Direct Imaging of Exoplanets around Alpha Centauri and Other Multiple Star Systems

The goal of the project is to develop a new method to directly image planets in multiple-star systems such as Alpha Centauri.

A (sun-like) star is 10 billion times brighter than an Earth-like planet that may be orbiting around it. In any real telescope, diffraction and glare from that star completely swamps the miniscule brightness of the planet, similar to how a searchlight swamps the light of a nearby firefly. Several starlight suppression systems have been demonstrated in the lab that can remove the leakage of light from a star close to 10¹⁰ contrast, enabling Earth-like planet detection, but only for single-star systems. The challenge with double-star (or multi-star) systems is that usually the off-axis companion star lies outside the nominal control region of existing systems and therefore its diffraction and glare cannot be controlled. Up till now, the “common wisdom” has been that removing the light of a companion star is impossible with current technology, and so binary star systems are often excluded from target lists of direct imaging missions. Our new method enables existing systems directly image exoplanets in multiple-star systems.

The main reasons why this question is important are:

• Most of Sun-like stars are in multi-star systems
• In particular, Alpha Centauri (a Cen) is a multi-star system and deserves close attention because:
  • Its habitable zone can be resolved by a small and potentially cheap (~25cm) telescope
  • a Cen is humanity’s next frontier beyond the solar system
  • Kepler recently announced that 22% of stars have Earth-like planets around them, so a Cen has about a 40% chance of harboring an Earth-like planet around either of its stars

Science/Technical Objectives:

We propose to demonstrate a new method to directly image planets in multiple-star systems such as Alpha Centauri.

A (sun-like) star is 10 billion times brighter than an Earth-like planet that may be orbiting around it. In any real telescope, diffraction and glare from that star completely swamps the miniscule brightness of the planet, similar to how a searchlight swamps the light of a nearby firefly. Several starlight suppression systems have been demonstrated in the lab that can remove the leakage of light from a star down to 10¹⁰ contrast, enabling Earth-like planet detection [1], but only for single-star systems. These systems employ a combination of a coronagraph to suppress the star and a wavefront control system based on a deformable mirror (DM) to remove residual starlight (speckles) created by the imperfections of telescope optics. Removal of speckles by the DM can only occur up to the spatial frequency range of the deformable mirror (half-Nyquist).

The challenge with double-star (or multi-star) systems is that usually the off-axis companion star lies outside the DM’s control region and thus the DM is unable to remove the speckles from the off-axis star. Up till now, the “common wisdom” has been that removing the light of a companion star is impossible with current technology, and so binary star systems are often excluded from target lists of direct imaging missions.

The main reasons why this question is relevant is that:

• Most of Sun-like stars are in multi-star systems
• In particular, Alpha Centauri (a Cen) is a multi-star system and deserves close attention because:
  • Its habitable zone can be resolved by a small and potentially cheap (~25cm) telescope
  • a Cen is humanity’s next frontier beyond the solar system
  • Kepler recently announced that 22% of stars have Earth-like planets around them, so a Cen has about a 40% chance of harboring an Earth-like planet around either of its stars

Innovation: Aliased wavefront control (ACW)

We propose to use a mild grating (or an existing pattern commonly found on many DMs left over from their manufacturing process) to remove light from the off-axis star. 

The grating creates a controlled diffracted image of the off-axis star close to the on-axis star, placing the diffracted off-axis image within the DM control region, enabling it to remove the speckles from both stars (which can be done independently at the cost of halving the size of each control region). This solution is innovative because it uses the DM in a regime outside its nominal control range, where the DM experiences what is typically a harmful side effect: spatial aliasing. Our innovation is that we use aliasing as a feature, rather than a bug. Taking advantage of aliasing is a trick from another field (signal processing theory) with could be the reason why this simple solution has not yet been proposed in astronomy. We call this method “Aliased Wavefront Control”, or ACW.

The impact of this solution is that it potentially enables the detection of biomarkers on Earth-like planets (if they exist) around our nearest-neighbor star, Alpha Centauri, with a small and cheap space telescope or even a balloon, potentially decades sooner than a large space coronagraph could do the same around a single star system (the closest one is much more challenging that Alpha Centauri). In addition, this solution enables a large space telescope to survey multi-star systems, magnifying its science return significantly.

Approach expected results?

We will start by demonstrating ACW theoretically and with the following simulations:

• 1. Create a dark zone beyond the OWA (in monochromatic light) for an off-axis star
• 2. Simultaneously create a dark zone close to a near star and far from an off-axis star (in monochromatic light)
• 3. Repeat #2 for broadband light.

We will then perform preliminary demonstrations of 1-3 in the Ames Coronagraph Experiment (ACE) laboratory [e.g. 2].

• Expected results: We expect the simulations to be successful down to 1e-10 contrast, and a 10%-wide spectral band; and preliminary demonstration to get down to ~ 1e-6 contrast in monochromatic light.

References

1. Lawson, P. R., et al., Proc SPIE 8864-50 (2013).

2. Belikov, et al., Proc SPIE 8442-6 (2012).

3. Thomas, et al., Pric SPIE 8864-50 (2013).