While 522 exoplanets have been discovered so far, we only have photos of about a half-dozen. The star pictured above is HR 8799. It's 130 light years from Earth and 1.5 times more massive than the sun. It's been obscured to better show the three planets - b,c and d - in orbit about it. Credit: C. Marois et. all, NRC Canda
I’ve been reading a great new book about the history of discovery of exoplanets – planets revolving around stars beyond the sun – called Strange New Worlds – The Search for Alien Planets by Ray Jayawardhana.
The three primary ways astronomers detect other planets is by measuring the small gravitational tugs they produce on their host stars revealed through the shifting patterns of light in the stars’ spectra, transits or mini-eclipses across the face of their host stars that cause a tiny but measurable drop in the star’s light and microlensing.
We examined microlensing last week when the topic of ‘free-floating’ planets came up. When a star lines up precisely with a distant background star, the foreground star acts like a lens and magnifies the light of the distant one, briefly making it much brighter. A planet orbiting the foreground star will cause deviations in the light that betray its presence.
To date, 522 alien planets have been discovered, most them through gravitational tugging, better known as the ‘radial velocity’ method. The planet in effect tugs the star forward and then backward during it orbital revolution causing a small change in the star’s velocity toward and away from Earth. Jupiter, the biggest planet in our solar system, shifts the sun to and fro by 40 feet per second, a very tiny amount.
"Hot Jupiters" are the most common type of alien planet discovered. They're big enough - and close enough - to produce measurable tugs on their host stars. Most orbit closer in than Mercury does to the sun, making their sun-facing sides exceedingly hot.
It should come as no surprise that the larger and closer an extrasolar planet is to its host star, the greater its gravitational pull will be and the larger it will appear during a transit. That’s why most of the new planets are Jupiter-sized or larger and located very close to their host suns. They’re the easiest planets to see because their effects are the most obvious. Although our instruments are extremely sensitive, exoplanet detection is still in its infancy. The first planet around a sun-like star was only discovered in 1995.
The smallest exoplanet found so far is Kepler 10-b which is only 1.4 times the size of Earth. As methods and instrumentation improve, we’ll soon be finding more earth-sized rocky planets. Already, astronomers have come up with clever ways to detect chemicals in a planet’s atmosphere. During a transit, a bit of the star’s light passes through the planet’s atmosphere and leaves a ‘fingerprint’ on its light. When the planet has finished its pass in front of the star, astronomers then measure the star’s light alone. Subtracting the two tells them what chemicals the planet’s atmosphere contribute to the light. Sodium, carbon monoxide and hydrogen are some of the materials they’ve found.
Four stars with planets are visible in the southern sky at nightfall. The brightest and easiest in the spring sky is the star Gamma in Leo the Lion. It's orbited by the planet Gamma 1 Leo b that packs nine times the mass of Jupiter and goes around once in 428 days.Â Maps created with Stellarium
With all these new planets popping up, I thought it would be fun to comb through the list to see which planet-hosting stars are visible with the naked eye on late May nights. I was pleasantly surprised to find 11 stars – two with multiple planets – that were brighter than the standard rural sky limit of magnitude 6.0.
Turning to face the northwest at nightfall, Gemini, Cancer and Ursa Major (UMa) all sport exoplanets. The planet 47 UMa b is a triple planetary system, while 55 Cancri has five in all!
While the planets themselves are utterly invisible – like trying to see a firefly next an arc light 1,000 miles away – we can see the stars well enough. One of them, Gamma Leonis, is even bright enough to see from a city. All the others are visible to varying degrees depending on your sky. Binoculars will show them all with ease.
Four additional stars greet sky watchers facing north in late May. The second brightest planet-bearing star in the spring sky is Gamma Cephei, located below the North Star.
The next time it’s clear and you have a chance to go outside, see how many of these planet-bearers you can see for yourself. To help you picture the invisible worlds in orbit, consult the list below for details. Planets take their star’s name with the letter ‘b’ appended for the first planet discovered followed by ‘c’, ‘d’, etc. if additional ones are found.
* Gamma 1 Leo bÂ / 8.8 Jupiter-mass planet orbits 1.2 x Earth’s distance in 428 days / Star brightness or magnitude is 2.0 – bright!
* 70 Vir b / 6.6 Jupiter-mass planet orbits 0.5 x Earth’s distance in 117 days / Star mag. = 5.0
* Tau Boo b / 4 Jupiter-mass planet orbits 0.046 x Earth’s distance in 3.3 days / Star mag. = 4.5
* Kappa CrB b / 1.8 Jupiter-mass planet orbits 2.7 x Earth’s distance in 1191 days / Star mag. = 4.8
* Tau Gem b / 18 Jupiter-mass planet orbits in 305 days / Star mag. = 4.4
* 55 Cnc b, c, d, e, f / range of 0.03 to 3.8 Jupiter-mass planets orbiting from 0.02 to 5.8 x Earth’s distance in 0.7 to 5218 days / Star mag. = 6.0
* 47 UMa b, c, d / 0.5 to 2.5 Jupiter-mass planets orbiting from 2 to 11.6 x Earth’s distance in 1078 to 14,002 days. / Star mag. = 5.0
* 4 UMa b / 7 Jupiter-mass planet orbiting 0.9 x Earth’s distance in 269 days. / Star mag. = 5.8
* 11 UMi b / 10.5 Jupiter-mass planet orbiting 1.5x Earth’s distance in 516 days / Star mag. = 5.0
* Gamma Cep b / 1.9 Jupiter-mass planet orbiting 2x Earth’s distance in 903 days / Star mag. = 3.0
* 42 Dra b / 3.9 Jupiter-mass planet orbiting 1.2x Earth’s distance in 479 days / Star mag. = 4.8