Because we only have direct images of a tiny number of planets orbiting other stars, we’re used to extrapolating as much as we can from our data and plugging in possible scenarios. But as the recent announcement of two ‘super-Earths’ around Kepler 62 demonstrates, we’re coming up hard against the limits of our knowledge. The comments on my recent story on the Kepler find bring up Greg Laughlin’s always interesting systemic site and a post he made in early April. Laughlin (UC-Santa Cruz) is worth reading not only for his shrewd analysis but for the sheer brio he brings to the exoplanet hunt. And here he sounds a note of caution:
I think we currently have substantially less understanding of the extrasolar planets than is generally assumed. Thousands of planets are known, but there is no strong evidence that any of them bear a particular resemblance to the planets within our own solar system. There’s always a tendency, perfectly encapsulated by the discipline of astrobiology, with its habitable zones and its preoccupation with water — to make wild extrapolations into the complete unknown.
Many Centauri Dreams readers don’t read the comments after each post, so I wanted to get that sentiment out in plain sight. One of the problems Laughlin notes is that if you plot the mass-radius relationships for super-Earths, you get numerous possibilities based on what we have found so far. We find what Laughlin calls a ‘huge range in apparent planetary structures’ here. Little more needs to be said: We’re tempted to look for clement conditions for astrobiology but we have to be careful that we don’t read our own hopes into still ambiguous results.
Image: A size comparison of Kepler 62 e and f as measured against the Earth. How much do the images contain our own hopes for astrobiology as opposed to what we really know? Credit: Planetary Habitability Laboratory.
Using data from the Allen Telescope Array, the SonATA program (SETI on the ATA) has continued to run a SETI search that focuses on Kepler candidates, especially those thought to be in the habitable zone of their stars. A recent news release homes in on the Kepler 62 finds, though with a caution that rings true in relation to Laughlin’s thoughts above. Clearly, Center for SETI Research head Gerry Harp would love to point his equipment at worlds with water on the surface, but in the case of Kepler 62 the mass of both habitable zone planets is unknown because they’re too small and too far away to analyze their gravitational effects on the host star.
Jon Jenkins, senior scientist at the Carl Sagan Center of the SETI Institute, is thus wise to take a measured approach:
“These discoveries move us farther down the road to discovering planets similar to Earth. While we don’t know if Kepler 62e and f are rocky or whether they have liquid water pooling on their surfaces, their existence shows that the incidence of small worlds in the habitable zone of sun-like stars is high. Thus we can look forward to the discovery and detailed characterization of Earth’s cousins in the years and decades to come by future missions and telescopes.”
And that seems like a legitimately cautious take-away from the Kepler 62 work. Of the five planets thus far identified around the star, two super-Earths seem to be in the habitable zone, but we can’t push definitively beyond that. The good news, as Laughlin notes on systemic, is that with the approval of TESS (Transiting Exoplanet Survey Satellite), we’re going to be discovering transiting super-Earths around the few million brightest stars close enough for detailed radial velocity follow-up, all of this within a scant ten years. TESS will provide the data we need to get a better understanding of the mass-radius relationships about which we have so many questions.
Blending Detection Methods with TRENDS
And while we’re talking exoplanets, let me take note of the TRENDS project (TaRgeting bENchmark-objects with Doppler Spectroscopy) being led by Justin Crepp (Notre Dame). Caltech’s John Johnson heads up Project Minerva at the university’s Exolab, recently described here as a telescope array dedicated to Earth-like planets around nearby stars, one capable of both radial velocimetry and transit studies. Crepp worked with Johnson as a postdoc and is now, like his mentor, combining exoplanet detection methods. TRENDS uses radial velocity measurements to identify promising targets for high-contrast imaging observations.
Initial results here are promising. The TRENDS team has discovered low-mass stellar companions orbiting HD 53665, HD 68017, and HD 71881 using NIRC2 adaptive optics (AO) observations. Its follow-up imaging has demonstrated common proper motion; i.e., co-moving companion objects with estimated spectral-types of K7-M0, M5, and M3-M4 respectively. These three M-dwarf companions could be found in the long-term Doppler trends of the host stars and the follow-up work at Keck then allowed their direct detection. From the paper:
Few mass, age, and metallicity benchmark dwarfs are known to date (Liu et al. 2007; Dupuy et al. 2010; Bowler et al. 2012a,b). The goal of the TRENDS high-contrast imaging program is to discover and characterize low-mass stellar and substellar companions with physical properties determined independently from spectro-photometric measurements, in order to calibrate theoretical atmospheric models and thermal evolutionary models. Each of the companions presented are amenable to direct spectroscopy using AO-fed integral-field units, such as OSIRIS at Keck or Project 1640 at Palomar.
This kind of news doesn’t drive the headlines, but we’re looking at important work. Crepp’s team is coming up with metallicity and age benchmarks, and further observation should produce mass benchmarks as well. Moreover, Johnson called my attention to another paper in which Crepp and colleagues characterize the brown dwarf HR7672B, establishing it as a “rare and precious ‘benchmark’ brown dwarf with a well-determined mass, age, and metallicity…” This is how you test your knowledge of the spectral and evolutionary models we need to make sense of what we see. Combining radial velocity and direct imaging clearly yields powerful new analysis.
The TRENDS work demonstrates improved ways to use our tools at the same time that it reminds us how much we have to learn. There is a raw excitement in pushing our methodologies in such synergistic ways. As we look forward to what projects like TESS will reveal in a few short years, we should keep in mind what while those Earth analogues are surely out there, tuning up our techniques will enable discoveries we haven’t begun to anticipate.
For more on TRENDS, see Crepp et al., “The TRENDS High-Contrast Imaging Survey. I. Three Benchmark M-dwarfs Orbiting Solar-type Stars,” accepted at The Astrophysical Journal (preprint). See also Crepp et al., “The Dynamical Mass and Three-dimensional Orbit of HR7672B: A Benchmark Brown Dwarf with High Eccentricity,” The Astrophysical Journal Volume 751, Issue 2 (2012), article id. 97 (abstract).
An issue with super Earths is well the mass -the gravity wells will not allow hydrogen to escape fast enough. If life (oxygen producing) gets a hold though it could convert the hydrogen using carbon dioxide into water making a water world.
Hi Paul,
As always your site and articles resonate for hours and days with thought
provoking ideas. I think that Laughlin may have coined the understatement
of the year;‘huge range in apparent planetary structures’ …very droll and rich in implication.
Best regards,
Mark
Regarding the PHL images, I really don’t like the orange cast they put on their renderings of planets orbiting cool stars. I’ve been in a room lit by filament light bulbs (which are roughly in the same temperature range as a mid-to-late M-dwarf) and haven’t been overwhelmed by the blatant orangeness of everything. The brain is pretty good at compensating for the spectrum of the illumination.
Heh, bearing in mind the enormous difference in time any possible civilization would have with us I doubt SETI will have any chance of success, especially in the narrow window of time civilizations use radio waves. And if there would be an advanced civilization there, capable of beaming enormous energy transmissions for whole thousands if not millions of years, then it would have surely detected us by more advanced telescopes and traveled for direct study(as such endeavor would be within its reach).
As to the studies, they are exciting, but yes, we do know little, despite the fact how much we learned. I am looking forward to Gaia, TESS, CHEOPS and others, but I am afraid to get real answers we will need at least new SIM and then hypertelescopes…
I wonder how exactly TESS will broaden our knowledge about the planets in the HZ of G and K stars.
It is said that it will be able to detect planets with orbits no larger than 60days. It is ok for M dwarves but… Earth twins will be excluded from the basket. Or maybe TESS is rather designed as a planetary systems detector that will leave the rest of research, icluding the detection of longer period planets for ground based follow up? Franky speaking I do not have a clue…
Is there any accepted standard for what would constitute habitable for humans as they are? What mass range, atmospheric density, temperature, amount of water, etc? If it had an oxygen atmosphere is would already be inhabited, but are there additional constraints we would need? While a Martian outpost may be possible, would anyone consider it habitable?
The lack of mass measurements is really frustrating: there’s no way so far to tell if the Kepler-62 planets are rocky or not. Not much hope for RV followup either.
Seems to me the best hope for an unambiguous HZ rocky planet from the Kepler mission would have to be one which is undergoing significant transit timing variations.
If so, I wonder what the local equivalent of the Milankovitch cycles would be like…
The Crepp et. al brown dwarf paper has a preprint at http://arxiv.org/abs/1112.1725. Apparently, the brown dwarf desert is still quite empty.
Greg Laughlin makes a very good point that we know far less about the exoplanets
than generally assumed. For example there were times when “Earth-like” conditions
meant a planet that was almost completely covered by miles of thick ice
and at other times, “Earth-like” conditions meant a planet with no ice with sub-tropical climates reaching the poles. Therefore since “Earth-like” conditions meant very different things
during Earth history, than even if we found an planet exactly like Earth orbiting a star
exactly like the Sun, it would be anyone’s guess what would be the conditions on that planet.
I still dont’t get the obssession with “sun-like” stars, there’s nothing convincing that make them better to produce habitable or Earth-like worlds. M stars should be much better to study since they are smaller, so it’s easier to get signals from the planets and star pertubations by the planets. Sun-like stars are just much harder to get results with today technology, takes lot of time.
I consider what the ATA is doing in terms of SETI with the Kepler data to be token efforts to appear viable. I know they got a three million dollar boost not too long ago, but how long will that last and are they receiving steady funding from somewhere?
Even more, last year Frank Drake said in an interview that they were collecting lots of data but had neither the money nor the staff to process it. What is happening there?
What are other SETI programs up to these days? It is hard to know who is doing what as they hardly ever seem to be trumpeting their work. Are they also in financial troubles?
Ironicall the general public still thinks there is this giant government program using huge radio telescopes listening to the stars 24/7. Instead it seems that 53 years after Project Ozma we are still seeing SETI programs begging and scraping for money and telescope time. Yet even professionals are shocked that we somehow haven’t gotten a cosmic greeting card or a string of prime numbers in our dishes and declare that we are alone in the galaxy.
@Rangel,
The cooler the star the closer a planet must orbit to be in the HZ and the closer it is to the star the more likely it will be tidally locked. I can’t see tidal locking of a planet been an issue though but it will place contriants on organisms living there.
Well, that would be Ok, if we were to throw away the requirement that
life needs H2O. Some models point to M stars tidal heating and flaring driving off the atmosphere of close in planets in < 1Gyr. So even if there are vast numbers of RedDwarves, their "fertility" to create life forms would appear to be pretty short. But the number of Reds is so great that I would agree that some lifeforms have taken hold there. But I think they in a doomed enviroment that may not allow the evolution of high lifeforms
and allow escape.
Our earth will enter this doomed enviroment when the sun strenght is enough disassocistes water molecules at a rate beyond the tipping point. The Earth will have been fertile for At least 4 Gya. Quite a difference.
the TRENDS project (TaRgeting bENchmark-objects with Doppler Spectroscopy)
Someone take that acronym out and shoot it; it’s suffered enough.
[TRENDS] Here we do not discriminate against projects that are acronymically-challenged, nor on the basis of font style or size. Well, except for Comic Sans in which case we punch its lights out.
TRENDS, that’s a good one. Stretched and fractured beyond belief.
It would be as if the mAssachusetts iNstitute of TEchnologY called itself ANTEY.
Rob Flores:
“Some models point to” is a pretty weak form of evidence that does not really allow for anything close to a conclusion. And consider this: Even if those models are true, that missing atmosphere can easily be brought back by making the planet just a little bit more massive.
Also consider: Flares and radiation make no difference at all under water, where life is thought to have evolved originally.
And: Tidal locking is nowhere near certain to occur, as is seen with Mercury. Nor is it necessarily detrimental to the development of life, if it does occur.
Discovery of first alien Earth threatened by budget cuts
NBCNews.com (blog)
Officials from NASA, the National Science Foundation and the SETI (Search for Extraterrestrial Intelligence) Institute gave testimony to the House Science, Space and Technology Committee on Thursday about the state of exoplanet research, saying …
http://science.nbcnews.com/_news/2013/05/13/18234173-discovery-of-first-alien-earth-threatened-by-budget-cuts?lite