Exoplanets pepper the night sky. We may not be able to observe them directly, but we can use their host stars as surrogates to envision them in our mind's eye.

Exoplanet first
This artist’s view shows the hot Jupiter exoplanet 51 Pegasi b, which orbits the star 51 Peg about 50 light-years from Earth. Found in 1995, it was the first exoplanet discovered around a normal star. It orbits just 4.7 million km from the star and has an estimated surface temperature of 1,000°C — hot!
NASA / JPL-Caltech

I usually talk about things you can actually see in the sky, but this time we're making an exception to include exoplanets, none of which are visible in a telescope. Sure, go ahead and laugh, but give me a minute to explain. I understand your skepticism, but the overwhelming number of exoplanets — 3,878 spread across 2,896 planetary systems as of November 14, 2018 — compels me to recognize them as legitimate observing targets.

Exoplanets, also called extrasolar planets, are planets that orbit stars other than the Sun. They're named for their host sun followed by the letter "b" for the first planet discovery, "c" for the second, and so on.

The vast majority spin around distant stars tallied in obscure catalogs, but several dozen accompany naked-eye stars. At the very least we can gaze at these stars and imagine massive Jupiter-sized planets cycling back and forth around them just like the eight marbled orbs entrained by our Sun.

Gentle tugs betray a planet
All but two of our featured exoplanets were discovered using the radial velocity method, which detects when a planet tugs on its host star and causes it to change its orbital speed in a cyclical way. Astronomers measure velocity changes by noting a slight shift in the color of the star's light when it approaches (blue) and recedes (red). 
Las Cumbres Observatory / CC BY-NC 2.0

Most of the ones on the list are hot Jupiters, massive, Jupiter-sized worlds that orbit close enough to their parent stars to get baked by stellar rays but also to exert a measurable gravitational tug. The planet's pull causes the star to describe a small circle about the system's center of mass. As it alternately approaches the Earth and recedes from it, astronomers measure the star's changing speed — called its radial velocity — to determine the planet's mass and other characteristics.

Rare direct image
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 show its surroundings better.

Massive planets closely orbiting their parent suns pull strongest and are more easily detected than smaller Earth-like planets in more distant orbits. Jupiter, for instance, yanks the Sun around to the tune of 13 meters per second while the much less massive Earth manages only a measly 9 centimeters/second. Current spectrometers, which measure the redshift and blueshift of light caused by variations in speed, can detect speed changes to just under 1 meter/second.

Direct image
This false-color near-infrared image has been processed to remove most of the scattered light from the star Kappa Andromedae (masked out at center). The "super-Jupiter" companion, Kappa And b (upper left), orbits its star at about 55 a.u., or about 1.8 times farther from it's host star than Neptune is from the Sun. Neptune's orbit is shown for comparison (dashed circle). The white blob marking the companion indicates a signal present in all near-infrared wavelengths, while colored blobs represent residual noise.
NOAJ / Subaru / J. Carson (College of Charleston) / CC BY-SA 3.0

A great many exoplanets have also been found by observing the periodic fading of the host star when a planet passes in front of it, using what's called the transit method. The recently defunct Kepler Space Telescope used this strategy to discover nearly 2,700 planets! There are many more ways of digging out planets buried in their host star's light, including gravitational microlensing, pulsar timing, and direct imaging.

Exoplanet abundance!
This map will help you identify some of the 19 stars orbited by exoplanets visible in the fall sky from mid-northern latitudes. Details about each are found in the table at the end of the article.
Stellarium with additions by the author

Two planets on our list have been directly photographed — Fomalhaut b and Kappa (κ) And b. All the rest were discovered via radial velocity variations. Fomalhaut b, one of the closest exoplanets at just 25.1 light-years distant, orbits within the edge of a debris disk (planetesimals?) 115 a.u. from its host star. Both Kappa Andromedae's and Fomalhaut's planets plod around their host suns very slowly with periods of 877 years and 589 years, respectively.

Autumn's invisible planets
Aldebaran is one of the brightest stars hosting an extrasolar planet. At 6.5 times the mass of Jupiter, it orbits about 45% farther from its star than Earth does from the Sun. Like many of our featured planets, it's a hot Jupiter with a temperature of around 1,230°C (2,240°F).
Stellarium with additions by the author

A glance at the table below shows that Tau (τ) Ceti has the least weighty companions with two of its four planets only twice as massive as Earth, which, by the way ,is 0.003 times as massive as Jupiter or 0.003 MJ. The most massive planet is Upsilon (υ) And d with a mass 23.6 times that of Jupiter. Sounds like it would be spectacularly hard to stand up on that planet, and yet it's still a far cry from being massive enough to fire up hydrogen within the core and become a star. That requires about 75 Jupiters worth of material.

Circumpolar exoplanets
Four exoplanet-orbited stars beckon in the northern sky, including bright Kochab in the Little Dipper.
Stellarium with additions by the author

Even if you can't see their planetary companions, gaze up at any of these stars the next clear night — all are visible with the naked eye — and relish the certainty of knowing that planets circle them as surely as the familiar planets whiz around the Sun. Ours is the first generation in history to know this as fact instead of speculation.

Planets below are listed in order of right ascension from west to east:

Name Discovery Date Method Mass (Jupiter) Orbital Period Star Mag.
11 UMi b 2009 Radial velocity 10.5 MJ 516 days 5.0
Beta (β) UMi b 2014 Radial velocity 6.1 MJ 522 days 2.0
42 Dra b 2009 Radial velocity 3.9 MJ 479 days 4.8
18 Del b 2008 Radial velocity 10.3 MJ 993 days 5.5
51 Peg b 1995 Radial velocity 0.5 MJ 4.2 days 5.5
Fomalhaut b 2008 Direct imaging 3.0 MJ 877 years 1.2
91 Aqu b 2003 Radial velocity 3.2 MJ 181 days 4.2
14 And b 2008 Radial velocity 5.3 MJ 186 days 5.2
Kappa (κ) And b 2013 Direct imaging 13.0 MJ 589 years 4.2
Gamma (γ) Cep b 2003 Radial velocity 11.1 MJ 903 days 2.5
Eta (η) Cet b,c 2014 Radial velocity 2.6 MJ, 3.3 MJ 404 days, 752 days 3.5
Ups (υ) And b,c,d,e 1996—2010 Radial velocity 0.6 MJ, 9.1 MJ, 23.6 MJ, 1.1 MJ 4.6 d to 10.5 years 4.1
Tau (τ) Ceti e,f,g,h 2012—2017 Radial velocity 0.006 MJ to 0.012 MJ 20 to 636 days 3.5
Alpha (α) Ari b 2011 Radial velocity 1.8 MJ 381 days 2.0
75 Cet b 2012 Radial velocity 3.0 MJ 692 days 5.4
81 Cet b 2008 Radial velocity 5.3 MJ 953 days 5.7
Epsilon (ε) Eri b 2000 Radial velocity 0.8 MJ 7.4 years 3.7
Sigma (σ) Per b 2014 Radial velocity 6.5 MJ 580 days 4.4
Aldebaran b 1998 Radial velocity 6.5 MJ 628 days 0.9


Image of Rod


November 14, 2018 at 11:17 am

Bob, very good report and list of host stars with exoplanets we can observe (the stars only). I viewed the star Aldebaran on 30-Dec-17 during the lunar occultation using my 90-mm refractor, much fun. I forgot this star is listed with an exoplanet and report I did not see it when Aldebaran came back into view from behind the Moon 🙂 I will try and observe other stars on your table here and log them. Weather where I am at in Maryland is currently cloudy with perhaps some winter mix arriving tomorrow morning 🙂

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Bob King

November 14, 2018 at 11:46 am

Thanks, Rod. Seeing the stars is the best we can do of course. Still, it's fun and a great thing to share with the public on observing nights.

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November 14, 2018 at 7:19 pm

That's a great list!

Besides, IMO, there's absolutely nothing to be laughed at (at least by amateur astronomers worth their EP collection): Specially when we take into account observational astronomy is more about CONTEXT than observing beautiful objects per se. For beautiful eye candies we still have Hubble images!.

I so pray what I preach that I've gotten used to having in my "To observe" list one or another star or DSOs I usually read about - usually in the news pages of S&T, just for the thrill of letting my mind (or imagination if you will) wonder while I observe the object I read about - be it a dust disc around U Mic star or the metal rich stars Hubble spotted in NGC 6496.

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Bob King

November 14, 2018 at 8:29 pm

You nailed it, Nilson. Come to think: whenever I'm looking at a galaxy made of billions of stars in my telescope, all that starriness appears as nothing more than fuzz, and yet I'm perfectly happy to perceive it as a star-studded galaxy.

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