The Los Angeles Times features a neat science story that, rara avis, gets its facts straight in a presentable manner. Nice work. The story reminds us that there is still much to be learned about the dwarf planet Eris, but that astronomers are nevertheless finding new information.
One item was glossed over sic:
With such small, far-off objects, astronomers wait for what’s known as a stellar occultation, in which the object will cross over a star, essentially casting a shadow over the Earth. The amount of starlight blocked by the object allows scientists to calculate the object’s size.
Witnessing this stellar occultation last year required being in the right place at exactly the right moment during the brief time window that Eris was scheduled to block the star.
There is nothing really wrong with this description of the process, but the Czar thought you might not appreciate how freaking clever this technique is.
All right. We can photograph Eris—although it looks little more than a dot of light—and can measure how fast it moves across the sky. Since there is a direct relationship between a planet’s orbital speed and its distance (another amazing story for another day), we can determine that Eris orbits about nine billion miles away from the sun, give or take.
So we wait for Eris to move in front of a star, which it did. This is called an occultation, and there is nothing odd or weird about it. The planet simply moves in front of a star; the star stays relatively fixed in the sky.
Like an eclipse, the planet blocks the star light. So astronomers watch the light of the star. When the light begins to dim, we know the planet is beginning to move in front of it. When the light is blocked, we know the planet is now completely in front of the star. And when the light returns to full brightness, the planet has moved away.
So this tells us a lot of things. First, we know the speed the planet moves through the sky, right? That means by measuring the time it took for the star to lose its brightness to when it returned, we can accurately determine the diameter of the planet: Eris is 1,445 miles in diameter—something we really didn’t know before.
Second, it confirms the distance of the planet to the sun. If we know how long it took, and we know its orbital speed, we can work out its distance at that point to the sun (remembering that planets orbit mostly in nearly perfect circles but not exactly perfect).
Third, we noticed that the light of the star winked out instantly, not gradually, and winked back on a few moments later. This tells us that, at the present time, Eris has no atmosphere. Planets with atmospheres (and from time to time, even Pluto has one) cause a star to fade out, not wink out.
So we learned three things, at least, from one simple coincidental event: Eris moving in front of a distant star and blocking its light.
This is very cool stuff.