FAQ

Lately I’ve been researching dark matter and dark energy, and I’ve been wondering, out of all the candidates that could be dark matter (brown dwarfs, neutrinos, black holes, etc.), which is most likely? I was also wondering the same about dark energy.

Thank you for your inquiry!  Here is what I can tell you about dark matter and dark energy:

Dark matter:
Actually, developments since the 1970s and 1980s have helped rule out all of the dark matter candidates you listed below (planets, brown dwarfs, black holes, neutrinos) as the primary dark matter candidate.  Planets, brown dwarfs, and black holes belong to a class of dark matter candidates called ‘MACHOS’, which stands for MAssive Compact Halo Objects, and the idea is that these objects are all quite massive compared to the amount of light they emit.  Microlensing surveys of the Galactic bulge, which detect massive bodies (i.e. MACHOs) bending the light of background stars, have found far too few of these MACHOs for MACHOs to be the primary dark matter candidate.  Meanwhile, neutrinos, which are “hot” (a jargon term meaning that they travel at almost the speed of light that has *nothing* to do with their temperature).  Because neutrinos move so fast, they never would have slowed down enough in the early universe to coalesce via gravity into the galactic dark matter halos we observe today.

The current theory, called Lambda-cold dark matter (or Lambda-CDM), favors a new type of particle for dark matter that, unlike neutrinos, moves slowly enough to coalesce and form galactic halos.  This particle is often called a ‘WIMP’, for Weakly Interacting Massive Particle.  Several theories of particle physics, including super-symmetry, predict the existence of the WIMP, although it has not been found yet.  Many different physics experiments are looking for the WIMP.  These all involve detectors made of heavy nuclei located up to a mile underground!  WIMPS should interact with other matter very rarely, so keeping the detector underground protects it from other particles like cosmic rays that would introduce noise to the experiment, while still allowing the WIMPs, which can pass through the entire Earth without interacting with a single atom, to (sometimes) interact with the detector.  There is also the possibility that the Large Hadron Collider ( http://lhc.web.cern.ch/lhc/ ) at CERN could produce dark matter particles; some scientists are actively looking for a dark matter signal in the LHC data.

Here are a few of the dark matter particle detector websites:
LZ Dark Matter Experiment
The DAMA Project

Dark energy:
The currently favored cosmology, Lambda-CDM, involves a component called dark energy that exerts a negative pressure.  Unlike matter, which attracts via gravity, dark energy is a term we use to mean that the fabric of space time does work on the universe to accelerate its expansion.  Lambda-CDM predicts that the acceleration of the expansion has always had the same value (i.e. the pressure has always been the same).

I think you would be interested in a fantastic website, Particle Adventure, in which you can explore the science and history of particle physics through interactive displays and quizzes.  The website has won several awards, and it is where I learned much of the particle physics I know.

Enjoy!

More Questions About Dark Matter

I have thought recently that the quality of bright day light appeared to be different to how I have always perceived it, but I dismissed it. However I’ve noticed that at midday shadows were very long, at that time shouldn’t they be almost zero or tiny; I then realised that the Sun wasn’t overhead but at a lower declination that I thought it should be – ie not over head. I then thought how much cooler it had been this year, and that if what I was seeing was correct that the Earth would have tilted, and in a way that meant we (the UK) were further North. I’ve tried to disabuse myself of this this rather worrying view, but having looked on the net & Youtube, it’s not just me that seems to be noticing this!!! It can’t be so, can it, wouldn’t it have been on the news, or at least ‘The Sky at Night’; What am I seeing???

You have made some excellent observations!  Indeed, in the U.K., the sun has not been directly overhead this summer.  Nor has it been directly overhead, in summer or winter, in previous years.

The reason for this is the tilt of the Earth’s axis.  Only places between latitudes 23.5 degrees North (the Tropic of Cancer) and 23.5 degrees South (the Tropic of Capricorn) will ever have the sun directly overhead.  The sun will be directly overhead for all locations on the Tropic of Cancer on the summer solstice at noon, and the sun will be directly overhead for all locations on the Tropic of Capricorn on the winter solstice* at noon.  Here is a picture illustrating the summer solstice.

A wise Greek named Eratosthenes used this fact to measure the circumference of the Earth.  He lived in a city called Alexandria, which was north of the Tropic of Cancer.  He knew that on the summer solstice, a city a bit further south called Syene had the sun directly overhead because the sunlight shone directly down a well.  In Alexandria, he placed a stick vertically into the Earth, and on the summer solstice at noon, he measured its shadow.  He used the length of the shadow and the length of the stick to calculate the angle of the sun (83 degrees above the horizon, or 7 degrees away from passing directly overhead, since there are 90 degrees in a right angle).  Here is a picture that shows the setup of his experiment.

Because the sun is very far away from the Earth, its light rays that reach the Earth can be considered parallel.  As shown in the diagram, the difference in angles of the shadows in Alexandria (7 degrees) and Syene (0 degrees) was the same angle as the angle of separation between the two cities, as measured from the center of the Earth.  Thus, if he knew the distance between Syene and Alexandria, Eratosthenes could measure the distance around the Earth by relating the the angle between the cities (7 degrees) to the number of degrees in a circle (360).

Eratosthenes estimated the average speed of a camel to calculate the distance between Syene and Alexandria as 5000 stadia, giving the Earth a circumference of 360/7 * 5000 stadia = 252,000 stadia.  The ancient unit of the stadion varied, and it is not clear which version Eratosthenes used.  Thus, his measurement corresponds to a circumference between 39,690 km and 46,620 km.  Considering that the Earth has a circumference of 40,008 km around the poles, Eratosthenes’ measurement was within 16% of the true value.  That’s pretty good considering he had to estimate the average speed of a camel!

The U.K. is north of the Tropic of Cancer (London is about 52 degrees North), so the sun will never be overhead.  However, you can repeat Eratosthenes’ experiment if you would like (and if the weather cooperates). Wait until one of the solstices or equinoxes (first day of spring/summer/autumn/winter) and put a pole vertically into the ground.  At local noon**, measure the length of the pole that sticks out of the ground and its shadow.  Find the angle between the sun’s rays and the pole by calculating arctan(length of shadow/length of pole).  You know that the sun is overhead at one of the following locations: Tropic of Cancer (summer solstice), equator (autumnal or vernal equinox), Tropic of Capricorn (winter solstice).  The angle you have measured is also the angle of separation between you and the location where the sun is overhead.  Look up the distance between yourself and that location, in kilometers.  Do (360/your angle) * (distance to that location) = circumference of the Earth.  Were you close?  You can also use the angle you measured to find your latitude: your angle + latitude of comparison location (23.5, 0 or -23.5) = your latitude.

As for the weather aspect of your question, weather cycles are complicated!  The weather can vary a lot from year to year.  I suggest looking at yearly averages *and* records, which will convince you that this year was not really that out of the ordinary.  

Various aspects of the Earth’s orbit do change, but on timescales of thousands or millions of years, which is much longer than a human lifetime!  

As for youtube, do not trust the internet!  Anyone can post whatever they want to say on youtube.  They do not need to be qualified or an expert.  By asking an astronomer, you did the right thing to get a scientific answer to your questions!

I hope this helped.

*The solstices are defined by the seasons in the Northern hemisphere.
** Local noon is when the sun is highest in the sky, and this is not necessarily 12:00pm on your watch!  You should look this up for your precise location on the date that you do the experiment.

More Questions About Earth Axis

Occasionally there is some coverage in the media about the continuing discovery of planets around other star systems in the Milky Way, through the work of the Kepler telescope, and other initiatives. Is there a place on the web I can go to, to check what the latest thinking on all of this work is? I’m thinking of an up to date record of things like numbers of candidate habitable planets discovered, where they are, stuff like that.

I am glad you are enthusiastic about keeping up with exoplanet discoveries!  

You can check the Kepler website for tables of the Kepler Mission’s discoveries: https://www.nasa.gov/mission_pages/kepler/main/ (note: not accessible during the U.S. government shutdown).  The table links to the relevant science papers.

In general, you can find information about confirmed planets and planet candidates from the Kepler Mission and other studies here: (the most comprehensive and specific about status of planet confirmation, but a little difficult to use)
exoplanets.org (rigorous in only including confirmed planets, and easy to use)
exoplanet.eu (less rigorous than exoplanets.org, but more up-to-date, also easy to use).

More Questions About Exoplanets

Do you have any recommendations or sources that might help me get ready for the Astronomy section of my Science Olympiad?

How nice to meet you!

1.) You should read an introductory astronomy textbook.  I used this one, which you can get pretty cheaply online.

2.) Start following Astronomy Picture of the Day.  If you need to learn about specific objects, you can search for them in APOD’s archive.

3.) Go outside.  Can you figure out which of the bright objects are planets?  How do the phases of the moon work?  Get a planisphere or star chart and try to identify some stars.  Unfortunately some locales aren’t known for their clear skies, so that might pose a challenge.

Best of luck!

More Questions About General Resources

Everything is made of molecules, and we see our world in color. Does that mean that molecules have a color? If not, where does the color come from?

Interesting question! Molecules do have “color” in a certain sense, but first I should say that not everything is made of molecules. Rocks and metals, for instance, are made of atoms that haven’t formed molecules. There’s also a lot of stuff in space that’s not made of molecules, including the Sun and other stars, as well as stuff called “dark matter” that doesn’t seem to be made of anything we see here on Earth!

Other than dark matter, though, just about everything else I mentioned has a color, even if it’s not made of molecules. That’s because when we see color, what we’re really seeing are little waves of light (called photons) that have different frequencies (kind of like notes on a piano, with each note being a different color). It turns out that our eyes can only detect a small fraction of those colors (just like our ears can’t hear certain sounds that dogs and other animals can hear).

We know those colors exist, however, because we can build special instruments to detect them. For instance, the radio waves that an antenna can detect are really just another form of the light we see with our eyes — our eyes just aren’t sensitive to the frequency (that is, color) of radio waves. Even things that seem to be completely colorless to our eyes (like the air that you’re breathing) has a color that can be measured by the right equipment; it just won’t be one of the normal colors you see in a rainbow or a box of crayons.

More Questions About Molecules

We’ve all seen the posters in school telling us the order of the eight planets and they’re all neatly put in a straight line; that seriously cannot be how the planets orbit the sun in a straight line some must be off in a tilt. So I went and tried to do some research and most sources do put all the planets in a somewhat near line not really varying from a straight rotation around the sun… Is that image correct do all the planets tend to rotate around the sun on an even plane if so then our solar system must be extremely flat with huge vast spaces closely above and below planetary rotations that are never occupied.

Indeed, the planets in our solar system orbit the sun in a relatively flat plane.  The largest mutual inclination between the orbital planes of two planets in our solar system is 7 degrees (out of a 360 degree circle).  There is even a special name for the plane in which the solar system planets orbit: the ecliptic plane.

We think the reason that the planets are in such a flat plane is tied to how planets form.  We think that our sun and its planets formed from a cloud of gas that collapsed.  The center of the cloud got hot enough to fuse hydrogen to helium; this part became our sun.  But what about the cooler outer layers of the cloud?  If the cloud had even a tiny bit of spin as it was forming, that spin would have been accelerated during the cloud’s collapse, like an ice skater who starts a slow spin, then brings her arms in to spin faster.  As the gas cloud collapsed and started spinning faster, much of the outer layers flattened into a disk in the same way that someone making a pizza crust spins the dough to flatten it into a pie.  After the gas cloud around the sun flattened into a disk, planets formed from that disk.  This is why the planets in our solar system have such flat orbits!  Here is a cartoon of the effect, and here is a short simulation video.

Best wishes!

More Questions About Orbit

I wonder if there is anyone on your staff, or anyone you can refer me to get the facts on the story about a rogue planet that has an orbit perpendicular to the elliptical plain of the other planets and which is Gmail Planet X and the year 2012 supposed to hit the Earth in 2012? As a former Astronomy student I know it’s a bunch of junk, but I’d like to get some more detail if possible. The issue I’m dealing with is my town has an end of the world cult preparing for the collision and we would like to get the real story out.

I have heard similar ideas about the Earth/Solar System/Universe ending in 2012 (due to the Mayan calendar running out and a variety of other such nonsense). After scouring the literature and doing some digging on the web it seems that Wikipedia actually has a pretty concise and very accurate and welldocumented version of the whole story behind the Undiscovered Planet and the destruction of Earth (or lack thereof). I recommend reading these two pages on Wikipedia: Planets Beyond Nepture and Zeta Talk. 

The first one is very scientific and presents both sides of the argument, as well as the final conclusion of astronomers today (“Today the overwhelming consensus among astronomers is that Planet X, as Lowell defined it, does not exist” i.e. there is no extra planet that we don’t know about or that is being kept secret from most humans and thus nothing will crash into Earth and destroy life in 2012). The second one is more about some of the history of these wild claims.

Between Dec 2011 and Jun 2012 I observed with the unaided eye a bright light in the western sky from the North American continent, far too bright to be a planetary body, that did not appear to move against the fixed stellar background (between Aquila and Ophiuchus?) and grew brighter as the months passed. As that group of stars made their seasonal procession into the day side of our sky, I lost sight of it.

Hi Astronomy Enthusiast,

You were seeing Venus!  Venus was up in the evening Western sky in the first part of 2012 before its transit in early June (you lost sight of it as it moved farther west and got lost in the brightness of the sun before its June transit).  Of all the planets, Venus appears the brightest because it is close to Earth and very reflective.  (Venus reflects 90% of the sunlight that hits it!)

At its Westernmost excursion, Venus does not appear to move very much relative to the background stars because its motion is mostly along our line of sight, rather than across it.  Venus appears to move most near the sun (such as during a transit) when its motion is across our line of sight.

Cheers!

Is there any work being done to try to identify such planets around either Alpha Centauri A or B? Is there somewhere I can go to on the web to see what the current thinking and evidence is from any work on these stars and their surrounding planets, and whether any might be in the ‘goldilocks’ zone for these stars?

There is a paper reporting the detection of an Earth-mass planet around Alpha Centauri B.  See Phil Plait’s blog for a great synopsis of the paper.

Alpha Centauri, as you can imagine being the nearest star system to our own Sun, has been extensively studied for many years to determine whether it has planets.  Last year a roughly Earth-mass planet was reported around Alpha Cen. B orbiting at 0.04AU (about a tenth of the orbital distance of Mercury), however there is considerable controversy surrounding this discovery and it has yet to be confirmed.  The existence of any planets larger than around Neptune has already been ruled out in the Alpha Centauri system, however finding, or ruling out, small, Earth-size, planets at Earth-like orbital distances is exceptionally difficult.  The Wikipedia article on Alpha Centauri Bb actually does a very good job of summarising the current situation.

How long it will take mankind to identify a habitable planet that would become the candidate for our first planetary colony? Do you think this is going to happen in the next, say 5-10 years with the TESS and E-ELT coming on line? Or, if it might take longer, how long?

Anything regarding colonisation of other worlds is of course highly speculative.  However I’m not above a little speculation so here are my thoughts.

Firstly I would say that the first permanent human settlement on another world would almost certainly be in our own solar system, so in that sense we already know of the planet or moon in question. Which solar system body would host that first colony is another round of speculation, the Moon or Mars are the most frequently touted, though I’ve always been rather fond of the idea of cloud cities on Venus.

In terms of identifying the planet that might eventually become the first extrasolar human outpost, I strongly suspect it would be more than 10 years away.  TESS is due for launch in 2017, but the E-ELT is not expected to be completed until 2022 and the James Webb Space Telescope is not expected to launch until late 2018/early 2019.  So already just in terms of getting these facilities ready we’re looking a fair distance into the future.  One then needs to consider whether any of these facilities are going to find a potential ‘Earth 2.0’ and if so how long that would take.

TESS will not find direct Earth analogs, the planets it finds will have orbital periods of no more than 2 months, so finding planets in the habitable zone of a sun-like star is not something it can do. It should however find Earth-size planets in the habitable zones of dimmer, less massive stars.  Where the Kepler mission went for finding smaller numbers of longer period planets, TESS is intended to find vast numbers of shorter period planets.  TESS by itself would also not be able to tell us whether any of these planets would be nice places to live, it can provide us with a sample of Earth-sized planets in the habitable zones of cool stars, however we will then need to study those planets further.  That will be where JWST and the E-ELT, as well as some new instruments on existing telescopes, come in, trying to study the atmospheres of these planets to give us a better idea of what they are like.  Studying the atmospheres of extrasolar planets is very difficult however, at the moment we have really only managed to get good atmospheric properties for a small number of hot-Jupiter type planets (massive planets very close to their host stars).  The new telescopes and instruments coming online in the next decade will certainly allow us to significantly improve on what we have now, but getting at the atmospheres of truly Earth-like, mostly rocky, planets will still be far from easy, and certainly I don’t think we’ll be able to tell whether humans would be able to live there.

We might get lucky and it may turn out that one of the planets that we discover in the next decade does turn out to be our ideal second home, though I think that is far from certain, and I don’t think we’ll be able to tell until at least the generation of facilities beyond those currently planned.  The way these cycles go that future generation of facilities won’t be here for 20-30 years.

That may sound a bit pessimistic, but we are edging closer every day, and compared even with the time since the invention of the telescope, 30 years is just round the corner!

More Questions About Planets

Can you tell me if Betelgeuse will move all over the sky or will it move in it’s immediate area?

It isn’t entirely clear what you mean.  Betelgeuse will appear to move across the sky over the course of a night due to the Earth’s rotation, as will all stars, but it will remain fixed relative to other stars.

I would like to know if you can tell me if this is Capella. Looking through binoculars, it changes shapes. It looked like a flower with bright red on the end, yellowish in the middle and green on the end.

Changes in the color and shape of an astronomical object are optical effects rather than real changes in the object.  The ring-shaped patterns could be caused by an improper focus.  The color gradient from red to green is due to chromatic aberration, which causes different colors of lights to focus at different distances behind a lens (here is an image).

More Questions About Stars

How would you choose a telescope? Lets say – if you want to see the surface of the moon very closely.

If you want to look at details on the surface of the moon, any binoculars or telescope will give you stunning views.  You will also be able to see the four Galilean moons of Jupiter and the rings of Saturn through just about any binoculars or telescope.  However, if you are interested in looking at fainter objects, such as galaxies and nebulae, you will want to invest in a higher quality telescope or giant astronomical binoculars.

There are a few criteria that distinguish really excellent from okay telescopes:

• The most important is the aperture of the telescope, which determines how much light the telescope can gather.  You will be able to see bright galaxies and nebulae, such as the Andromeda Galaxy and the Ring Nebula, very clearly with a 6″ diameter telescope.

The following criteria are all important, but some are more or less important based on how you plan to use your telescope:

• The second most important thing is a good mount.  Your telescope must be stable so that it doesn’t wobble.  This is not as important if you want to look at something big and bright like the moon, for which a pair of hand-held binoculars will do the trick.
• The third most important thing is good optics.  A reflecting telescope, which uses a mirror as its primary optic, is lighter-weight, cheaper, and suffers from fewer optical imperfections and aberrations than refracting telescopes, which use a glass lens as the primary optic.  However, you can invest in a telescope or a pair of binoculars with high-quality glass.
• Another criterion that I find important is the weight of the telescope.  I am a small person and don’t particularly fancy carrying heavy loads by myself.  I just bought a pair of giant binoculars and a tripod.  Since the binoculars only weigh 10 lbs., I anticipate bringing them on many camping trips!

For a more detailed discussion of telescope features, see this Sky & Telescope article.

More Questions About Telescopes