Smaller stars pack big X-ray punch for would-be planets

June 14, 2016, Chandra X-ray Center
Credit: Chandra X-ray Center

Young stars much less massive than the Sun can unleash a torrent of X-ray radiation that can significantly shorten the lifetime of planet-forming disks surrounding these stars. This result comes from a new study of a group of nearby stars using data from NASA's Chandra X-ray Observatory and other telescopes.

Researchers found evidence that intense X-ray radiation produced by some of the young stars in the TW Hya association (TWA), which on average is about 160 light years from Earth, has destroyed disks of dust and gas surrounding them. These disks are where planets form. The stars are only about 8 million years old, compared to the 4.5-billion-year age of the Sun. Astronomers want to learn more about systems this young because they are at a crucial age for the birth and early development of planets.

Another key difference between the Sun and the stars in the study involves their mass. The TWA stars in the new study weigh between about one tenth to one half the mass of the Sun and also emit less light. Until now, it was unclear whether X-ray radiation from such small, faint stars could affect their planet-forming disks of material. These latest findings suggest that a faint star's X-ray output may play a crucial role in determining the survival time of its disk. These results mean that astronomers may have to revisit current ideas on the formation process and early lives of planets around these faint stars.

Using X-ray data from NASA's Chandra X-ray Observatory, the European Space Agency's XMM-Newton observatory and ROSAT (the ROentgenSATellite), the team looked at the intensity of X-rays produced by a group of stars in the TWA, along with how common their star-forming disks are. They split the stars into two groups to make this comparison. The first group of stars had masses ranging from about one third to one half that of the Sun. The second group contained stars with masses only about one tenth that of the Sun, which included relatively massive brown dwarfs, objects that do not have sufficient mass to generate self-sustaining nuclear reactions in their cores.

Credit: Chandra X-ray Center

The researchers found that, relative to their total energy output, the more massive stars in the first group produce more X-rays than the less massive ones in the second. To find out how common planet-forming disks in the groups were, the team used data from NASA's Wide-Field Infrared Survey Explorer (WISE) and, in some cases, ground-based spectroscopy previously obtained by other teams. They found that all of the stars in the more massive group had already lost their planet-forming disks, but only about half of the stars in the less massive group had lost their disks. This suggests that X-rays from the more are speeding up the disappearance of their disks, by heating disk material and causing it to "evaporate" into deep space.

A typical star and planet-forming disk from each of these two groups of stars are shown in the illustrations. The illustration above depicts one of the relatively , which has a large number of flares and spots. This is a sign of its enhanced X-ray production, which is thinning and destroying the remnants of its planet-forming disk.

X-ray Image of TW Hya association binary star. Credit: Chandra X-ray Center

Another illustration (below) shows one of the lower mass, fainter stars. Because it is not as active in X-rays, it has retained a thicker disk that represents a more suitable environment to form planets. The planet formation process would cause gaps, not shown in this illustration, to appear in the disk. The streams near the center show how matter from the disk is still falling onto the star. These illustrations, which are not to scale - the stars are actually miniscule in size when compared with their surrounding disks - are accompanied by a Chandra image of young binary star system that was included in the new study of the TWA.

In previous studies, astronomers found that 10-million-year-old stars in the Upper Scorpius region, another star-forming group, displayed a similar trend of an increase in the lifetime of disks for lower mass stars. However, the Upper Scorpius work did not incorporate X-ray data that might offer an explanation for this trend, which is one reason why this new study of the 8-million-year-old TWA is important. Another reason is that theoretical models of the evolution of planet-forming disks generally predict that the lifetimes of disks should have very little dependence on the mass of the star. The new results for the "puny" TWA stars point to the need to revisit disk evolution models to account for the range in the X-ray outputs of very low-mass stars.

In searching for planets outside of our Solar System, many astronomers have focused their efforts on observing stars less massive than the Sun, like those described here. Such stars may offer some of the best targets for direct imaging of exoplanets in the so-called habitable zone, the star-to-planet distance range where liquid water could exist and life may eventually flourish. These low mass are also attractive targets because they are relatively faint and planets in their habitable zones should be easier to detect and investigate.

Explore further: T-Tauri Stars

More information: M Stars in the TW Hya Association: Stellar X-rays and Disk Dissipation. lanl.arxiv.org/abs/1603.09307

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Chris_Reeve
Jun 14, 2016
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Chris_Reeve
Jun 14, 2016
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Chris_Reeve
Jun 14, 2016
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Chris_Reeve
Jun 14, 2016
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torbjorn_b_g_larsson
4 / 5 (4) Jun 14, 2016
@Chris Reeve: I wouldn't say disarray, since the old ones work well up to the point that they have now solved the "one meter gap" problem.

But they seem to be incomplete ("what the old theory was missing"), as should be expected when more data becomes available. Re super-Earths, it may be that our Earth is the superEarth we are looking for. Two recent unbiased methods has resulted in that: http://www.planet...ets.html .

Also, our Saturn seems to be a giant neptune rather than a small jupiter [ibid]. Then our system is likely smack in the middle of the normal range of planets of various kinds, with or without Planet Nine.

So, now I can A the Q: No, the article doesn't ignore the scientific context. The Chandra collaboration wouldn't get away with that. The work builds perfectly well on the background of incomplete but well working older models.

torbjorn_b_g_larsson
4 / 5 (4) Jun 14, 2016
[ctd] I don't know how much planetary science you have read. A followup Q may be, how do you know that your A is the way to interpret the quoted text. Isn't that an opinion?

Yes, it may be an opinion, but I hope a well informed one. I am not a planetary scientist, only involved in astrobiology. But the trick is, and I was confused too in the beginning as always in a new subject, to read a few interesting main stream papers, look up their references to common subjects, and if there is no well working context provided take time to read a recent review. In other words, let the scientists present their best case, and *then* see if it works/is useful. reading one paper that describes a problem isn't very illuminating, and doesn't get to the balance of the work in the area.

Also, in my case I cheated: I studied astrobiology at the nearest university to get a quick walk through at the professional level. Planetary science was a topic, obviously.
Steelwolf
not rated yet Jun 14, 2016
I would tend to think that the protoplanetary disc, when it is lit by stronger and stronger x-rays and light energy in general from the new star it orbits, that the dust and larger bodies that had to have been forming in the same manner that the star did, however, the star outgrew the others and so took the majority of the mass and became the center of gravity of the system.

The rest of the smaller masses, the x-rays would blow away much of the excess atmosphere of most rocky cored gaseous planets, allowing the gas a chance to gather on the farther masses from the star and since the light and x-rays would produce their equivalent of a shock-wave I would expect that much of the disc actually becomes the equivalent of asteroid belts and Oort clouds.

One would think that much of the energy would actually help form the proto planets as it pushes the gas and dust that it gathered back away from it, there will be a flow collision mixing and chance for formation then too.
Steelwolf
not rated yet Jun 14, 2016
I would tend to think that the protoplanetary disc, when it is lit by stronger and stronger x-rays and light energy in general from the new star it orbits, that the dust and larger bodies that had to have been forming in the same manner that the star did, however, the star outgrew the others and so took the majority of the mass and became the center of gravity of the system.

The rest of the smaller masses, the x-rays would blow away much of the excess atmosphere of most rocky cored gaseous planets, allowing the gas a chance to gather on the farther masses from the star and since the light and x-rays would produce their equivalent of a shock-wave I would expect that much of the disc actually becomes the equivalent of asteroid belts and Oort clouds.

One would think that much of the energy would actually help form the proto planets as it pushes the gas and dust that it gathered back away from it, there will be a flow collision mixing and chance for formation then too.
Steelwolf
not rated yet Jun 14, 2016
Apologies for bad mouse creating repeat posts
tblakely1357
not rated yet Jun 14, 2016
If I read the article right, it looks like life-bearing planets around red dwarf stars are extremely unlikely.
Chris_Reeve
Jun 14, 2016
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Chris_Reeve
Jun 14, 2016
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Chris_Reeve
Jun 14, 2016
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Chris_Reeve
Jun 14, 2016
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Chris_Reeve
Jun 14, 2016
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Chris_Reeve
Jun 14, 2016
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