Exoplanets for the Mind’s Eye

You might call it wishful thinking, but here's how to "see" a dozen exoplanets in the fall evening sky.

Artist's illustration of exoplanet

Artist's conception of an extrasolar planet orbiting a star.

When I was a boy, I read a book that inspired a love of science, outer space, and adventure. The Wonderful Flight to the Mushroom Planet by Margaret Cameron featured two young boys, Chuck and David, who met up with the brilliant but eccentric Tyco Bass. Bass invented a special "stroboscopic polarizing filter" that allowed him to see a hidden planet named Basidium very near Earth. With his help, the boys built a spaceship, traveled there to solve a crisis, and returned safely home.

While it's unlikely Earth harbors a hidden planet, amateur astronomers of the far future may one day focus their filter-equipped telescopes on distant stars to spy planets that today require the most advanced techniques to photograph, let alone see. But we needn't walk away empty-handed in the present. Instead, we can better picture these alien worlds, if only indirectly, through acquaintance with their host stars. I'll explain in a minute.

For every blip there is a dip

In the transit method, a repeating dip in the star's brightness is caused by an orbiting extrasolar planet.

Extrasolar or exoplanets orbit stars other than the Sun. As of October 22, 2014, we know of an astonishing 1,832 of them. 1,151 gave themselves away when transiting or orbiting in front of their host stars. Astronomers look for regularly repeating dips in the star's light — usually less than 1% — that indicate shading from a transiting planet. Knowing the distance and diameter of the star, as well as the length of time the passing object dimmed the star’s light, they can determine the planet’s size and mass.

Twisting the night away

In the radial velocity method, the exoplanet's gravity causes the host star to move back and forth. Measuring the shift of the star's light toward blue and red yields information about the planet's mass and orbit. Both methods favor the discovery of massive, Jupiter-like worlds.

The radial velocity method, with 559 exoplanet discoveries to its credit, is the next most popular way to snag an alien world. Here, astronomers measure the slight wobble of the host star around the planet–star center of gravity caused by the gravitational tug of an orbiting planet. As the star swings toward the observer, the light leaving it is compressed toward the blue end of the spectrum. As the star moves away, its light is stretched or shifted toward the red end of the spectrum. Measuring these small shifts leads to information about a planet's orbit and mass.

Three bright northern stars orbited by planets

Three relatively bright stars visible in the northern and northwestern sky in autumn have planetary companions including Kochab (Beta UMi), Gamma Cephei, and Epsilon Corona Borealis. Exoplanets take the star's name, followed by a lower case letter (b, c, d, etc.) if more than one planet is found orbiting the star. The map shows the sky facing north around 7 p.m. in early November. Click to enlarge.
Source: Stellarium

We can see six of the Sun's planets without optical aid, and the remaining two require us only to use binoculars. Though no one's seen an extrasolar planet, a small number have been photographed. As you'd expect, because of their distances and sizes, all appear as points of light. That shouldn't stop us from looking up and at least picturing them in our mind's eye, though.

Exoplanet in the flesh

Fomalhaut b is one of the few planets for which we have a direct image. These images were taken by the Hubble Space Telescope and show the exoplanet and rocky debris disk around Fomalhaut. Light from the star itself has been blocked out to better see its surroundings.

A surprising number of naked-eye stars have a planetary companion(s), including 1st-magnitude Fomalhaut and three 2nd-magnitude stars — Kochab in the Little Dipper, Hamal in Aries, and Leo's Algieba. There are presently about 30 in all.

Planets aplenty pack the evening fall sky

The southern half of the fall sky has five, easy-to-see stars with extrasolar planets — Epsilon Tauri in the Hyades, Aries' brightest star Hamal, 4th-magnitude Upsilon Andromedae (with no fewer than four planets), a pair of planets around Epsilon Ceti, and bright Fomalhaut with its single planetary companion. The map shows the sky facing south around 8 p.m. local time in early November. Click to enlarge.
Source: Stellarium

For your viewing pleasure, here are maps showing a select group of 4th-magnitude and brighter stars that are accompanied by one or more exoplanets and visible in the autumn evening sky. The table below includes a few details about each to whet your appetite.

Wishing you a wonderful flight to a hidden world!

Name Mass  Discovered Detection method
Orbital Period Distance
Epsilon CrB b 6.7 Jupiters 2012 Radial velocity 418 days 221 l-y
Beta UMi b 6.1 J 2014 Radial velocity 522 days 131 l-y
Gamma Cep Ab 1.85 J 2003 Radial velocity 903 days 45 l-y
Epsilon Tau b 7.6 J 2007 Radial velocity 595 days 147 l-y
Alpha Ari b 1.8 J 2011 Radial velocity 381 days 66 l-y
Upsilon And b 0.6 J 1996 Radial velocity 4.6 days 44 l-y
"              " c 1.8 J 1999 Radial velocity 238 days "     "
"              " d 10.2 J 1999 Radial velocity 1,303 days "     "
"              " e 1.1 J 2010 Radial velocity 3,849 days "     "
Eta Ceti b 2.5 J 2014 Radial velocity 403 days 124 l-y
"         " c 3.3 J 2014 Radial velocity 752 days "     "
Fomalhaut b 3.0 J 2008 Direct imaging 320,000 days 25 l-y

Use a Sky & Telescope Star Wheel to guide your journey through the autumn skies!

Astronomy Blogs, Explore the Night with Bob King, Observing
Bob King

About Bob King

Amateur astronomer since childhood and long-time member of the American Association of Variable Star Observers (AAVSO), Bob King also teaches community education astronomy and writes the blog Astro Bob. The universe invites us on an adventure every single night. All we need do is look up. My book "Night Sky with the Naked Eye" was just published and is now available on Amazon and BN. It covers all the great things you can see at night with just your eyeballs. No equipment needed!

One thought on “Exoplanets for the Mind’s Eye

  1. Anthony BarreiroAnthony Barreiro

    Thank you. Seeing stars with exoplanets makes for a very cool observing project, interesting and doable if you have a clear sky and a decent chart.

    One small clarification. The star chart and table both feature Eta Ceti, but the caption for the star chart refers to Epsilon Ceti. Eta Ceti is correct, right?

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