direct imaging of extrasolar planets

Coronographic imaging with the Advanced Camera for Surveys, on the Hubble Space Telescope, has revealed a jupiter mass planetary companion, with confirmed common proper motion with its parent star, Fomalhaut.

This would be the first robust bona fide direct imaging detection of an extrasolar planet.
Good catch from the mouth of the fish of the south.


From Kalas et al (Science, Nov 14th 2008).

Hubble press release now up

Perspective article in Science (pdf)

click for large image

In a paper to appear on friday Nov 14th in Science, Kalas et al (pdf) show data from the Advanced Camera for Surveys on the Hubble Space Telescope, providing coronographic imaging of a planet orbiting the bright, young, nearby star Fomalhaut.

Exoplanet Encyclopedia entry

The planet has a mass less than 3 times that of Jupiter, and probably more than 0.4 times that of Jupiter. It is in an orbit that probably has an orbital radius (semi-major axis) of about 120 AU, and a moderate eccentricity of about 0.1. Orbital period is then just under a 1000 years.
The orbit estimate is from theory, and consistent with the observed separation.

Chiang et al have a companion paper on the theory and models (pdf).
Eugene also has provided a very nice with more explanation, images and a fun (but large) animation illustrating the model for the system.

Fomalhaut, the brightest star in the constellation Piscis (southern fish), is a young star, about 2-300 million years old, which was discovered in 2004 to have a wide annular dust disk around it.
It is an A star, with a mass of about twice that of the Sun, and about ten times more luminous, at a distance of a little over 20 light years away.

Followup observations taken more recently (also here) confirmed that a faint apparent companion to Fomalhaut is in fact physically associated with the star.
The companion as common proper motion with Fomalhaut, and the (much smaller) partial arc of orbital motion about the star was also detected.
Quillen, in 2006, predicted that the morphology of the Fomalhaut disk annulus was caused by a jovian planet orbiting near the inner edge of the disk in a moderately eccentric orbit. This data confirms that prediction.

There are some interesting aspects about this discovery.

There have been several reports of low mass companions recently, like HR 8799 here (also here), and 1RXS J160929.1-210524 here - but these are objects with masses close to or above 10 Jupiter masses, which is the range where you worry that we are really seeing brown dwarfs.
Marois et al paper on HR 8799

HR 8799 image from Marois et al Science 14 Nov 2008

The Fomalhaut object has a mass well below the brown dwarf limit, if the data is interpreted correctly. The mass is no more than about 3 Jupiter masses, and may well be less than a Jupiter mass. So it is unambiguously a bona fide planet. Finally.

However, while the brightness and colours of this object, Fomalhaut b, are consistent with such a planet, they are inconsistent in detail.
There are several possible explanations for this - the simplest being that our models for young giant planets are inadequate; a second explanation is that there is emission from the planets atmosphere, or, more interestingly from a protolunar disk around the planet.
Jovian planets in the solar system have lots of moons, and one scenario for the formation of the more massive "regular" moons, is that the young planet had a dust debris disk around it, like a miniature version of the protoplanetary disk from which the planet formed around the star.
It would be very, very interesting to catch such a disk in formation.
However, theory suggests such disks evolve rapidly, and should be gone in much less than a hundred million years, which is inconsistent with the age of the system.
A more speculative possibility is that there is a warm disk around the planet, but that it reformed later due to dynamical instabilities and collisions in the system of moons around the planet.
That would also be pretty cool.

Spectra next.

doesn't look like much, eh?

the artist's conception

Hubble video (67 Mb mp4)

so, was this worth foreshadowing?

Kalas press release web site from 2005

STSCI press release 2005


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Don't forget that Fomalhaut b is only one of the planets announced today that have been found directly. HR 8799 has a system of three planets with masses ranging from 7 to 10 Jupiters. Interestingly, its planetary disk is most massive within 300 light years. So, lots of materials available to make hefty planets.

PS. Fomalhaut is the brightest star in Piscis Austrinus, not Pisces.

By Dunkleosteus (not verified) on 13 Nov 2008 #permalink

Already corrected pisces to piscis, typing reflex- not used to those funny southern constellations...

The timing of the HR8799 release was amusing.
I'm calling those brown dwarfs meself. M/sin(i) suggests probably masses over 10 jupiters. Maybe even for the lowest mass one.

Are there any possibilities that this is a big dust cloud? I worry about the blue colors of the object.

Hm, well Alice's prediction of the object go a long way to assuage such worrier, but then I am a theorist and predisposed to such.

Orbital speed out there is few km/sec, so an unbound dust clump would shear at ~ 100 m/sec. That is 10^10 m over the observing period, or ~0.1 AU - that is borderline detectable at a distance of 7 pc.
So, we'd have to catch the dust at a very special time for it not to have sheared back out again.

If it is self-gravitating dust with radius less than 0.1 AU, then is it arguable even more interesting than a planet! But even less likely.

I'm dangerously close to the "what is a planet?" nonsense, but the structure of the system suggests that the objects were formed from the protoplanetary disk like planets but unlike proper brown dwarfs. The other previously announced systems are much more (sub-)brown dwarfish.

By Duncleosteus (not verified) on 13 Nov 2008 #permalink

Yeah, could be. They're quite far out though, might have formed from separate clumps that were bound to the primary.
We just don't understand all this stuff well enough.

Well, they did a good job not hinting the result, given the need for two well spaced epochs of observation. It's a drag that all of the NIR observations are upper limits. This is going to make it hard to get spectra, though clearly they could justify getting All the Observing Time in the World for it.

Cornography easily beats least on
some days :)

I'm calling those brown dwarfs meself. M/sin(i) suggests probably masses over 10 jupiters. Maybe even for the lowest mass one.

Given the planets of HR 8799 were detected by imaging, I wouldn't have thought that there would be a sin(i) degeneracy in the masses... plus the architecture of the system looks more like a planetary system than a stellar one.

HR 8799 adds more evidence that planetary systems of A-type stars don't have the metallicity dependence that solar-type stars do: [Fe/H]=-0.47, only a handful of extrasolar systems have lower metallicity host stars.

Sure does make one pine for the ACS coronograph.

Just remember to feel for anyone who managed to post a non-planet paper today on astro-ph.

Once you have three similar-mass objects in a nonhierarchical arrangement around something much more massive, then I think you have to start considering the P word (and to hell with the deuterium burning limit anyway). There is various emerging evidence that planets tend to/should have higher masses around higher-mass stars, and I could see an argument that for intermediate-mass stars, you could get gravitational instability in the outer parts of their initial protostellar disks (in the areas where the envelope is accreting to the disk).

In fact, there's a model floating around (Stamatellos/Whitworth) that is trying to argue that most brown dwarfs form in exactly this fashion. I don't really buy it as a significant BD formation channel for several reasons, but one of its key predictions is that you should see a swarm of brown dwarfish objects in the outer system because a massive disk will instantly fragment into a set of ~10-15 BDs that then dynamically evolves. I guess the question you then ask is whether core accretion, gravitational stability in a CTTS disk, and gravitational instability in a Class 0/I disk are really all methods of forming "planets", or if you really want to call one of them something else.

Sure does make one pine for the ACS coronograph.

Yeah, but these folks just bought us all a completion bond (or insurance policy) on Servicing Mission 4 being carried out. Nice work.

Just think of all the extra readers who will be attracted to astro-ph today.

Sorry about the sin(i) comment - brain fart on the mass estimates, I was thinking of another multi-planet system.
Mass ambiguity for HR8799 basically boils down to understanding hot vs cold collapse in young giant planets.

I agree that fragmentation of outer disks is intriguing, and certainly low end of brown dwarf mass function overlaps the high end of the planet mass function.
We're going to have to tackle the definition problem some day soon - but astronomy is phenomenology driven, we can not label based on unobserved mechanisms.

It is posts like these one lives for... and I can see worse places to be in astro-ph than immediately after Chiang et al paper...

JvP - PLEASE do not put entire NYT articles in the comments!
It is blocks people reading downstream and it can get me in trouble.

Finally someone has suggested in e-mail we need to worry about really unlucky high proper motion background star.
Unlikely a priori, a third epoch will settle it.

I actually just caught the metallicity of HR 8799, and -0.5 dex is really low for a star that presumably formed pretty recently. Almost all recent star formation in the solar neighborhood has had a metallicity right around zero, so why is this star metal poor? Its kinematics look thin-disk to me, with a radial velocity of 11 km/s and proper motion of ~130 mas/yr (~25 km/s), so it's not like this is some weird refugee from the outer disk or something.

Gehren, in 1977 (A&A 59 303) notes its peculiarity and suggests it is a type I blue straggler - former binary that merged through a McCrea mass transfer

That would have interesting implications for the planets - if they formed recently in an excretion disk rather than through a protoplanetary disk

I don't see much more recent stuff on this, but I only glanced at ADS so far

Finally someone has suggested in e-mail we need to worry about really unlucky high proper motion background star.
Unlikely a priori, a third epoch will settle it.

Presumably that is true only for Fomalhaut, not for HR8799. That latter would have to have three high proper motion background stars.

I agree with E, that Fe/H seems kinda weird (but what do I know?) I asked the resident star person and that person agrees it is unusual for such a young system.

Why in the Hubblefolk's own press release is Piscis Austrinis referred to as Piscis Australis? Is that a proper variant? I've never seen it spelled that way.

I suppose when you've just made a bonzer discovery like that, you can take all the poetic license you please.

Regarding the low metallicity of HR 8799, it is classified as a Lambda Boötis star, which if I understand the concept correctly, have depleted surface layers but more normal abundances in their interiors.

it is a lamba Boo? I didn't see it on the very short list of known lamba Boo's but then I am probably a decade out of date.

lambda Boos have normal C/N/O/S but low mean metallicity, they could be consistent with blue stragglers, though their origin is really not understood.
Anyone have a cite on good modern metallicity measurement of HR8799?

It is a gamma Doradus variable
Those are generally thought to be blue stragglers, maybe getting ready to turn onto the subgiant branch

That can seriously change the interpretation of the planets.

Seriously change, indeed. What's the current explanation for blue stragglers? Last I knew, the answer was either a merger of two low-mass stars or post-main-sequence mass transfer from a higher-mass star to a lower-mass star.

If the former, then it probably doesn't change the interpretation too much. For the purposes of large outer protostellar disks, it doesn't matter if the central star fragmented into a very close binary.

If the latter, then the sky's the limit for crackpot formation models. RV monitoring of A stars is always difficult, but anything close enough to transfer mass (or rather, the compact remnant of such a star) would induce a pretty hefty reflex motion. Is there enough RV data available to rule out such a companion? If so, can you find some way to supernova-kick a neutron star out of the system without the kick unbinding the remaining star from the planets? main sequence mass loss could explain why the planets are out at very wide orbits. All the orbital radii would expand adiabatically by the ratio of M_initial/M_final. Stipulating a much higher initial mass would also explain how you get a very massive protostellar disk to form three objects in the 10-30 MJup range.

Crackpot theory #1!

Regarding the Lambda Boötis classification of HR 8799, see e.g. Gray & Kaye 1999 (AJ 118 2993). According to the list in Kaye 2007 (Communications in Asteroseismology 150 91) it is the only known Gam Dor+Lam Boo star, so quite an odd one.

Are Gamma Doradus stars actually thought to be blue stragglers? I haven't come across this idea mentioned in the papers I've seen about the Gamma Doradus phenomenon.