So you were probably expecting the second part of our "Tale of Two Worlds" this week, but my train of thought was interrupted by a couple items in the news coming out of the professional astronomy community about two other recently discovered worlds — orbiting stars other than our own.
The search for planets orbiting other stars seldom involves observing them directly. Planets only shine by the light reflected from the stars they orbit, and even the largest planets are a very small fraction of the size of a star. The planets are therefore extremely dim objects, and are always located so close to their parent star that the brightness of the star greatly overwhelms the planet, making it (usually) impossible to view in even the largest telescopes.
However, there are two greatly successful techniques for indirectly detecting the presence of planets orbiting stars in our local region of the galaxy. The planets announced this week were discovered using each of these methods, giving a great opportunity to understand how modern astronomers are able to gain so much detail about objects that we may never directly see in a telescope.
Any two masses in the universe are attracted to each other by the force of gravity. This force increases with the mass of the objects, and as the distance between them decreases. The Sun's gravity pulls upon Earth and keeps us in our orbit. The Earth in return pulls ever so sightly on the Sun, and will very, very slightly cause the Sun to wobble back and forth in its path as it orbits the center of our galaxy. So, a star with a planet will show a continual periodic motion, with one period completing with every orbit of the planet around the star.
The next piece of the story is a bit more complicated, and starts with an analogy. Think of what a passing train's whistle, or racing car's engine, sounds like as it comes by you. The sound of the whistle or engine starts at a high pitch, and then drops to a lower pitch after passing you. This change in the pitch of the sound is the Doppler Shift, creating higher frequency sound waves from an approaching object, and lower frequencies as the object moves away from you.
The same phenomenon occurs with light waves. Objects moving toward us have their light "shifted" to higher frequencies — toward the blue end of the visible spectrum; objects travelling away appear shifted toward the red, lower frequency, end. The faster the object is moving, the greater the amount of the shift.
Modern astronomers separate the light of stars into spectra using extremely high precision optical instruments that can measure vanishingly small shifts in the light emitted. This allows us to know the speed and direction of motion (toward or away from Earth) of the stars down to a few feet per second.
In an amazing result announced last week, researchers using this technique have detected an Earth-sized planet orbiting Alpha Centauri B — in the star system nearest to our own, located a mere 4.3 light years from Earth. The Alpha Centauri system consists of three stars - two stars (boringly named A and B) similar in brightness and structure to the Sun, and a much dimmer third star (Proxima), closer to Earth, but more distant from the brighter pair.
The discovered planet orbits extremely close to Alpha Centauri B — at only 3.7 million miles from the star, about 1/10th the distance between Mercury and our Sun, and completes one orbit in a mere 3.2 days. The velocity change caused by the pull of this small planet upon its star amounted to only about 4 feet per second - right at the limit of this incredibly sensitive technique.
With a surface temperature estimated at 2,200 degrees (Fahrenheit), this is not a planet we would be wanting to colonize! However, using models of the physics involved in solar system formation, we find that small rocky Earth-like planets tend to be formed in batches. So, the discovery of one such planet at Alpha Centauri B may very well indicate that other such planets, farther from the star and therefore cooler, are also present.
The second discovery announced this week used a completely different approach to planet detection, and with a fascinating twist. If a planet's orbit around its star is oriented such that it passes between Earth and the star during its orbit, the planet will block a very small portion of the light coming to us from that star during each "transit". If you recall, Venus passed between Earth and the Sun back in April, blocking about 1/1000th of the light from the Sun during its
transit — not particularly noticeable.
Using extremely sensitive cameras on an orbiting telescope — the Kepler Mission — light blockages as small as 1 part in a million can be detected. The Kepler spacecraft has been imaging 100,000 stars in the constellation Cygnus continuously for the past 4 years, collecting immense amounts of data which are to be searched to find extremely small dips in light output in each of the target stars, occuring on a regular period, that would indicate the presence of a planet.
Kepler has already detected 77 planets which have been confirmed through additional observation, however the sheer volume of data being generated by this instrument has presented a difficult problem for the astronomical community - but that challenge has been met with a very creative solution.
The human mind has a highly evolved capability for detecting patterns, far more sophisticated than our best computer algorithms. The Kepler data sets can be presented visually to any human mind, and with a minimum of training, the types of patterns marking possible planet blockages can be recognized. This fact lead the Kepler scientists to an awesome idea - letting the interested public become directly involved in the search for planets.
The website is www.PlanetHunters.org. After registering and reading some very simple instructions, you are presented with measurements of light vs time coming from target stars. You will indicate sections of the measurements that look like planet transits. Software will compare your observations with dozens of others. Those observations fitting a pattern will be sent on to researchers who will then closely examine these candidate observations.
The announcement came this week of the first discovery completed through this process, and what a discovery it was! The identified candidate star system was a pair of pairs of stars — two binary star systems orbiting each other; a total of four stars. Using the massive telescopes the Keck Observatory in Hawaii, the presence of a Neptune-sized planet, about 8 times larger than Earth, orbiting one of the pairs of stars was confirmed, by predicting and then observing additional transits of the planet.
The planet has been catalogued as "PH1", after the website's name. Although even now the discovery of planets beyond our solar system has become routine, we can look forward to many more opportunities for what I will term "community science" to both engage the public in scientific research, and to enable discoveries that may be otherwise lost without the use of our uniquely human intelligences joined through modern technology.
Before leaving you this week, I'll mention that I am now registering families for my Spring 2013 Introduction to Astronomy class, held here at my home in Southbury. If you are interested in this class, please visit www.turnerclasses.com for more information.