The Plot Thickens: Habitable-Zone Exoplanets around Proxima Centauri and TRAPPIST-1

Introduction

As the time approaches to commission HPF on the Hobby-Eberly Telescope, we are learning that the spectrograph will be coming online in truly exciting times for exoplanet science!  The detection of habitable-zone exoplanets around two nearby M dwarf stars—including around the Sun’s nearest stellar neighbor—puts the science case for HPF into stark relief.  Let’s take a quick look at these systems, and discuss where HPF fits into the picture.

TRAPPIST-1: little star, big deal!

TRAPPIST is a robotic 60-centimeter telescope at La Silla Observatory in Chile.  It is entirely dedicated to observations of transiting exoplanets and minor bodies in the solar system.  One of the telescope’s first discoveries, announced in May, is TRAPPIST-1, a nearby M dwarf star with three transiting planets.  Each of the three planets have radii similar to that of Earth, and orbit near the star’s habitable zone.

 

Artist's impression of TRAPPIST-1 as seen from the outermost of its three transiting planets. Image credit: ESO/M. Kornmesser

Artist’s impression of TRAPPIST-1 as seen from the outermost of its three transiting planets. Image credit: ESO/M. Kornmesser

While we now know of exoplanets orbiting thousands of stars, the TRAPPIST-1 system is remarkable because of just how small and cool the star is.  TRAPPIST-1 is a member of a class of stars sometimes referred to as “ultracool dwarfs”. It has just 8 percent the mass of our Sun, and is only 5 ten-thousandths as luminous.  For comparison, if TRAPPIST-1 were as bright as a 100-watt incandescent light bulb, the Sun would be about as bright as a whole set of floodlights used at an outdoor sports complex.  TRAPPIST-1’s mass is near the minimum required to trigger hydrogen fusion in its core, making it just barely a star at all!

Ultracool dwarfs are so faint at visible wavelengths that they have almost never been targeted by exoplanet surveys.  Kepler observed almost none of them during its primary mission, and they are similarly underrepresented in Doppler planet searches to date.  Thus, the discovery of the TRAPPIST-1 planets is a real first.  To find that ultracool dwarfs host systems of multiple small planets like their more massive stellar siblings is an exciting step forward for the science of exoplanets around low-mass stars.

Proxima Centauri b: the habitable-zone planet next door

Proxima Centauri (Proxima for short) is part of the triple-star Alpha Centauri system, and the closest star to the Sun.  It is a small, distant companion to the binary stars Alpha Centauri A and B.  At a distance of about 4 light-years from the Sun, the Alpha Centauri system is so close that some people are already discussing sending robotic probes there.  From a more near-term perspective, stars as close as Proxima are prime targets for a huge number of astronomical instruments, including space-based telescopes such as the upcoming James Webb Space Telescope (JWST).  Thus, the time is now to identify and characterize any exoplanets around the Alpha Centauri stars.

Proxima has long been a target of exoplanet surveys.  HPF science team member Michael Endl was among the first astronomers to search for exoplanets around Proxima, observing it with the UVES spectrograph at the Very Large Telescope in Chile as far back as 2000.  As a result, we have known for some time that any planets orbiting close to Proxima must be small, or we would have found them by now.  This allowed for the tantalizing possibility that a small, terrestrial planet might reside in the star’s habitable zone.

It appears that this possibility has now proven to be a reality.  In January, a group of astronomers launched the Pale Red Dot program, a campaign to observe Proxima intensively with the HARPS spectrograph with the goal of discovering any terrestrial planets that might reside within its habitable zone.  The program also included near-simultaneous measurements of Proxima’s brightness as part of an effort to mitigate the effects of stellar magnetic activity, which we have discussed as a challenge for the discovery of low-mass exoplanets around M dwarf stars.

This week, the team announced they had in fact discovered a potentially Earthlike planet: Proxima b, which has a minimum mass just 27 percent greater than Earth’s and orbits in the very middle of the habitable zone.  At a distance of just 5 percent the separation of the Earth from the Sun, the planet completes an orbit every 11.2 days.  Proxima b has not yet been observed to transit its star, but it has not been ruled out, either.

Artist's impression of the Alpha Centauri stellar system as viewed from the surface of the habitable-zone planet Proxima b (Image credit: ESO/M. Kornmesser)

Artist’s impression of the Alpha Centauri stellar system as viewed from the surface of the habitable-zone planet Proxima b. Image credit: ESO/M. Kornmesser

Proxima b represents one of the most significant exoplanet discoveries to date.  Proxima Centauri is close enough to enable follow-up observations not feasible for many other stars.  If the planet does transit, JWST could potentially characterize its atmosphere.  If not, it may still be possible to detect molecules in its atmosphere with upcoming 30-meter-class telescopes.  Astronomers are a creative bunch, and it is likely that they will find ways to explore Proxima b that have not even been discussed yet!

How HPF fits in

It is important to keep in mind that the F in HPF stands for “Finder.” The primary objective of HPF is to discover new exoplanets around low-mass stars. However, the discovery of the TRAPPIST-1 system emphasizes another important role for HPF, which is to characterize planets that are found to transit nearby stars.  Experiments searching for transiting exoplanets such as TRAPPIST, Kepler (now operating as K2), and TESS will find many systems like TRAPPIST-1 in the coming years.  With its combination of near-infrared wavelength coverage and a large-diameter telescope, HPF is uniquely suited to provide Doppler follow-up on the coolest, dimmest nearby stars.

As for Proxima, the discovery of Proxima b reiterates the idea that there are many exciting worlds still waiting to be uncovered.  Proxima is a benchmark for the type of star HPF will observe most frequently, an idea we will discuss in more detail on this blog shortly.  The instrument is nearing completion, but the work is just beginning.

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NEID, HPF’s sister spectrograph

Recently, the HPF team was selected to build the NEID spectrograph, the next generation spectrograph for the 3.5m WIYN telescope at Kitt Peak National Observatory, located on the Tohono O’odham reservation in Arizona. The word neid means ‘to see’ in the language of the Tohono O’odham, where we as a community, are privileged to be doing astronomical observations.

NEID is funded by the joint NASA/NSF partnership NN-EXPLORE program, administered by the Jet Propulsion Laboratory (JPL) in Pasadena, with the goal to support and advance exoplanet research through an ultra-precise radial velocity observational program. NEID is designed from existing heritage to address some of the most pressing and exciting exoplanet questions over the next decade—many of which can only be answered with extreme precision radial velocities. In particular, NEID leverages direct technology heritage from HPF, adopting many subsystems that have been discussed in detail here on the HPF blog. Like HPF, NEID will have its own dedicated blog, which will be kept updated with news on the build, assembly, tests, and new results. Go check it out here: http://neid.psu.edu/ !

wiyn

The 3.5m WIYN telescope at Kitt Peak.

NEID_logo

The NEID Logo

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The HPF cryostat test drive: sub-milliKelvin temperature stability

Background: the need to cool HPF down to 180K

One of the most frequently discussed topics on this blog has been the need to enclose the HPF instrument in a stable, cold environment.  Because HPF will observe stars in infrared light, it is essential to remove as much of the thermal infrared radiation from the system as possible, and keep the rest absolutely constant.  We achieve this by placing the entire instrument in a vacuum chamber (known as a “cryostat”) and cooling it to 180K (-93.15° Celsius).

Previous posts on the HPF blog have covered the theory of our thermal control system and the design of our vacuum cryostat.  Since posting those articles, we have been hard at work bringing this system off the spec sheet and into the lab.  The cryostat is now pumped down, and the environment control system (ECS) is running.  Let’s look at the results!

Easier Said Than Done

Very briefly, our cryostat is designed to maintain a constant temperature by cooling the instrument to about 170K using copper thermal straps connecting the optical bench to a tank of liquid nitrogen, then warming back up to 180K with electrical heaters controlled by a custom computer system.  Below is a schematic of the basic ECS operation.

A schematic of how the HPF Environment Control System (ECS) maintains a constant 180K instrument temperature.

A schematic of how the HPF Environment Control System (ECS) maintains a constant 180K instrument temperature.

Every step of this process requires specialized hardware and careful attention to detail.  We showed you the construction of the thermal straps and electrical heaters in a previous entry, but let’s look at some other aspects of the system that are now in place.

While we are unable to verify officially, we are reasonably confident that HPF has more internal electronics and wiring than any previous astronomical spectrograph with no moving parts.  The temperature sensors and heaters seen in the schematic above are simple enough devices on their own, but managing the wiring for the approximately 65 such units in a vacuum chamber can be quite the tangle.  We made dozens of custom wiring harnesses with specialized vacuum feed-throughs to connect our cryostat to the outside world.

Eric Levi and Paul Robertson make the final electrical connections before closing the radiation shield lid.

Eric Levi and Paul Robertson make the final electrical connections before closing the radiation shield lid.

Vacuum feed-throughs for the HPF cryostat. Note the number of wires!

Vacuum feed-throughs for the HPF cryostat. Note the number of wires!

Additionally, achieving vacuum pressures of 10-7 Torr is not as simple as hooking up your household dust buster.  We need two separate pumps: one “roughing” pump to remove the first 99.99999% (!) of the air, and then a high-speed “turbo” pump to get us the last three orders of magnitude.  When you add in the accompanying safety valves and vacuum gauges, the cryostat and its vacuum components start to take up quite a bit of space and power.

The HPF vacuum pumps

The HPF vacuum pumps and electronics rack.

So, does it all work?

A system this complex—with so many components that have to work in concert to achieve the desired results—gets road-tested in incremental stages.  Our first full-scale environment control test was a “warm test”, designed to test the vacuum and electronic systems.  In this mode, we disconnected the copper thermal straps from the optical bench and attempted to maintain temperature stability near room temperature.  Rather than use the liquid nitrogen tank as a heat sink, we used the heaters to raise the internal temperature about 10 degrees above room temperature, and used the entire ambient environment as the heat sink.  The liquid nitrogen tank is still filled, but only to cool the activated charcoal getters that maintain our exquisite vacuum.

A secondary motivation for the warm test is our ongoing instrument concept design for NASA’s upcoming Extreme Precision Doppler Spectrograph, or EPDS.  Our HPF team is one of two finalist groups competing to build the instrument.  Because the EPDS spectrograph will use optical wavelengths (as opposed to HPF’s near-infrared coverage), it can be operated near room temperature, as the thermal background does not affect the visible spectrum.  However, EPDS still requires excellent temperature stability, hence our adaptation of the HPF concept to warmer temperatures.

We just completed a two-week warm stability test, and we are beyond excited with the results.  Over the entire two-week run, the cryostat was stable to less than a milliKelvin!  Better yet, over a typical 24-hour period, our RMS stability is closer to a tenth of a milliKelvin.  For HPF, this exceeds our needed stability by about a factor of ten, giving much-needed assurance that the cryostat will provide a suitable environment for precision Doppler spectroscopy.

Without further ado, here are the data!

Results from a 2-week warm temperature stability test. All 14 active control heaters remained stable to within 0.1 milliKelvin, while the optical bench maintained sub-milliKelvin stability, and exceeded 0.15 mK stability over 24 hours.

Results from a 2-week warm temperature stability test. All 14 active control heaters remained stable to within 0.2 milliKelvin, while the optical bench maintained sub-milliKelvin stability, and exceeded 0.15 mK stability over 24 hours.

Even with this encouraging result in hand, we know we can do better.  The longer-term fluctuations seen in the plot above are caused by ambient pressure changes, which we will mitigate by installing a pressure regulator on the liquid nitrogen tank.  And remember, this system is designed to operate at 180 Kelvin!

Next up: the cold test.  The thermal straps are reattached, and the liquid nitrogen is flowing.  Stay tuned!

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