Rocky Planet Discovered

Fraser Cain writes "Astronomers have discovered a rocky, terrestrial planet orbiting a nearby star, Gliese 876. The planet has approximately 7.5 times the mass of the Earth, double its radius, and orbits its parent star once every two days. This is the most Earthlike extrasolar planet discovered so far." Reader Karthik Narayanaswami points out that "the planet was discovered by the famed Berkeley astronomer Geoff Marcy," and adds a link to the news release from Berkeley.

Comments

Let's do the time warp again!

[Anonymous Coward]

Oh wait... you said Rocky Planet.

Berkeley Press Release

[metlin]

Here is the link to the Berkeley press release and information on Berkeley astronomer Geoff Marcy.

And oh, looks like Slashdot is continuing to mirror Boing Boing.

We could never colonise this planet....

[aXis100]

With a new year every two days, everyone would be broke buying birthday cakes.

Re:We could never colonise this planet....

[EnsilZah]

On the bright side, you can use one bit to record people's birthdays.

Once every two days?

[istartedi]

The thing has gotta be mighty close to the star. Mercury orbits in 60 days, right? This thing may not be a gas giant, but it must totally bake on the sunny side, and aren't there going to be some horrendous tidal forces with an orbit that close? It probably has no shortage of volcanism. Hey! It's Vulcan, maybe... if it can hold an atmosphere without having the stellar wind blow it all away. Whatever, it can't be Earth-like.

Re:Once every two days?

[chaotixx]

Its Earth-like because it is a rocky, rather than gaseous planet. Astronomers have to find new planets by detecting the wobble in the path of the star being orbited, which is caused by the orbiting planet. The bigger the planet, the bigger the wobble, so big gas giants were the first planets found. The fact that it completes an orbit in two days also helps as you don't need to collect years of data in order to see the wobble. So it's really not very Earth-like, but its the closest thing found so far, outside of our solar system.

Hey SETI

[grub]

which is only 15 light years away

So why not send some radio traffic which would obviously not be of natural origins. Surely 30ish years isn't that long to wait for a reply? (assuming the place has lifeforms which developed radio...)

Re:Hey SETI

[nacturation]

They could send them CNN or Fox News. Surely those signals are as unnatural as any.

Re:minimum mass

[iostream_dot_h]

The planet has approximately 7.5 times the mass of the Earth, double its radius, and orbits its parent star once every two days.

g_newplanet = G(7.5M_Earth)/4(r_Earth^2), where g is the gravitational field strength and G is the gravitational constant. This is less than 2g_earth.

Seems close enough for life. Although I'm not sure why a stronger gravitational field would necessarily be a huge constraint on the development of life.

And to preempt the trolls, my little convenient formula is good enough. Relativistic effects can be ignored.

Re:minimum mass

[nacturation]

Is life possible at all under such gravity? Any examples?

And where do you propose we find such examples? In space perhaps? :) But seriously, check out this article.

YOU can live under such gravity!

[marat]

Gravity is only 1.8 from normal - I believe you can get used to it. Meanwhile surface is 3.2 times larger, so if it could be terraformed it will hold a lot of people from our overcrowded Earth.

Of course I put many questions aside like how would they get there, does it have any continents, how sensitive processes like childbirth are to the gravity, does its atmosphere shield properly from radiation, isn't it too cold/hot there (although this can be fixed) etc etc...

Doesn't sound very earthlike

[Pinefresh]

I am not an astronomer, but isn't mars more earthlike than that?

Re:Doesn't sound very earthlike

[Pinefresh]

No you idiot, it said extra solar there, replied to myself.

Re:Heh

[forceflow2]

The wobble method is nothing compared to the weeble method. Of course, this has severe consequences when the planets fall down.

Rocky Planet Discovered

[VitrosChemistryAnaly]

Adrian!!!

What does it mean to discover a planet?

[Edmund Blackadder]

After reading the article it seems like they "discovered" the planet simply by observing the star and two very large jupiter type gas giants that are circling the star. By the orbits of the planets and the "wobble" of the star they have determined that there must be another planet of the specified size and orbit.

So essentially this planet was discovered solely on observation of its gravitational effect on other planets. In other words the scientists built a computer model which includes the star and two visible gas giants, and found a planet which they could insert in it so it causes the star and the gas giants to behave as they in the model as they do in observation. Then they declared that they have discovered a new planet.

How did they know it was a rocky planet? Well, correct me if i am wrong but it seems like they decided that by elimination -- the planet is too small to be a gas giant and too close to the star to have anu liquid water on it. Therefore, it must be a rocky planet.

Admittedly I do not know much about modern astronomy but all of this is a little troubling. I mean should we not obtain direct observation from something before we proclaim it "discovered"?

I am sure modeling solar objects is very useful but modeling is limited to our current knowledge. If rely too much on modeling we will never discover anything that we do not already know about.

Re:What does it mean to discover a planet?

[aprosumer.slashdot]

The majority of scientists consider the 'wobble' to be an effect of the planet(s) revolving around the star. If you don't accept that planets are the most probable cause, then what your best guess as to what is the cause of the 'wobble'? If you're answer is "I don't know, but they don't know either.", then I think you simply don't know enough to be critical of these PhDs who have made this discovery. (But then again, this is Slashdot, right?).

I think you over-simplify your estimation of how they decided it was rocky. They use, as a model, what they've found in our solar system, as well as basic physics.

As far as I know, this star is simply too faint to be directly observed for planets and detecting the wobble is the best method of detecting the likelyhood of a star system containing planets. If you have a better method of detection other than declaring "your method is not good enough!", please let the scientific community know. They could then use your help and your insightful powers of posting on Slashdot.

I watched the live webcast of the announcement of their discovery as they answered questions from the rest of the astronomical community (via phone/e-mail). Believe me, these people who announced this discovery aren't stupid, and they are using the best scientific theory they have at hand (with 3 years of data supporting their theory, yeah they waited that long before announcing this).

Re:Orbital Velocity - significant acceleration?

[StupendousMan]

No.

A body moving in a circle of radius R at a uniform speed V experiences an acceleration a = (V*V)/R towards the center of the circle. In neither of the cases you mention does any centripetal acceleration come close to the local gravitational acceleration at the surface of the planet.

Case 1: The Earth: orbital speed V = 30 km/s, and R = 150 million km, so (V*V)/R is of order (10^8)/(10^11) m/s^2, or about 10^(-3) m/s^2. The local gravitational acceleration is about 10 m/s^2, of course. If you speak of the Earth's rotational motion at the equator, then very roughly V = 500 m/s and R = 6,400,000 m, so (V*V)/R has magnitude roughly (2.5 x 10^5) / 6.4 x 10^6 = 0.03 m/s^2; again, much less than 10 m/s^2 due to the gravitational pull of the Earth.

Case 2: The new planet. Its orbital radius is about 2 billion meters, so the circumference is about 7 billion meters; if it travels that distance in a period of 2 days = 170,000 seconds, then it speed is about V = 40,000 m/s. The orbital centripetal acceleration is therefore of order (16 x 10^8)/(2 x 10^9) = 0.8 m/s^2. That's much larger than the Earth's orbital centripetal acceleration, but still far less than the likely gravitational acceleration at the surface (or cloudtops) of this planet.

Whatever

[iamdrscience]

Tell me when they find the Bullwinkle planet. Bullwinkle was always funnier.

Adrian!! Adrian!!

[richardoz]

I can hear it screaming now...

Re:Interesting, but method is flawwed

[helioquake]

Instead of moderating you down, I'm just going to comment here...

Yes, our Sun wobbles like that. Its wobble is done mostly by Jupiter, but the Earth contributes as much as +/- 3 meters/s, if I recall correctly. And for these guys, it's not impossible to detect such perturbations.

That said, the wobble method (Dopper detection) is good for all sizes of planets. If it is not a single planetary system, that will show up in the radial velocity curve (like it does here..non sinosoidal curve, I mean).

What you should be asking is this: how the hell do they know about the radius of the planet? The mass isn't too hard to determine (Kepler's law would tell you); but the radius isn't. Not in the accuracy claimed here (2 earth radius). Since I don't have an access to the article yet, it's hard for me to judge the accuracy of the radius value. Nontheless, that's where all of you should be pondering about, not about the wobble method.

Amazing that FARK readers are pointing this out better than Slashdot readers...what did we go wrong?

Parent is wrong

[p3d0]

This is pretty simple. Surface gravity for spherically-symmetrical masses scales linearly with mass and inverse-square with radius. The mass makes gravity 7.5 times higher, while the radius would make it 4 times lower, for a total surface gravity of about 1.9G.