Re: How many moons does Quaoar have and what is the temperature?

Date: Wed Apr 2 13:55:42 2003
Posted By: John W. Weiss, Grad Student in Planetary Science
Area of science: Astronomy
ID: 1045021935.As

Message:

Two very good questions! Let me begin with the temperature. For most Kuiper-Belt Objects ("KBOs" for short), we do not actually know the temperatures. We can guess at them by guessing an albedo (Albedo is the fraction of the Sun's light that an object reflects. A lower albedo means more light is absorbed and so you have a warmer planet.), but it turns out that we might have been guessing way too low for a most of them. Quoar is one of the KBOs whose albedo we actually have measured: 0.12. Given that peice of information and knowing how far Quaoar is from the Sun: 44 AU (short of "Astronomical Unit", where 1 AU is the distance between the Earth and Sun, 93 million miles or 150 million kilometers). That said, how do we get the temperature? The easy way is to plug into the following formula:

Temperature = 280 Kelvin * ((1-Albedo)/Distance²)^1/4.
For Quaoar, that comes out to 41 Kelvins. Of course, Kelvins are a great scientific unit for temperature, but no one uses them in ordinary life. (I think it would be fun if the weather men would give forecasts in Kelvin, so that we could say, "Hey, it's going to be 280 tomorrow!" even in the winter. Fortunately, no one listens to me.) Kelvins are the same as degrees in Celsius, except they start at absolute zero (-273 degrees Celsius). So Quaoar is at -232 degrees Celsius. That seems a lot colder, doesn't it? How about Fahrenheit? The conversion there is
degrees Fahrenheit = 1.8*(degrees Celsius) + 32,
so we get -386 Fahrenheit. Brrr! At that temperature, water isn't just ice, the ice basically acts just like a rock.

I should point out a bit of silliness here. I was unable to find the temperature of Quaoar listed anywhere, which is why I computed it. What's silly is that the temperature of Quaoar is what was actually measured to get the albedo (along with size of Quaoar and it's orbital distance). That's just silly. By the way, I found the albedo in several press-releases archived online, such as at space.com. Also, Dave Jewiit, an astronomer in Hawaii has a these listed at on his website. I should also point out that the original estimates of the albedo of Quaoar were more like 0.10, but these estimates seem to have been moved up a bit. If you use the lower value, then Quaoar is about a degree Fahrenheit warmer. Which doesn't make it all that much nicer to visit.

Now for moons. I have not found any reports of any moons around Quaoar, so far. But this doesn't mean that there aren't any! It took nearly 40 years after the discovery of Pluto before someone noticed it had a moon. And Pluto's moon, Charon, is a very large moon, too! The trouble is that it is vary hard to see much this far away from Earth. The smaller the object, the harder it is to see. (Just like you can see your house from several blocks away, but you can't see the ant crawling on the front steps.) If Quaoar has a moon, it will be smaller than Quaoar, so it will be hard to spot. (What we se as "Quaoar" might actually be Quaoar and a moon!) The best answer to this question is "stay tuned". The question is an excellent one and astronomers are wondering the same thing. You can bet that people will try to spot moons are Quaoar in the months and years to come.

I wish I could recommend a good book about Kuiper-Belt Objects for you, but we've only just started finding them. The only books I know of which are up to date on this field are college textbooks. We can barely get college students to read those, so I won't recommend that you read them. (Yet. I'm betting you'll be ready to dig in to them in a few more years.) An excellent website which stays pretty up-to-date on these things is The Nine Planets Website. I hope this has helped you!

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