Guest Post from NASA Sagan Postdoctoral Fellow Dr. Jonathan Gagné

Today's post comes from my collaborator and current NASA Sagan Postdoctoral Fellow, Dr Jonathan Gagné.  Our work is featured in a press release from this week.   

In other news, another one of my collaborators, Andrew Vanderberg, had his paper accepted for publication.  Andrew, a graduate student at Harvard, analyzed the relevance of M dwarf stellar rotation periods in the search of exoplanets with the radial velocity method.  More on this new paper in another post...


When exoplanets orbit their host star, they exert a gravitational force on it, causing the star to revolve around a tiny ellipse that is usually smaller than the star itself. Astronomers can then detect this tiny stellar motion and deduce the presence of a planet indirectly, through what is called the radial velocity method. This is the method that allowed scientists to discover the first exoplanets, called Hot Jupiters, around other stars in our Galaxy.

It is however very difficult to detect small planets with the radial velocity method, as they have a much weaker pull on their host star.  One way to do this is to search around smaller stars, which are more easily affected by their planets. These small stars, also called low-mass stars, are also much more numerous in our Galaxy, which makes them even more interesting as targets for planet hunters.

Astronomers have thus tried identifying small planets around small stars with the radial velocity method, but have stumbled upon a major obstacle: stellar activity. Much like the Sun, other stars have magnetic fields that evolve through time and cause dark stellar spots to appear and move around on their surface. These dark spots, combined with the rotation of the star, can imitate the signature of a planet in our data, an effect that is called "stellar jitter". The signature of these stellar spots is strong at the visible wavelengths of light, which have been used in the radial velocity method.

The team of Dr. Plavchan have started exploring another avenue that might crack this obstacle of stellar activity: infrared light. Effectively, the effect of dark stellar spots is expected to be much weaker at the longer wavelengths of light, in a domain that is called the near-infrared. The signature of a true exoplanet, however, would cause exactly the same signature at all wavelengths of light. Obtaining data in the near-infrared could thus be used to differentiate true exoplanets from stellar activity.

Measuring the small motions of stars in the near-infrared is a technical challenge, since this requires very precise and well calibrated instruments. Dr. Plavchan has worked with the NASA Jet Propulsion Laboratory (JPL) to develop a piece of equipment, called an isotopic methane gas cell, that is used to achieve a much better precision in our radial velocity measurements at near-infrared wavelengths.

The methane gas cell developed by Dr. Plavchan and NASA/JPL.

Using this updated equipment, our team has started a follow-up of 27 bright low-mass stars, some of which are known to have exoplanets, and others of which are known to have very strong stellar activity. The CSHELL spectrograph at the NASA IRTF 3-meter telescope was used to carry out this survey; our gas cell survey was added to improve this instrument's radial velocity capabilities. The IRTF medium-sized telescope was ideal to obtain more observing time, which was needed to demonstrate the efficiency of this new equipment.

This was the start of a 5-years survey of tireless data accumulation and fight against systematics of the CSHELL instrument. The CSHELL camera uses a detector that is 25 years old and started to slowly deteriorate with age. We had to develop very specialized data analysis tools in order to successfully extract our data despite these newly appearing problems.

Our team has also developed a new method to extract the data in an efficient way, while taking account of the specialized equipment that we added to CSHELL. This method is described in a scientific paper that was led by Peter Gao.

The NASA/IRTF telescope located on the summit of Mauna Kea in Hawaii.

The results of the survey itself were presented in another scientific paper that I led. We demonstrate in this paper that the gas cell has much improved the ability of CSHELL to detect smaller exoplanets around low-mass stars. Our gas cell allowed obtaining radial velocity measurements that were more precise than all other surveys which were undertaken with CSHELL!

Our survey has also allowed us to detect the known exoplanet system called GJ 876 bc, which had only been previously detected at visible wavelengths. A few other stars showed unexpected variations that could be due to exoplanets, but an additional follow-up will be needed to confirm their existence.

Another important feature of our survey is  that we measured the level of stellar jitter on some very young and active low-mass stars. Our results indicate that this jitter seems smaller than what is typically seen at visible wavelengths on some of the most active stars in our solar neighborhood. Our data provides a few bricks to the wall of understanding stellar jitter, as other near-infrared and visible-light surveys will need to observe the same stars before we can really understand jitter in a more precise way.

The radial velocity variations of the star GJ 876 which were measured in our survey. The expected disruption of the star’s velocity from its known planets is displayed with the black curve.

In summary, the work that we presented in this new scientific paper demonstrates the high potential of using near-infrared light to detect small exoplanets around small stars, and in particular to beat the obstacle of stellar activity. The same method and equipment will be used on new instruments such as iSHELL at the IRTF telescope, which will potentially allow the detection of Super-Earths, rocky planets that are much larger and more massive than the Earth, near the habitable zone of their host star where water can exist in its liquid form.

This is only the beginning of the quest for small planets using near-infrared light and the radial velocity method, and the methods and equipment that the team of Dr. Plavchan have developed will be extremely useful to answer fundamental questions that will direct the future of exoplanet research.