Great observatories work together to stretch the boundaries of what is possible for each. Data from the Chandra X-ray Observatory were used in tandem with the James Webb Space Telescope, for example, to observe the death of a star as it was consumed by a black hole. JWST’s infrared look at this Tidal Disruption Event (TDE) helped show the structure of stellar debris in the accretion disk of the black hole, while Chandra charted the high-energy processes at play in the cataclysmic event.
Or have a look at the image below, combining X-ray and infrared data from these two instruments along with the European Space Agency’s XMM-Newton, the Spitzer Space Telescope and optical data from Hubble and the European Southern Observatory’s New Technology Telescope to study a range of targets.
Image: Four composite images deliver dazzling views from NASA’s Chandra X-ray Observatory and James Webb Space Telescope of two galaxies, a nebula, and a star cluster. Each image combines Chandra’s X-rays — a form of high-energy light — with infrared data from previously released Webb images, both of which are invisible to the unaided eye. Data from NASA’s Hubble Space Telescope (optical light) and retired Spitzer Space Telescope (infrared), plus the European Space Agency’s XMM-Newton (X-ray) and the European Southern Observatory’s New Technology Telescope (optical) is also used. These cosmic wonders and details are made available by mapping the data to colors that humans can perceive. Credit: X-ray: Chandra: NASA/CXC/SAO, XMM: ESA/XMM-Newton; IR: JWST: NASA/ESA/CSA/STScI, Spitzer: NASA/JPL/CalTech; Optical: Hubble: NASA/ESA/STScI, ESO; Image Processing: L. Frattare, J. Major, and K. Arcand.
Working at multiple wavelengths obviously pays dividends, and with Chandra in the news because of proposed budget cuts, it’s worth noting that its observations have a role to play in the search for habitable worlds in our own stellar neighborhood. Currently the observatory is being used in conjunction with XMM-Newton in an ongoing exploration of habitability in terms of radiation. Which stars close enough to Earth for us to image planets in their habitable zones are also benign in terms of the radiation bath to which these same planets would be exposed?
High levels of ultraviolet and X-ray radiation can break chemical bonds and damage DNA in biological systems, not to mention their effect in stripping away planetary atmospheres. Thus ionizing radiation becomes a key factor in habitability in the kind of planets whose atmospheres we are first going to be able to study, those orbiting nearby M-dwarfs where the habitable zone is dauntingly close to the parent star.
The new study is led by Breanna Binder (California State Polytechnic University), who told the American Astronomical Society’s recent meeting in Madison, Wisconsin that of some 200 promising target stars, only a third have previously been examined by X-ray telescopes. Of these, many appeared quiet at visible wavelengths but quite active in X-ray emissions. Clearly we need to learn more about this critical variable.
With a generation of Extremely Large Telescopes soon to come online and future space observatories like the Habitable Worlds Observatory in the works, we will be able to take apart light from individual planets in search of biosignatures. Binder’s team will try to identify stars where the X-ray background is not dissimilar to Earth’s, so that the chances of life evolving within a protective atmosphere are enhanced. Ten days of Chandra observations along with 26 days of data from XMM-Newton were collected to home in on the X-ray behavior of some 57 nearby stars, ranging from 4.2 to 62 light years away.
This continuing effort is in a sense a shot in the dark, for while some of the stars are known to have planets, many are not. The necessary geometry of planetary transits means that many stars are likely orbited by habitable zone planets we can’t yet detect, and radial velocity methods are likewise not tuned for planets of this size. Edward Schwieterman (University of California Riverside) points to what comes next:
“We don’t know how many planets similar to Earth will be discovered in images with the next generation of telescopes, but we do know that observing time on them will be precious and extremely difficult to obtain. These X-ray data are helping to refine and prioritize the list of targets and may allow the first image of a planet similar to Earth to be obtained more quickly.”
Image: A three-dimensional map of stars near the Sun that are close enough to Earth for planets in their habitable zones to be directly imaged using future telescopes. Planets in their stars’ habitable zones likely have liquid water on their surfaces. A study with Chandra and XMM-Newton of some of these stars (shown in blue haloes) indicates those that would most likely have habitable exoplanets around them based on a second condition — whether they receive lethal radiation from the stars they orbit. Credit: Cal Poly Pomona/B. Binder; Illustration: NASA/CXC/M.Weiss.
More on this continuing study as results become available. Meanwhile, note in relation to the budgetary woes Chandra is experiencing that a letter to NASA administrator Bill Nelson went out on June 6 from several U.S. senators and members of the House of Representatives urging that restrictions on its funding be rescinded. The letter bears all too few signatures, but it’s at least a step in the right direction. Quoting from it:
Premature termination of the Chandra mission would jeopardize this critical workforce, potentially driving talent to other countries. Chandra should serve as a bridge to a promising future in high energy astrophysics at NASA, including the development of its eventual flagship-scale successor, as recommended by the 2020 Decadal Survey in Astronomy and Astrophysics.
We strongly urge NASA to maintain full FY25 funding for the Chandra mission at the $68.7 million level, as outlined in NASA’s FY24 budget request, and to halt plans for significant reductions in FY25 until Congress determines Chandra’s appropriations. The proposed budget cuts would cause damage to U.S. leadership in high energy astrophysics and prematurely end the mission of a national treasure whose most significant discoveries may still be ahead.
Extrapolate to the scale of our galaxy: if X-rays can be harmful to a planet and therefore not allow the appearance of life as we know it, and the density of stars increases this potential of X-rays, Does that mean we have to look for life neither too close nor too far from the center of our galaxy, in other words in a belt where we are approximately on in the extension of Orion’s arm ?
Obviously the distances are no longer the same…
https://phys.org/news/2024-06-hot-nasa-chandra-habitability-exoplanets.html
Out of curiosity I had fun looking at some famous galaxies with X-ray + ultraviolet with color treatment EOSB on GAIA. Two superimposed layers: the background represents the ultraviolet with slightly dull colors; the X-ray is in red and bright orange shifting to green.
We notice that the distribution of X-rays is very uneven: on M31 and M81 (spirals similar to our galaxy) it is essentialand the galactic center that radiates; we can understand it given the density of the stars. Same on M83 but more punctually. On M55; M74 and M101 there is strangely almost no radiation (?) On the other hand M77 – other cluster – radiates considerably in X-ray
I let everyone propose conclusions to this short study, but if we consider that X-rays are fatal to the appearance of life, I would tend to look for it preferably towards the periphery of spiral galaxies. Does the celeste mechanics do things well? ;)
Here is a small contact sheet of the studied galaxies that everyone can find on GAIA.
https://ibb.co/YL0H7xC
The X-rays that Chandra sees are of too low energy to pass thru more than a meter of atmosphere so have no effect on planetary life
While direct X-ray penetration is blocked, that doesn’t mean they could not have indirect effects. By ionizing the atmospheric gases, they might well generate molecules that will be toxic, such as nitrogen oxides, which in turn can form acid rain. The atmosphere could erode as well, reducing the effective depth, and eventually allowing the X-rays to reach the surface. Those acidic rains mean that the oceans will not be protective of marine life either. Unless the X-rays are produced uniquely by the source, they are likely to be accompanied by other wavelengths, such as UV, which we know has similar effects on the atmosphere and organic molecules.
Therefore I would not a priori rule out strong X-ray sources reducing the habitability of planets.
While Chandra’s capability of imaging all of the X-ray spectrum may be limited, that doesn’t mean that the X-ray sources it can view are not also producing em wavelengths outside of its detection range.
While I am a researcher in a foreign country, I fully support the continued use and operation of the fully functional Chandra telescope and instrument for the next year. Adding that serious consideration should be made for maintaining funding also in coming years.
Just this morning, found a link to Monthly Notices concerning Gliese 12b at 12 parsecs distance.
https://academic.oup.com/mnras/article/531/1/1276/7679807?login=false
This is a link to a pdf copy of the paper
Gliese 12 b, a temperate Earth-sized planet at 12 parsecs discovered with
TESS and CHEOPS
I am curious if the planet noted here shows up in the grid examining the x-ray risk
in the discussion above. At about 42 light years distance it would be within the
grid shown.
Also, while there can be a galactic background radiation ( for a given galaxy or region of a galaxy, since these stars are our local neighbors, I would presume that the x-ray risk ( or lack thereof) would be related to the stars’ generation of such energy, characteristic of their spectra. A main sequence red dwarf would have a disproportionate amount of x-rays vs. its peak radiation, and especially as a young star.
Looking a simplified models such as Sun-Earth and present day, Black body temperature for the sun expanded to 1 AU is about 400 K. But shirtsleeve here on the earth is 273 + K with a lot of explanations:
1. we are on a sphere where the dark side radiates heat away.
2. we have an atmosphere that both reflects and retains heat.
3. the surface and oceans have contributions too.
4. it’s been about 4.5 billion years since the Earth got under way.
So, if a planet orbits another G star or a K or an M, there is also the issue of whether it gets a rotisserie effect like Earth and Mars do, or whether the rotation is synchronous with the year. How to handicap such circumstances, I am not really sure, but it is worth investigation.
That’s why I like that 400 K starting line related to Earth. Maybe someday, based on stellar age, spectra and planet distance, shifts of this HZ demarcation could be established. In a case such as Trappist 1, a calculation of the 400 K line might identify what is nominally the planet ( b, c, d, e, f, g…) closest to that Earth index, for example.
@wdk
This went right over my head. Can you explain it in simple terms for me? Thanks.
Hello, A.T.
Before I spotted your inquiry, I submitted the material on the seven Trappist-1 planets below. I hope that they help as an illustration.
What the “400 Kelvin” idea is all about is an attempt to devise a unified approach to studying exoplanets and habitability. The idea is to use the star’s black body temperature as a yardstick (meterstick?) for giving an Earth analog an orbital radius equivalent to that of Earth’s. The “equivalence” would be based on the effective temperature of a point source with radiative flux equivalent to the star. Since the surface temperature and area of Trappist 1 are both considerably smaller than our sun’s, where effective temperature becomes the same as the sun’s at 1 AU with a surface temperature of 5800 K, it is going to significantly smaller radius. And it can be calculated.
Now since even with strict black body distribution of energy, there will be differences in the radiation reaching Earth or an exoplanet, but the notion of
effective temperature would be the same. And when you look at the following
table of Trappist 1 planets, the assumptions about surface conditions appear to adhere to this idea implicitly. Between c and d would be where a more exact Earth analog would apparently be placed.
Now for a case like Earth or Mars, the reductions from 400 K for surface temperatures are dictated to first order by spherical geometry ( irradiated hemisphere) and then by albedo of the atmosphere or surface. Planets with larger atmospheres ( e.g., Jupiter or Venus) have further departures from this model, either due to trapped heat ( green house) or internal heat sources.
What does this have to do with x-ray dosage?
Well, since Trappist 1 d or e do not orbit our sun, but a red dwarf where they are exposed to such discharge, maybe the 400 Kelvin point is a good reference point for further “handicapping” habitability. If evidence of significant atmosphere at Trappist d or e is obtained, then maybe we need not change our red dwarf HZ bounds significantly. If they are stripped, then at least we could say that Trappist 1 with its inherent age, might have run the clock out and the bar has to be moved further out. “Should have been around here a couple billion years ago.” So it’s a step toward sorting incoming data.
Since I brought up the example of Trappist 1 above, it seemed appropriate to pursue further the implications ( and needed some time to check the calculations).
Luminosity or stellar flux can be treated with a control volume approach.
If the “luminosity” is treated as a constant than as it is spherically attenuated with increased distance, the resulting effective temperature associated with the surface reduces with distance as follows.
L = T^4 x R^2
Hence, the 5800 K surface temperature for our sun adjusts to 400 K at 1 AU – and then you have all the qualifications of temperature living on a spherical rotating planet with an atmosphere and less illuminated regions ( the poles and dark side) where this temperature can be reduced. Venus clearly does not want to let go.
So, given that Trappist 1 ( by Wikipedia accounts) has 0.1192 solar radius and effective surface temperature fo 2554 K owing to being a very dim red dwarf (M10), my calculation for equivalent 400 K distance from the primary is
3,401,700,7 kilometers, about 0.02273 AUs.
This is a comparison of Trappist and Sol based on black body temperatures.
Departures from the curves such as spurts of high energy or lack of relatively lower light in the red to blue band – will cause additional differences in one’s stroll during the day.
Now what about planetary conditions ( a through h)?
Here’s are some results. Temperature is calculated as above.
Note that 400 K is equivalent to solar flux at 1 AU in the solar system. (Earth).
Planet distance ( million km) temperature (K)
b 1.720 562.
c 2.36 480
d 3.30 412
e 4.3 355
f 5.7 309
g 7.0 278
h 9.3 242
Were these planets at equivalent thermal distances in the
solar system…
b: 0.5065 AU
c: 0.6944
d: 0.9426
e: 1.2695
f: 1.6757
g: 2.0702
h: 2.7320
In effect, our view of Trappist 1 takes us between solar system’s Mercury and Venus to the lower bounds of the asteroid belt. Since this particular red dwarf
is assumed billions of years older than the solar system, the planets would be
ancient in our own terms. But since they are closer to Trappist-1, their exposure to flares and X-ray radiation due to M dwarf behavior is significantly higher.
Moreover, their rotational velocities are higher which has “impact” considerations too. And these are the planets detectable by transit…
In summary, the 400 K index is based on a particular planet’s position ( ours and now) orbiting a G star of 4.5 billion year vintage. Examining our circumstances and history, life’s appearance likely occurred when this G star was somewhat less
luminous and the significant events in our biosphere could have been stretched over billions of years, unique to this particular case. Our search for living earth analogs will go on anyway, but we need to look for ways to screen candidates due to pattern recognitions of some sort.
@wdk
Are you able to determine what the expanded sun’s surface temp would be when it was less luminous 4.5 bya? Presumably less than 400K.
What I don’t understand is what to make of this difference between this 400K for our sun expanded to 1 AU and our actual average surface temperature. Is the 400K just an easy calculation that is associated with Earth in the [C]HZ and supporting a rich biosphere and therefore a trivially easy BoE calculation that can be calculated for all stars with detected planets to indicate whether they might be living worlds?
I would think that a comparison of traditionally calculated HZs to this calculation would be most instructive if clear differences emerge.
We know that billions of years ago, there is the “paradox of the faint young sun” being insufficient to make Earth habitable with the possible atmospheric gases composition and thickness. Yet here we are. What conditions are we missing – other gases to trap heat, greater internal heat, or something unknown? Does the simple expanded sun calculation offer any extra insight about habitability ranges?
A.T.,
You know, I was just thinking about that too. From what I recall, the sun has changed in luminosity by about “half a magnitude”. So, roughly speaking, if it is now a 4.4 magnitude star, it was once a 4.9. And a magnitude would be a diminishment to about 40%.
…We could cheat a little and say it was about 70% of its current luminosity a couple billion years ago. But given that, we would like to know what its temperature and/or radius was. Still, we can just say it was 70% less luminous whatever its effective surface temperature was and still obtain a local effective temperature at 1 AU based on 70% less flux.
Looking at my scratchpad I think the answer is taking the our previous answer for conditions at 1 AU distant from the source, the R squared in the relation remains the same, but the T^4 factor based on 400 is reduced by 70%.
Double square root of [0.7 x 400 ^4] = 365.87 K
Using a different derivation approach, one of my old textbooks refers to 395 K as the Earth’s subsolar point temperature. Then, disregarding albedo, the solar absorbing area is treated as a disk, and the unilluminated hemisphere surface is treated as a radiator, responsible for the low ball equilibrium “black body” temperature of about 277 K. Significantly that value is just above water freezing. Same was applied to the moon. But the Earth equilibrium temperature observed runs between 250-300 K, according to table – and the moon between 120 and 390 K. Presence or absence of atmosphere and other factors… One might be nearly a month-long lunar day if the readings are local.
Yet it is another way to illustrate possible contrasts possible in the habitable zone.
Of course, getting back to the red dwarf habitable zone and x-ray exposure for planets therein, we have to consider both the planetary formation epoch and then the on-going radiative influence after a planet is designated as formed.
When I look for models, understandably most are based on having primaries similar to our sun ( to explain our own existence). Originally, in the 1990s, when there was little proof of exoplanets due to inadequate sensors, it was assumed that the circumstellar disk during the stellar formation phase would allow a window of about 30 million years from after the point when the protostar formed and began to radiate energy. By the end of that, the circumstellar disk would be largely cleared and the protostar would settle into the main sequence.
Well, this basic picture would be speeded up by brighter more massive stars and slowed down for less massive ones. Red dwarfs time lines for formations would be about 300 million years instead of 30 million. And that still applies.
Now correspondingly, the picture for planets forming around G-stars corresponds to this initial picture. But whatever the x-ray emissions are for an M, the scenario begs the question of how long it takes for planets to form around an M dwarf.
Some of the dynamics would be slowed down if the smaller mass star takes ten times as long to emerge out of the protostar cloud. But on the other hand, planets within the nominal HZ is one 50 to 100 times closer. Since our solar system begins at Mercury, we would tend to dismiss the idea of G stars having closer terrestrial planets – but transits show some evidence for their existence.
So, to take the Pollyanna point of view for illustration, some of the x-ray exposure possible in an early red dwarf planetary system could be before the planets form into terrestrial size bodies -if it takes longer for the protostar to coalesce and go main sequence. Thereafter, since they are so close to the “screen”, if the red dwarf is an x-ray emitter, it will have a significant effect and habitability would be a trade between low temperature and intense x-ray and charged particle flux.
Whether it’s through theoretical models or genuine exoplanet measures, we probably won’t need to wait decades for answers.
And as for the 400 K index? From a habitability standpoint, red dwarfs likely have some x-ray flux patterns, initially and over lifetime. Well, if we can characterize red dwarf x-ray flux intensities over time, we might even be able to adjust the BB temperature line
accordingly through M0 to M10 and their age.