand extrasolar planets and the number is growing rapidly. There are two primary methods used to identify the extra solar planets: the back-and-forth motion of the star due to the orbiting planet and the slight dimming of the star as a planet passes in front. These detections techniques are biased in favor of giant planets like Jupiter that are very close to the parent star. The techniques are not well suited to detecting Earth-sized planets in the habitable zone - the range of distances from the star where life as we know it might thrive.

Only one or two of the thousand known planets might be habitable. Even worse, we have no means of determining whether they actually are habitable (for example having water in liquid form) or of determining whether they bear life. The situation is not likely to improve in the near future and, without a new approach, two decades from now we will know a great deal more about how planets form and how common they are but nothing about whether any extrasolar planet is actually habitable.

ATLAS has a special capability to leap-frog the existing barriers to determining whether a planet is habitable because it will look at 10,000 white dwarf stars every night. White dwarf stars are the collapsed remnants of stars that have burned up their supply of nuclear fuel; our Sun will become a white dwarf in about 5 billion years. Although nuclear reactions in the center of the star have ceased white dwarfs take up to 10 billion years to cool down. Prof. Eric Agol at the University of Washington has shown that the key ingredients to evolution of life are present in such systems (ApJ 713: 31, 2011) but it is unknown whether these white dwarf stars have planets and whether the planets could have liquid water that is necessary for life.

Detecting a habitable planet around a white dwarf is far easier than around a normal star because the collapsed star is nearly the same size as Earth. In this scenario an extrasolar planet transit is a dramatic event - like a solar eclipse by the Moon. More important, the light that passes through the planet's atmosphere during eclipse can be a substantial fraction of all the light that we observe on Earth, even for partial eclipses (1% for a white dwarf versus 0.0001% for a normal star). This means it is possible to detect strong molecular lines from oxygen and water during the brief eclipse using the world's largest telescopes or perhaps the future JWST.

While astronomers think it is more plausible that a habitable planet would be found around normal, main-sequence stars, it is completely unknown whether white dwarf stars have planets and ATLAS is a way to find out.

Gravity Waves

Ripples in spacetime generated by fast orbiting stars (neutron stars, white dwarfs or black holes).

Einstein's General Theory of Relativity tells us that space and time are linked in a space-time 'fabric' that has 'dents' in it where there are massive objects like stars.

Imagine a taut painter's canvas lying parallel to the ground with a cannonball in the center. Now imagine rolling another cannonball towards the first one and as they collide and bounce back and forth the canvas will bounce around as well and anything attached to the canvas will experience the effect of the cannonballs' collision. The animation to the left illustrates the rippling of space-time in 2 dimensions. Of course, space is 3-dimensional so it's a little more difficult to imagine.

Neutron stars are the remains of a supernova explosion in which the entire mass of star like the Sun gets crushed into a volume no bigger than 12 km across (that's about 8 miles in diameter). A single teaspoon of neutron star material weighs as much as an entire mountain like Mt. Everest. Now imagine that two neutron stars collide (and coalesce) - they are our cannonballs and their collisions will create ripples in the fabric of space-time that may be detected by sensitive gravity wave detectors like LIGO, Advanced LIGO and EGO.

The ripples in space-time that are caused by the mind-blowing mergers of neutron stars travel vast distances before being detected even by Advanced LIGO. The typical merger will occur at distances on the order of 1.5 billion light years! That distance corresponds to seeing things 10% of the way to the edge of the observable universe! Having never actually seen a neutron star - neutron star merger taking place we rely on calculations and computer simulations to inform us how much energy would be released. Furthermore, we don't know how much of that energy will be emitted in the form of visible light rays. But if just 1 part in 100,000 of the energy appears in visible light than ATLAS will be able to just detect the typical events observed by Advanced LIGO. Of course, ATLAS will have no difficulty detecting the many events that are brighter than typical and these detections combined with the Advanced LIGO results will provide important constraints on the formation of gravitational waves and on the General Theory of Relativity.

Active Galactic Nuclei

Hubble Space Telescope image of a 5,000 light-year long jet being ejected from the active nucleus of the galaxy M87.

Most astronomers now think that there are massive black holes at the center of every large galaxy. A black hole is a region in space where there is so much mass that the gravity is so strong that even light can not escape. A black hole that had the mass of the Sun would have a diameter of only about 30km (about 20 miles)! That may not seem so surprising until you know that the actual diameter of the Sun is about 1,400,000 km (or about 1,000,000 miles)! Imagine squeezing all the mass of the Sun into a billionth of a billionth of its current volume. Or maybe it's easier to imagine squeezing the entire Earth into the head of a pin?

The supermassive black holes at the center of galaxies have the mass of a million to a billon of our Suns and diameters that are about the size of the Earth's orbit around our Sun. They became massive by gobbling up material that fell into them long ago. And by 'material' we mean entire stars - millions of them! They also consume the dust and gas from which stars are formed (and the material that's left over once they've formed). As the supermassive black hole assimilates material the objects heat up and emit energy that we can detect on Earth. When a supermassive black hole consumes a star it generates a specific signature in the type of emitted light and in how the light changes with time. Similarly, if the black hole is being fed by a disk of dust and gas that is swirling around it we can also tell what is happening.

An Active Galactic Nuclei (AGN) are black holes at the center of massive galaxies that we see consuming material. By identifying AGNs and monitoring their brightness we can learn a lot about the supermassive black holes and the environments in which they live at the center of galaxies. One thing we know is that there are more AGNs the further away we look in the universe. And looking further away is equivalent to looking backwards in time. So this means that long ago, back when the universe was much younger than it is now, there were more AGNs. The supermassive black holes are still at the centers of galaxies (there is one in our own Milky Way galaxy) at the current time but they are no longer usually active because they've already consumed all the available stars, gas and dust. Thus, finding and studying AGNs at different distances from us tells us how black holes and galaxies evolve in time.

With ATLAS's all-sky survey coverage we will monitor more than 100,000 AGNs every night. Furthermore, because ATLAS is watching 40 million galaxies twice every night we will detect when a star falls into one of the central black holes. Our estimates suggest that ATLAS will witness a black hole consuming a star about 20 times each year.

Lensing

The massive galaxy in the center of the illustration (the one highlighted by the blue glow) acts as a gravitational lens for the more distant cluster of galaxies on the right.

Just like a glass optical lens can bend and focus light, the gravity due to an object like a planet, star or even a galaxy can bend and focus light. There are three general broad categories of this 'gravitational lensing': micro, weak and strong. ATLAS will not be sensitive to weak lensing so we will not discuss it here.

Micro-lensing occurs when 'the lens', a small or faint object, passes directly between us and a more distant bigger or brighter object, 'the source'. (see the illustration to the left) The last sentence is deliberately vague because the lens could be anything that has gravity and the source could be anything that emits light - and that covers the range of many different types of beasts in the universe. Typically though, the lens is a big planet or a small brown dwarf star. Both planets and brown dwarfs are dark relative to stars. The source is typically a bright and more distant star. When we see the lens line up with the source the lens' gravity focusses light from the source on us and causes the apparent brightness of the source to increase by maybe 100x. The brightness increases gradually to a maximum and then decreases in a characteristic way that uniquely identifies the brightening as being due to a micro-lensing event. The lensing effect is not constant because everything in the universe is always moving relative to everything else and the precise alignment between the lens and source only lasts for a short period of time.

Thus, to detect micro-lensing you need to regularly monitor the brightness of a lot of stars - and this is one of the things that ATLAS is great at doing! There are about a billion stars in the sky bright enough to be detected by ATLAS so that means that there are about half a billion visible to ATLAS on any given night. Based on what we know about the frequency of lensing events from other surveys we estimate that ATLAS should detect about 30 micro-lensing events per year and about 10 of those will be very interesting because they will be bright and have good time coverage.

Strong gravitational lensing occurs when the lens is an entire and massive galaxy. The galaxy lens magnifies and distorts the light from even more distant source galaxies that are far beyond the lensing galaxy. Since the lens increases the apparent brightness of the source we can use the gravitational lens to examine galaxies that are much further away than normally possible. You might think that the probability of galaxies lining up so precisely for lensing to occur must be very small - and you are correct! But there are many, many galaxies in the universe so the probability of some of them lining up is pretty good. In fact, we expect that some where on the sky there are about 40 AGN sources (see above) lined up with a massive gravitational lens. Of these 40 sources there will about 7 that have magnifications of 10 or more! The strong lensing allows us to examine the behavior and properties of these very distant AGNs. At the highest magnifications there can be micro-lensing of the source by stars within the lensing galaxy that can cause 'flickering' of the distant AGN's light.

Stars that blow up:

Core Collapse Supernovae (CCSN)

This image shows the entire region around Supernova 1987A - a core collapse supernova.

At the end of a star's lifetime the most massive stars blow themselves to smithereens in a colossal explosion that releases an unimaginable amount of energy. The event is known as a supernova and when it happens the energy from this single exploding star can be more than the energy released by all the stars in a galaxy containing 100 billion stars. For a few weeks a single star can be visible to telescopes on Earth from across distances comparable to the size of the Universe.

The details of what leads up to a star blowing itself apart are too complicated to describe here but a quick web search will turn up many useful explanations. e.g. Core collapse Supernovae. Very briefly, a star is nothing more than a big ball of (mostly) hydrogen gas. They are so massive that the pressure and temperature at their center is high enough and hot enough to cause thermonuclear fusion. Yes, all stars are powered by self-sustaining thermonuclear fusion reactions just like those that power the bombs in the world's nuclear arsenals. The energy produced by the fusion in the star's center creates a pressure that balances the weight of all the material above it. But eventually, after just a few million years for the most massive stars, the energy from fusion in the center of the star begins to run out and the weight of the overlying material bears down on the center. The energy from the fusion reactions eventually stops and the star literally collapses in on itself and then much of the material 'bounces' off the center and gets launched into space as most of the star blows itself apart.

Perhaps the most interesting type of CCSN to the modern astronomer are those that result in a collimated explosion that creates what is known as a Gamma Ray Burst (GRB). It is collimated because most of the energy from the explosion is delivered in two beams lying along the star's original north and south poles. It is a GRB because much of the energy is delivered in so called 'gamma rays' which are really nothing more than very high energy light waves.

A typical GRB might release as much energy in a few seconds as the Sun will generate in its entire 10 billion year life span. It is unlikely that ATLAS will happen to be looking at a GRB in the few seconds that it release all that energy but many of them then display an 'afterglow' that can last for minutes to days. ATLAS should be able to detect about 10 of these afterglow events per year which will help astronomers better understand the GRB production mechanism and their environments.

Supernovae Type 1a

This composite image of the Tycho Type 1a supernova remnant combines infrared and X-ray observations obtained with NASA's Spitzer and Chandra space observatories, respectively, and the Calar Alto observatory, Spain.

Somewhat surprising is a particular type of supernova known as Type Ia (SNIa for short) that explode with nearly the same intrinsic brightness. They are universal light bulbs that burn with the same wattage. Some appear fainter than others only because they are further away, not because they are shining less brightly. Thus, these SNIa supernovae allow astronomers to measure the distances to galaxies across the universe. The ATLAS PI, John Tonry was on the High-Z team that was recently awarded the Nobel prize in physics. They used SNIa to discover that there is a 'pressure' in the universe causing distant galaxies to move apart faster than expected according to Einstein's theory of General Relativity - this 'pressure' is something called Dark Energy. We don't know what it is but the race is on to figure it out.

To really understand what the supernova are telling us though we need to accurately know how bright they're shining - they're wattage. Just like not all 100 watt bulbs are exactly 100 watts, not all SNIas emit exactly the same amount of energy. It depends a little on what kind of star it was before the explosion, what kind of galaxy the SNIa lives in, etc. Scientists call these effects 'systematics' and they're very important to our detailed understanding and use of SNIa to understand our universe. But to calibrate the systematics requires a large sample of bright SNIa - and this is exactly ATLAS's forte. We need to find the bright ones because these can be examined in detail using spectrographs on big telescopes. i.e. it is not particularly useful to find many faint SNIa because they are too faint to be observed even with big telescopes.

ATLAS expects to find about 300 SNIa per year with V magnitudes <17 and 30 with V magnitude <15! With this large sample of bright supernovae ATLAS will provide a treasure chest of shiny objects for detailed study by the biggest telescopes in the world.

Novae, Outbursts and Variable Stars

We all tend to think of stars as being fixed in brightness because we are used to the Sun that seems to shine down on us, weather permitting, at the same intensity day-in and day-out. But many stars can change their brightness regularly and also suffer from explosions that are not big enough to rip them apart but would be big enough to wipe out life on any planet unlucky enough to be orbiting one of those stars.

Novae are sub-critical stellar explosions that are still quite bright. ATLAS will be able to see all novae that occur in our Milky Way galaxy, in our neighboring galaxy, M31, and other large and nearby galaxies like M81 and M101.

Another type of explosive stellar fireworks are the cataclysmic variables. These explosions are not as big as the novae but they are still interesting from the perspective of understanding stars. There are about 2,000 known cataclysmic variables near the Sun and probably others that are not known and ATLAS will be keep an eye on all of them twice every night. Should any of them light up ATLAS can send out an alert for followup by other dedicated observers.

The variable star U Geminorum in its bright state (left) and its faint state (right).

Finally, many stars rhythmically change their brightness in predictable patterns. They are simply known as variable stars and they have been known for centuries. Some stars change brightness because they are literally expanding and contracting becoming respectively brighter and fainter. Others change in brightness because they are actually two stars in orbit around one another and the apparent brightness of the pair changes as one passes in front of or behind the other star. Perhaps the best known example of one of these eclipsing-binaries is the star Algol that you can see change in brightness with the naked eye. Others can change in brightness because they have large 'star-spots' on their surfaces that rotate with the star and cause periodic changes in brightness. ATLAS will observe all the stars in the night sky each and every night so any star that changes brightness will be seen by ATLAS. But there is still a place for amateur astronomers to regularly monitor the brightness of stars that are too bright to be detected by ATLAS or provide many observations on a night for detailed followup of particularly interesting stars..

Earth's Minimoons

The path of a simulated minimoon that is temporarily captured by Earth.

In 2006 a 5-meter diameter asteroid was discovered (2006 RH120) that was temporarily captured by the Earth and went into orbit around our home planet: for about a year the Earth had two known moons. Recent simulations by ATLAS co-I Robert Jedicke and his colleagues Mikael Granvik (U. Helsinki) and Jeremie Vaubaillon (Paris Observatory) suggest that the Earth always has a retinue of 'minimoons' along with the much larger Moon. The largest minimoon is probably only about one meter (yard) in diameter compared to the Moon which is 3.5 million meters in diameter (about 2,000 miles).

Why are minimoons interesting? Well, the primary reason is that they could be targets of robotic spacecraft that could travel to and bring back the entire asteroid to Earth! Astronomers and geologists would love to have a one meter diameter chunk of rock that has never traveled through the atmosphere sitting in their laboratories. The scientific opportunities for detailed analysis would be unprecedented. You might think that astronomers have lots of rocks from space, the moon and Mars, and to some extent, you're right. But all the meteorites in collections around the world got heated when they passed through the atmosphere and then damaged by weathering and erosion while they sat on the Earth's surface for many, many years. The rocks from the Moon were invaluable but 1) rocks from the Moon are just like rocks from the Earth's surface because the Moon was formed from the same stuff and 2) the total mass of rocks brought back from the Moon is much less than the mass of a single one meter diameter minimoon.

Minimoons are also interesting because even if we can't send a spacecraft to snag one and bring it back to Earth we can still use telescopes on Earth to study it in detail. If we know where the minimoons are then we can determine how fast they rotate and, to some extent, what they're made of using spectrographs. Planetary astronomers have thousands of spectra of large asteroids but none of the smallest objects. While we certainly expect them to be made of the same type of stuff, the details of surface types can tell us a lot about how asteroids move from the main belt of asteroids into the region near Earth.

The problem with finding minimoons is that they are usually faint and we don't know where they are. That's where ATLAS comes in, again. Since ATLAS surveys the entire sky every night it doesn't matter that we don't know where the minimoons are - ATLAS can still find them! We think ATLAS can find a few minimoons per year. Once we get really good at finding them it might be possible to devise a spacecraft mission to bring one back to Earth.

The Static Sky

Left: A 0.1 × 0.08 deg portion of the sky obtained by the (top) more than 50 year old Palomar Observatory Sky Survey in the R band and (bottom) the same view in the recent Sloan Digital Sky Survey in the r band. Right: (top) a simulation of the how the same field will appear in a single ATLAS red observation (two 30 sec integrations) and (bottom) after combining 5 clear nights of ATLAS images.

In a human lifetime there are are barely perceptible changes in the night sky. The Sun, Moon and planets move relative to the background stars, with really good eyes you might be able to detect the motion of a couple of the brightest asteroids, maybe a naked eye comet appears a few times in a lifetime, you can track changes in the brightness of a few stars with your naked eye and, if you're really lucky, maybe catch a supernova. But most people are unaware of these subtle changes and they watch the constellations of stars march by in their fixed arrangements from the first time they look into the sky as children till their aged eyes last close.

Over longer periods of time, or with more sensitive light and position measuring devices, we can see the sky change - it is not static but dynamic. To see the changes often requires a long time baseline but with this information in hand we can measure the slow motion of the stars relative to one another in the sky, detect the changing brightness of stars and galaxies, and have a reference for anything else that appears unexpectedly like a new asteroid or comet.

In the early 1950s the National Geographic Society's Palomar Observatory Sky Survey (POSS) used a 48" (about 122 cm) Schmidt telescope to image the entire sky visible from southern California. This survey was invaluable to astronomers at the time in providing interesting astronomical targets for detailed followup. It is invaluable to modern day astronomers as a reference source to monitor changes in the sky.

It took POSS about six years to image the sky. While the ATLAS telescopes are much smaller than the POSS telescope we can add images together from multiple nights (we call it 'stacking') to detect fainter sources. In just one week of observing ATLAS will collect enough images to stack them together and be a better survey than POSS! That is just 1/300th of the time that POSS took due mostly to the sensitivity of modern telescopes cameras that use CCDs rather than photographic plates to capture the faint light from distant objects.

We will build an image of the entire sky only half a year after ATLAS starts surveying and we will identify new asteroids and comets, supernovae and novae, variable stars and GRBs by comparing each new image to our reference 'static sky'. Furthermore, we will upload our static sky image to the web for anybody to access at any time.

Space Junk

Computer generated illustraion of objects in Earth orbit that are currently being tracked.

One thing humans are good at is polluting their environment and space is no different - there is a growing amount of 'space junk' in orbit around the Earth that will cause confusion with real objects detected by ATLAS and, more importantly, constitutes a real hazard to orbiting spacecraft. In early 2009 a U.S. Iridium satellite and a Russian Cosmos 2251 satellite collided producing a tremendous amount of space junk. One big problem is that once space junk is there it's really hard to clean up and every bit of space junk creates more space junk in what is known as a 'collision cascade' that soon could make some satellite orbits impossible. In other words, there will soon be so much space junk that we can not possibly place a spacecraft in those orbits because they would be destroyed quickly through collisions with the left over debris.

So it is important to monitor how much space junk is in orbit and where it is located. NASA, the U.S. Air Force, and other international organizations track all the objects they can but it's a constant struggle because there are some many of them and their orbits change in a (usually) predictable but rapid way. Plus, they can only track what they know about... what's needed is a survey that images the entire sky every night - ATLAS.

ATLAS can see an object that is only 10 cm in diameter (about 4 inches) in Low Earth Orbit (LEO) while it can detect objects of about 60 cm in diameter (about 2 feet) when they are in Geostationary Earth Orbit (GEO). Objects in LEO are typically somewhere in the range of 200 to 2000 km (about 100 to 1,200 miles) above the Earth's surface while those in GEO are about 36,000 km high (about 22,000 miles). It's really incredible when you think that the satellites that you can see with your naked eye moving slowly across the sky are hundreds of miles away from you and that ATLAS can detect something the size of a large beach ball when it is 1/10th of the way to the Moon!