Just a few decades ago, when we glanced skyward, we didn’t know for certain whether any star visible to the naked eye hosts planets like our own. Now, NASA scientists estimate that, on average, there may be at least one planet for every star in the Milky Way. That’s billions of extrasolar planets in our island universe alone — not to mention the billions of stars in the trillions of other galaxies in the cosmos.
Of the more than 5,000 exoplanets that have been confirmed so far and many more thousands of candidates, most orbit stars the mass of the Sun or lower. The spring night sky, however, offers us several examples of bright stars larger than the Sun thought to host planets. Let’s look at a few and use our imaginations to envision these secret worlds.
The yellow-orange 1st-magnitude Pollux (Beta [β] Geminorum), at only 34 light-years distant, is the closest giant star to the Sun, and the brightest gem in Gemini the Twins. Slightly under 2 solar masses, Pollux may also be one of the brightest intermediate-mass stars known to host a candidate planet. Properly known as Thestias, or Pollux b, this unconfirmed gas-giant world has a minimum mass and size twice that of Jupiter. It lies 1.7 astronomical units (AU; 1 AU is the average Sun-Earth distance of 93 million miles [150 million kilometers]) away from its parent star, completing an orbit once every 590 days.
Moving eastward, 2nd-magnitude Algieba (Gamma [γ] Leonis) in the Sickle of Leo asterism lies 130 light-years distant. Algieba is actually the combined light of a binary star system: a 2nd-magnitude orange giant star (Gamma1) and a 4th-magnitude yellow giant (Gamma2), which orbit each other over a 510-year period. However, our interest is mainly in Gamma1 Leonis; it has a confirmed gas-giant planet, Gamma1 Leonis b, slightly larger in size and at least nine times the mass of Jupiter. The planet is 1.2 AU from its host and takes 428½ days to complete an orbit. A possible second companion remains unconfirmed.
Slipping over to the north, we find 2nd-magnitude Kochab (Beta [β] Ursae Minoris) — the brightest star in the bowl of Ursa Minor (or Little Dipper) and one of members of the ancient “guardians of the pole” asterism. Also lying 130 light-years distant, this orange giant harbors a substellar gas-giant planet, Kochab b, with a minimum mass of six Jupiters. The planet orbits its host star once every 522 days from a distance of 1.4 AU.
Still facing north, let’s now turn our attention to Errai (Gamma Cephei), the star marking the rooftop of the house-shaped asterism in Cepheus the King. This orange giant star, 45 light-years away, appears as a single 3rd-magnitude star, but it is the combined light of two stars forming a true binary pair. As early as 1988, astronomers believed Errai had a planet, Gamma Cephei A b, orbiting it, but that suspicion was not confirmed until 2002. Some still consider Errai to be the star with the first discovered exoplanet. It has slightly more than nine times the mass of Jupiter and lies at an average distance of 2.1 AU from its host. This places it in the star’s habitable zone, where water can exist in liquid form and life could have a possibility of taking hold — especially if the planet has a moon.
Our last extrasolar gem also resides in the north. Shining at 3rd magnitude from a distance of 100 light-years, Edasich (Iota [ι] Draconis) is an orange giant star. It’s about the same mass as the Sun and hosts two confirmed worlds that may be planets. (Given the uncertainty in their measurements, there is also a possibility they are brown dwarfs.) Iota Draconis b is properly known as Hypatia (after the Greek scholar of ancient Alexandria) and weighs between 12 and 26 Jupiter masses. It lies at a distance of 1.5 AU in the star’s habitable zone, swinging around its host once every 1.4 years. Edasich’s second known world, Iota Draconis c, is between 12 and 30 Jupiters. It lies somewhere between 12 AU and 30 AU from its host, taking 30 to 120 years to complete one orbit — well away from the habitable zone.
The exoplanets listed above are but a small sample of the many wonders available to see.
This will be my last column for Astronomy, and as always I wish you well, with many starlit nights to fill your hearts with joy.
