# Musings on a Youthful Planet

People seem to be getting younger all the time. I’m told this is a common perception as you get older. In any case, it wasn’t so long ago that I met the son of an acquaintance at an informal gathering. He looked to me to be about fourteen years old, but something warned me not to assume this. I said “What do you do? Are you in school?” His reply: “No, I’ve got my own dental practice downtown.” I don’t know how old you have to be to become a dentist, but I do know it’s a lot older than fourteen!

Exoplanets and the stars they circle, on the other hand, seem to be mostly of a certain age, the denizens of relatively mature systems. Which is why TW Hydrae is so interesting. It’s an infant in stellar terms, at eight to ten million years old only a fraction of the Sun’s age. Like other stars in its age group, it is surrounded by a circumstellar disk of gas and dust, the sort of place where planets can form. And indeed, what seems to be the youngest planet yet detected has now been located within a gap in that disk. TW Hydrae b is about ten times as massive as Jupiter, orbiting in 3.56 days at a distance of some six million kilometers, or 0.04 AU.

Image: The newly discovered giant planet orbits around its young and active host star inside the inner hole of a dusty circumstellar disk (artist view). Credit: Max Planck Institute for Astronomy.

This work comes out of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, where a team led by Johny Setiawan used European Southern Observatory equipment at La Silla (Chile) to make the find. TW Hydrae b turns out to be quite a catch. Starspots analogous to the sunspots on our own star can distort radial velocity readings, a particular problem with young stars whose surface is still relatively unstable. But MPIA’s Ralf Launhardt seems sure of the result, saying:

“To exclude any misinterpretation of our data, we have investigated all activity indicators of TW Hydrae in detail. But their characteristics are significantly different from those of the main radial velocity variation. They are less regular and have shorter periods.”

Bear in mind that none of the known extrasolar planets have, until now, been found around stars young enough to still have their circumstellar disks. Here again we’re looking at a limitation in our methods, younger stars having been excluded from many searches because of the difficulty of measurement caused by the above mentioned solar activity. Now we’re seeing some constraints on planetary formation, learning that a planet can form within a ten million year timeframe and, presumably, migrate inward as it interacts with the circumstellar disk to its present position.

Discoveries like this one add to our knowledge of planetary formation. Is this how all ‘hot Jupiters’ form? Things we need to learn more about include the average lifetime of a circumstellar disk, now thought to be somewhere between ten and thirty million years. And the core accretion model we’re talking about here is still challenged by the gravitational instability alternative, which theoretically allows much faster formation of such giant worlds. The new planet becomes a helpful test case in which to simulate both scenarios as we look for still younger planets.

The paper is Setiawan, Henning et al., “A young massive planet in a star–disk system,” Nature 451 (3 January 2008), pp. 38-41 (abstract). New Scientist also offers an article on TW Hydrae b.

Comments on this entry are closed.

• andy January 3, 2008, 17:16

This system offers some very interesting insights, not least giving information about how far the planet has migrated (as I understand it from secondhand information about the paper’s contents, the inner disc out to 0.5-4 AU from the star contains less material than the outer part, which could indicate the region through which the planet has travelled), and also how much material is left in the solar system to form planets from. This could have a significant bearing on whether hot Jupiter systems can form habitable planets or not.

In addition, TW Hydrae b seems to be rather more massive than the typical hot Jupiter. Such “hot superjupiters” seem to be quite rare. I wonder if the TW Hydrae environment will shed any light on why this may be.

• Dylan January 3, 2008, 18:11

It sounds like this might also be how binary stellar systems form – at ten time the mass of jupiter it sounds like this could be a candidate for stellar evolution of it’s own, however it’s proximity of the main star sounds extremely close for a binary star system.

Can anybody comment regarding how this model relates to the formation of secondary stars in a system?

• Administrator January 3, 2008, 19:39

I’ll let someone else comment on the formation of binaries in this scenario, but I want to ask both andy and Dylan, and anyone else who may know, whether there is a simple breakdown somewhere showing estimated stellar ages for the exoplanet host stars we now know about? I realize that calculating stellar ages is itself a subject in flux, but it would be a useful link nonetheless if anyone has it.

And if I’m remembering the recent comment here on Struve’s work correctly, wasn’t the formation of close binaries the impetus for his original ‘hot Jupiter’ theory?

• andy January 4, 2008, 7:41

A good starting point for exoplanet host star ages is Saffe, Gómez and Chavero (2005) “On the ages of exoplanet host stars” – it isn’t an exhaustive list for exoplanet host stars (even of the ones known at the time), but it has a fair few in there.

• ljk January 7, 2008, 12:21

Tidal Evolution of Close-in Extra-Solar Planets

Authors: Brian Jackson, Richard Greenberg, Rory Barnes

(Submitted on 4 Jan 2008)

Abstract: The distribution of eccentricities e of extra-solar planets with semi-major axes a greater than 0.2 AU is very uniform, and values for e are relatively large, averaging 0.3 and broadly distributed up to near 1. For a less than 0.2 AU, eccentricities are much smaller (most e less than 0.2), a characteristic widely attributed to damping by tides after the planets formed and the protoplanetary gas disk dissipated. Most previous estimates of the tidal damping considered the tides raised on the planets, but ignored the tides raised on the stars. Most also assumed specific values for the planets’ poorly constrained tidal dissipation parameter Qp.

Perhaps most important, in many studies, the strongly coupled evolution between e and a was ignored. We have now integrated the coupled tidal evolution equations for e and a over the estimated age of each planet, and confirmed that the distribution of initial e values of close-in planets matches that of the general population for reasonable Q values, with the best fits for stellar and planetary Q being ~10^5.5 and ~10^6.5, respectively. The accompanying evolution of a values shows most close-in planets had significantly larger a at the start of tidal migration. The earlier gas disk migration did not bring all planets to their current orbits. The current small values of a were only reached gradually due to tides over the lifetimes of the planets. These results may have important implications for planet formation models, atmospheric models of “hot Jupiters”, and the success of transit surveys.

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.0716v1 [astro-ph]

Submission history

From: Brian Jackson [view email]

[v1] Fri, 4 Jan 2008 19:05:18 GMT (846kb)

http://arxiv.org/abs/0801.0716

• ljk January 15, 2008, 9:14

Two Unusual Older Stars Giving Birth To Second
Wave Of Planets

ScienceDaily (Jan. 15, 2008) — Hundreds of millions —
or even billions — of years after planets would have
initially formed around two unusual stars, a second
wave of planetesimal and planet formation appears
to be taking place, UCLA astronomers and colleagues
believe.

“This is a new class of stars, ones that display conditions
now ripe for formation of a second generation of planets,
long, long after the stars themselves formed,” said UCLA
astronomy graduate student Carl Melis, who reported the
findings today at the American Astronomical Society
meeting in Austin, Texas.

http://www.sciencedaily.com/releases/2008/01/080109173738.htm

• ljk January 17, 2008, 1:40

Grain Sedimentation in a Giant Gaseous Protoplanet

Authors: Ravit Helled, Morris Podolak, Attay Kovetz

(Submitted on 16 Jan 2008)

Abstract: We present a calculation of the sedimentation of grains in a giant gaseous protoplanet such as that resulting from a disk instability of the type envisioned by Boss (1998). Boss (1998) has suggested that such protoplanets would form cores through the settling of small grains. We have tested this suggestion by following the sedimentation of small silicate grains as the protoplanet contracts and evolves. We find that during the course of the initial contraction of the protoplanet, which lasts some $4\times 10^5$ years, even very small (greater than 1 micron) silicate grains can sediment to create a core both for convective and non-convective envelopes, although the sedimentation time is substantially longer if the envelope is convective, and grains are allowed to be carried back up into the envelope by convection. Grains composed of organic material will mostly be evaporated before they get to the core region, while water ice grains will be completely evaporated.

These results suggest that if giant planets are formed via the gravitational instability mechanism, a small heavy element core can be formed due to sedimentation of grains, but it will be composed almost entirely of refractory material. Including planetesimal capture, we find core masses between 1 and 10 M$_{\oplus}$, and a total high-Z enhancement of ~40 M$_{\oplus}$. The refractories in the envelope will be mostly water vapor and organic residuals.

Comments: accepted for publication in Icarus

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.2435v1 [astro-ph]

Submission history

From: Ravit Helled [view email]

[v1] Wed, 16 Jan 2008 06:10:49 GMT (22kb)

http://arxiv.org/abs/0801.2435

• ljk January 23, 2008, 10:03

Angular Momentum Accretion onto a Gas Giant Planet

Authors: Masahiro N. Machida, Eiichiro Kokubo, Shu-ichiro Inutsuka, Tomoaki Matsumoto

(Submitted on 22 Jan 2008)

Abstract: We investigate the accretion of angular momentum onto a protoplanet system using three-dimensional hydrodynamical simulations. We consider a local region around a protoplanet in a protoplanetary disk with sufficient spatial resolution. We describe the structure of the gas flow onto and around the protoplanet in detail. We find that the gas flows onto the protoplanet system in the vertical direction crossing the shock front near the Hill radius of the protoplanet, which is qualitatively different from the picture established by two-dimensional simulations. The specific angular momentum of the gas accreted by the protoplanet system increases with the protoplanet mass. At Jovian orbit, when the protoplanet mass M_p is M_p less than 1 M_J, where M_J is Jovian mass, the specific angular momentum increases as j \propto M_p. On the other hand, it increases as j \propto M_p^2/3 when the protoplanet mass is M_p greater than 1 M_J. The stronger dependence of the specific angular momentum on the protoplanet mass for M_p less than 1 M_J is due to thermal pressure of the gas. The estimated total angular momentum of a system of a gas giant planet and a circumplanetary disk is two-orders of magnitude larger than those of the present gas giant planets in the solar system. A large fraction of the total angular momentum contributes to the formation of the circumplanetary disk. We also discuss the satellite formation from the circumplanetary disk.

Comments: 39 pages,13 figures, Submitted to ApJ, For high resolution figures see this http URL

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.3305v1 [astro-ph]

Submission history

From: Masahiro Machida N [view email]

[v1] Tue, 22 Jan 2008 04:24:58 GMT (1354kb)

http://arxiv.org/abs/0801.3305

• ljk January 25, 2008, 10:45

Disks, young stars, and radio waves: the quest for forming planetary systems

Authors: Claire J. Chandler, Debra S. Shepherd (NRAO)

(Submitted on 24 Jan 2008)

Abstract: Kant and Laplace suggested the Solar System formed from a rotating gaseous disk in the 18th century, but convincing evidence that young stars are indeed surrounded by such disks was not presented for another 200 years.

As we move into the 21st century the emphasis is now on disk formation, the role of disks in star formation, and on how planets form in those disks. Radio wavelengths play a key role in these studies, currently providing some of the highest spatial resolution images of disks, along with evidence of the growth of dust grains into planetesimals.

The future capabilities of EVLA and ALMA provide extremely exciting prospects for resolving disk structure and kinematics, studying disk chemistry, directly detecting proto-planets, and imaging disks in formation.

Comments: 10 pages, 6 figures, to appear in the proceedings of the NRAO 50th Anniversary Science Symposium “Frontiers of Astrophysics”, ASP Conf. Series

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.3684v1 [astro-ph]

Submission history

From: Claire J. Chandler [view email]

[v1] Thu, 24 Jan 2008 00:01:11 GMT (346kb)

http://arxiv.org/abs/0801.3684

• ljk April 2, 2008, 11:12

Astronomers see ‘youngest planet’

By Paul Rincon

Science reporter, BBC News, Belfast

An embryonic planet detected outside our Solar System
could be less than 2,000 years old, astronomers say.

The ball of dust and gas, which is in the process of turning
into a Jupiter-like giant, was detected around the star HL Tau,
by a UK team.

Research leader Dr Jane Greaves said the planet’s growth
may have been kickstarted when another young star passed
the system 1,600 years ago.

Details were presented at the UK National Astronomy Meeting
in Belfast.

The scientists studied a disc of gas and rocky particles around
HL Tau, which is 520 light-years away in the constellation of
Taurus and thought to be less than 100,000 years old.

The disc is unusually massive and bright, making it an excellent
place to search for signs of planets in the process of formation.

The researchers say their picture is one of a proto-planet still
embedded in its birth material.

Dr Greaves, from the University of St Andrews, Scotland, said
the discovery of a forming planet around such a young star was
a major surprise.

“It wasn’t really what we were looking for. And we were amazed
when we found it,” she told BBC News.

“The next youngest planet confirmed is 10 million years old.”

If the proto-planet is assumed to be the same age as the star it
orbits, this would be some one hundred times younger than the
previous record holder.