When it was discovered in 2012 as a new comet coming from deep space, some observers predicted Comet ISON would become so bright in our skies it would be the "Comet of the Century." Wiser heads knew, as comet hunter David Levy likes to say, "Comets are like cats. They have tails and they do precisely what they want!"
For a while, the comet seemed disappointing. But this week, on its way in to a close encounter with the Sun, Comet ISON started putting on a somewhat better show (as you can see in the image, taken by the skillful UK astro-photographer Damian Peach.) The comet now has two nice tails, one made of dust, the other of gas, pointing in the direction away from the Sun.
The comet is currently visible in the pre-dawn sky, but only barely and only when it's really dark. It's better with binoculars or telescopes. The excitement, however, is just beginning. Comet ISON is what astronomers call a "sungrazer" -- a comet that comes indecently close to our Sun. It just so happens, the closest encounter -- only about 3/4 of a million miles from the Sun's surface -- will be on Thanksgiving Day 2013.
The solid "nucleus" of this comet -- a chunk of frozen ices and rock -- is now estimated to be somewhere between 1/4 and 3/4 of a mile across. The Sun's heat could vaporize much of its ice and rock and the Sun's gravity could tear it apart into smaller chunks. Past sungrazing comets have had one or both of these things happen. So it could emerge from its date with the Sun one smaller but strongly evaporating comet, or as several comets spread out over a wider area, or as nothing more than a subtle trail of gas and dust.
If Comet ISON survives Thanksgiving, it will swing away from the Sun and emerge into our dark skies going northward from the plane of our solar system. Should there be enough of it left to make a show, that show will be visible to us in December and January. Around January 8th, for example, it will be near the north star, remaining in our view all night long. (But by then it will likely be much fainter.)
As we say on the radio, "Stay Tuned!" I will give you more updates on this interesting new visitor to our cosmic neighborhood in future posts.
If your want bulletins on Comet ISON between my posts, or need more technical information, see the "Current Status" page at NASA’s Comet ISON Campaign: http://isoncampaign.org/Present and then look around on that site.
[If you search the web for Comet ISON information, beware of the nutty websites predicting a collision with Earth or some other reason for the end of the world. The closest the comet will get to Earth is about 40 million miles on Dec. 26th. That's far enough that we can all sleep soundly at night.]
This week, the University of California, Berkeley and NASA's Kepler Telescope project jointly announced that the ongoing discovery of planets around other stars had yielded some exciting statistics: It now appears that one out of five Sun-like stars has an Earth-like planet!
One out of five! This means there are likely to be BILLIONS of earth-sized planets orbiting at comfortable distances from BILLIONS of stars in our Milky Way Galaxy. All of us involved with SETI -- the Search for Extra-Terrestrial Intelligence -- are, of course, thrilled to hear this news. Our hope is that among all those planets, there are some where intelligent creatures with an interest in astronomy have developed and perhaps enjoy their own blogs with astronomy news like we do.
The latest stats about known planets around other stars are also record-breaking. As
of early November 2013, we have found 1039 planets around 787 stars beyond our solar
system! (There are 173 stars so far where we have discovered more than one
planet in the same system.) In addition, the
Kepler mission (which is searching for such planets from space) already has over 3000
candidate planets which are still being checked out! And they still have a whole year's worth of data to go through.
I was interviewed on KQED, the San Francisco Bay Area public radio station, by Michael Krasny, the host of the Forum program, about all this -- and we were joined by the Berkeley graduate student who had done the basic work of making the estimates. If you want to hear the interview, it is available at:
http://www.kqed.org/a/forum/R201311060930
Astronomers from two continents made a startling announcement last week. They had found a world similar in size and composition to the Earth that orbited its star in only eight and a half hours.... Just think about that for a minute. Our planet takes 365 and a quarter days to complete its orbit. The innermost planet in our system, Mercury, takes 88 days to circle the Sun. The new planet, designated Kepler 78b, takes only about a third of one of our days to orbit its star. In other words, a year on Kepler 78b is only 8.5 hours long -- a two-Earth-year-old toddler on this alien world would already be 2,063 years old in local time!
The planet is a bit larger in size than our Earth, but made of dense rock like our inner planets (and not gas and liquid, like our outer planets.) Since it circles so close to its star, it must be torridly hot, so we imagine its surface is molten rock and not solid like our own crust. Some are calling it a lava planet.
How can astronomers know so much about a distant world like Kepler 78b? As its name implies, the planet was discovered around a faint star in the constellation of Orion by the Kepler telescope in space. Kepler's camera measures the size of a planet when the planet is seen going across the face of its star and diminishing the star's light briefly. But that can only tell us how big the planet is across, and not what kind of material it's made of.
But once Kepler found the planet, astronomers in the U.S. and Europe used giant telescopes on the ground to find the tiny wiggle the pull of the planet causes in the motion of its star. This "wiggle method" tells us how much pull (gravity or mass) the planet has.
When astronomer combine the size of the planet from Kepler and the mass of the planet from the wiggle method, they can calculate the planet's "density" (mass per unit volume). In this case, all the measurements made it clear this was a dense world, made of rock, just like our Earth.
The mystery is: how did an Earth get SO outrageously close to its star. If it was falling in, what made it stop? We know it couldn't have been born so close to the star, because the star was larger when it was young, and the planet would have been inside the star, where no planet can exist. Kepler 78b is part of a group of strange planets Kepler has been discovering -- all of them too close to their stars for their own good and for our peace of mind.
