December 16, 2019

Astro-Challenge: What’s so Special About 51 Pegasi?

It’s hard to imagine a time before we knew of worlds beyond our own solar system. These days, extra-solar (or “exoplanets”) are back page news, as discoveries occur almost daily.  But scant decades ago (Waaay back in the pre-Internet Stone Age of the early 1990’s) no exoplanets were known, and the entire field was open to great conjecture. This was also one of the great variables underpinning the famous Drake Equation which attempts to quantify how many intelligent civilizations might exist in our galaxy; i.e. “how many stars possess solar systems?” That all changed in the 90’s, and the discovery of a planet in October 1995 by Geoffrey Marcy and Paul Butler orbiting the star 51 Pegasi in the constellation Pegasus was pivotal in opening the flood gates.

51 Peg wasn’t the first system harboring an exoplanet discovered; that fame goes to the exoplanetary system orbiting the pulsar PSR B1257+12. 51 Pegasi b (also informally known as Bellerophon) was, however, the first exoplanet discovered orbiting a main sequence star. And what a bizarre a world it turned out to be… 51 Peg b has a minimum mass half that of Jupiter, and orbits its host star in only 4 Earth days! In the coming years, a whole host menagerie of “hot Jupiters” were to follow, and 51 Peg showed astronomers that clearly we have yet to understand everything there is to know about solar system and planetary formation.

So, why, might you ask, are we re-hashing decades old news? Well, at magnitude +5.5, 51 Peg stands as one very few exoplanet host stars that are within reach of average optics. Visually unassuming, this star lies just off of the lower left corner of the “box” of Pegasus. The planet itself is on the current short list for potential direct imaging, and Hal Clement even used it as a setting for his sci-fi tale “Exchange Rate.” The system itself is 50.9 light years distant. For those with setting circle capability, the coordinates of 51 Peg are as follows;

Right Ascension: 22 Hours, 57 Minutes & 28.0 Seconds.

Declination: North +20° 46’ 08”

As the crescent Moon slides back into view, I urge you to pin-point this seemingly unassuming star that did, in fact, change modern astronomical history!

The astro-term for the week is radial velocity. This was the method by which 51 Peg b was discovered. As an unseen planet orbits its parent star, it “tugs” the star slightly out of place. This line of sight motion can be measured using a sensitive spectrograph, and can give us an idea of the minimum mass for the unseen object, assuming that the orbit is inclined nearly along our line of sight. Spectrographs able to measure shifts on the order of 70 meters per second came online in the mid-90’s, and account for the discovery of 51 Peg b. Of course, the flood of “hot Jupiter” discoveries was an artifact of this method; more sensitive techniques and longer durations are just now starting to tease out smaller worlds in more leisurely orbits.

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